Dissertations / Theses on the topic 'COMBUSTION STUDIES'

This thesis presents fundamental research about the combustion gas products and solid phase residue of switch grass combustion. To identify the compounds released during the combustion phase, tests were conducted using a Thermogravimetric Analyzer (TGA) coupled to a Fourier Transform Infrared Spectrometer. These test revealed that aromatic compounds as well as carbon dioxide and water were released.
High Pressure Liquid Chromatography (HPLC) and GCMS/GCFID were also used to identify and semi-quantify polycyclic aromatic hydrocarbons (PAH) and benzene derivatives. From these analyses it was concluded that thermal synthesis was not occurring within an oxidative environment and as such no PAHs were found.
Finally an infrared microscope and a scanning electron microscope were used to study functional group, morphology and metal content change resulting from the combustion process.
This research provided information about the combustion mechanism of switch grass and laid the foundation for pilot-scale testing.

The combustion of clouds of fuel droplets is of great importance in many industrial applications, such as gasoline and diesel engines, gas turbines and furnaces. Here, efficient combustion has to be combined with minimum noxious emissions. Aerosols also might produce a particularly hazardous explosion risk. To optimise their performance a fundamental understanding of the complex processes in aerosol combustion systems is necessary. A fundamental study of aerosol combustion has been conducted to quantify the parameters of importance. For this, a novel aerosol combustion apparatus was developed, that offers a well controlled environment with respect to aerosol properties, temperature, pressure and turbulence. Aerosols were generated using the Wilson cloud chamber principle of expansion cooling, which produces a homogeneously distributed, near monodisperse droplets cloud. Drop sizes of 10 to 30μm, pressures between 100 and 360kPa and temperatures of 263 to 292K were used. Laminar mixtures between the overall equivalence ratios of 0.8 and 1.2 were studied. A considerable burning velocity enhancement of up to 420% was observed. This enhancement was shown to be a function of drop size and liquid fraction. From the study, it was concluded that burning velocity enhancement probably is caused by the increase in surface area due to wrinkling, caused by the development of instabilities. At low temperature (<275K) the formation and destruction of wrinkles and cells was random. At higher temperatures (>290K) cell formation and division was progressive and traceable, like that observed in gaseous flames. Cellular acceleration at these temperatures was similar to that of gaseous flames. Stretch appeared to have a damping effect on the instabilities, caused by the aerosol. Oscillating flames were observed for some experimental conditions and these also showed enhanced flame speeds. These oscillations were possibly caused by aerodynamic interaction between droplets and gas motion ahead of the flame. Also Stretch and radiation probably influenced these oscillations. Inert glass particles in a gaseous fuel-air mixture had no effect on flame speed or structure. However, water aerosols caused significant burning velocity enhancement (50%). These findings contradict the hypotheses that fuel rich pockets, flame propagation through "easy-toburn" regions or a "grid-effect" trigger instabilities in aerosols. Comparison with a linear stability analysis of heat loss from the flame (Greenberg et al.,1998), yielded good qualitative agreement with the data of the present work.

The thesis comprises a fundamental study of spherical premixed flame propagation,originating at a point under both laminar and turbulent propagation. Schlieren cine photography has been employed to study laminar flame propagation, while planar mie scattering (PMS) has elucidated important aspects of turbulent flame propagation. Thrbulent flame curvature has also been studied using planar laser induced fluorescence (PLIF) images. Spherically expanding flames propagating at constant pressure have been employed to determine the unstretched laminar burning velocity and the effect of flame stretch, quantified by the associated Markstein lengths. Methane-air mixtures at initial temperatures between 300 and 400 K, and pressures between 0.1 and 1.0 MPa have been studied at equivalence ratios of 0.8, 1.0 and 1.2. Values of unstretched laminar burning velocity are correlated as functions of pressure, temperature and equivalence ratio. Two definitions of laminar burning velocity and their response to stretch due to curvature and flow strain are explored. Experimental results are compared with two sets of modeled predictions; one model considers the propagation of a spherically expanding flame using a reduced mechanism and the second considers a one dimensional flame using a full kinetic scheme. Data from the present experiments and computations are compared with those reported elsewhere. Comparisons are made with iso-octane-air mixtures and the contrast between fuels lighter and heavier than air is emphasized. Flame instability in laminar flame propagation become more pronounced at higher pressures, especially for lean and stoichiometric methane-air mixtures. Critical Peclet numbers for the onset of cellularity have been measured and related to the appropriate Markstein number. Analyses using flame photography clearly show the flame to accelerate as the instability develops, giving rise to a cellular flame structure. The underlying laws controlling the flame speed as cellularity develops have been explored. PMS images have been analysed to obtain the distributions of burned and unburned gas in turbulent flames. These have enabled turbulent burning velocities to be derived for stoichiometric methane-air at different turbulent r.m.s. velocities and initial pressures of 0.1 MPa and 0.5 MPa. A variety of ways of defining the turbulent burning velocity have been fruitfully explored. Relationships between these different burning velocities are deduced and their relationship with the turbulent flame speed derived. The deduced relationships have also been verified experimentally. Finally, distributions of flame curvature in turbulent flames have been measured experimentally using PMS and PLIF. The variance of the distribution increases with increase in the r.m.s. turbulent velocity and decrease in the Markstein number. Reasons for these effects are suggested.

The important conclusion was reached that when combusting H2/CH 4 fuel mixes flashback behaviour approaches that of pure methane for equivalence ratios less than about 0.65, all pressures investigated up to 7 bara and air inlet temperatures of 300 and 473K. Significant deleterious changes in flashback behaviour for H2/CH4 fuel mixes occurred for air inlet temperatures of 673K, although operation at weak equivalence ratios less than 0.65 was still beneficial.

Investigation of forward in situ combustion have been carried out in a 7.3 cm diameter tube having a length of 0.869 m. Experiments at pressures up to 50 psig were made to study combustion characteristics and enhanced oil recovery of three different crude oils, namely North Sea Forties (36.6 °API), Maya Isthmus (32.4 °API) and Maya (22.1 °API). Sand packs were prepared with oil saturations in the range 38-44.32%. Close adiabatic control of the combustion tube was achieved for both dry and wet combustion modes. Detailed production history and overall mass balances are presented. Correlation in both graphical and tabular form is given for air-fuel ratio, oxygen utilisation and normalised combustion velocity. In this respect, the results of the present work show good agreement with those of other workers. Normal wet, partial quenched modes of combustion were produced using WARs up to 3.75 m3/Mm3 (STP). The combustion front temperature was not significantly affected by the cooling effect of the injected water. Under partially quenched conditions, high combustion-steam zone temperatures were achieved. For wet combustion, the oxygen utilisation generally improved slightly. Air requirement, air-oil ratio and fuel consumption all decreased with increased water-air ratio and increased with increased clay content. The velocity of the combustion front (normalised with respect to the air flux) increased in a linear manner as the WAR increased. Increasing the clay content, however, gave rise to a decrease in the combustion front velocity. High oil recovery, at 79.37%, was achieved during normal wet combustion of Forties oil. In sand mixtures containing amorphous silica powder, the combustion exhibited virtually 100% oxygen utilisation, with higher carbon burning rates compared with runs using clay addition. These effects are attributed to the nature and magnitude of the surface area of solid additives, which play an important role in the oxidation mechanisms.

Bibliography: leaves 119-121.
This thesis deals with the effects of cooling and heat losses in internal combustion engines. The object of this work was to examine and research various cooling concepts and methods to reduce heat loss to engine coolant, improve thermal efficiency and to predict heat transfer values for these alternatives. The optimum system to be considered for possible application to small rural stationary engines. A literature survey was undertaken, covering work performed in the field of internal combustion engine cooling. Besides the conventional cooling system, two concepts emerged for consideration. These were the precision cooling system and the new heat pipe concept, the latter being relatively unknown for internal combustion cooling application. The precision cooling system, consists of a series of small bore tubes conducting coolant only to the critical areas of an engine. The theory being that in the conventional systems many regions are overcooled, resulting in excessive heat loss. The heat pipe is a device of very high thermal conductance and normally consists of a sealed tube containing a small quantity of fluid. Under operating conditions the tubular container becomes an evaporator region in the heat input area and a condenser region in the heat-out area. It is therefore basically a thermal flux transformer,attached to the object to be cooled. The heat pipe performance is also capable of being modulated by varying its system pressure. This is a positive feature for internal combustion engine application in controlling detonation and NOx emissions. Various facts were obtained from the literature survey and considered in the theoretical review. These facts were extended into models, predicting the heat transfer performance of each concept in terms of coolant heat outflow and heat transfer coefficients. The experimental apparatus was based on an automotive cylinder head with heated oil passing through the combustion chamber and exhaust port to simulate combustion gases. Experiments were conducted on this apparatus to validate the predicted theoretical performance of the three concepts. Tests were also made to observe the effect of heat pipe modulation and nucleate boiling in the precision system. Concept theory was validated as shown by the experimental and test results. The performance for each system approximated the predicted heat transfer and heat loss values. By comparison of the heat input, coolant heat outflow values and heat transfer coefficients it was found that the precision system was the most efficient, followed by the heat pipe and the conventional system being the least efficient. It was concluded that the heat loss tests provided a valuable insight into the heat transfer phenomenon as applied to the three systems investigated. This work also illustrated the effects of the variation of coolant flow, velocity and influence of nucleate boiling. This thesis has shown the potential of the systems tested, for controlling heat losses in internal combustion engines. The research work has created a data base for further in-depth evaluation and development of the heat pipe and the precision cooling system. Based on the findings of the experimental work done on this project, several commercial applications exist for the heat pipe and precision cooling systems. Further in-depth research is recommended to extend their potential in the automotive industry.

Thesis (Ph. D.)--University of California, San Diego, 2006.
Title from first page of PDF file (viewed June 14, 2006). Available via ProQuest Digital Dissertations. Vita. Includes bibliographical references (p. 126-135).

Vision-based measurement as a useful tool has been applied successfully in many applications. The aim of this thesis is to apply vision-based measurement in both combustion and vibration studies. The purpose is to process and analyse the recorded light information for understanding combustion and vibration conditions. The chemiluminescence emission from a flame contains fundamental information on combustion, and the reflected light from an object’s surface can also provide information on the condition of the measured object. These types of information can be recorded quantitatively into images through a camera. Further processing and analysis of the image data can explore useful information. In this work, a high-speed stereo colour imaging system is employed for both combustion and vibration studies. In each study, a suitable methodology is developed. In a premixed hydrocarbon flame, the blue-green flame colour is mostly attributed to the presence and mixture of chemiluminescence emissions of CH* and C2*. The modern colour camera with the colour filter array (CFA) scheme inherently encodes with red, green, and blue wide-band wavelength filters. According to the aforementioned principles, a flexible image colour model is proposed to detect flame chemiluminescence emissions of CH* and C2*. A sensor calibration process is employed to refine the CH* and C2* concentration expressions based on different camera sensor spectral sensitivities. The detected CH*/C2* ratio is utilised to analogue the fuel/air ratio for combustion diagnostics. Two cases of flame propagation in tubes and flame ignition to impinging are studied using this proposed image colour-based flame chemiluminescence measurement. Combined with stereo imaging and high-speed imaging, the ability of the proposed method to perform multi-dimensional measurement is demonstrated. The reflected light from the measured object is the result of the interaction between the incident light and the object’s surface. A camera captures the illumination of the reflected light as intensity in an image. When the positions of the light source and camera are fixed, any image intensity variation from the reflected light could indicate the object’s movement. Hence, the measured images of a vibrating object would show intensity fluctuations. Based on this, an image intensity fluctuation-based vibration measurement is proposed. Two cases, wind turbine blade vibration monitoring and industrial coupling rotation-vibration testing, are studied using the proposed method. The ability of the image intensity fluctuation-based vibration measurement to perform one-dimensional and two-dimensional measurements is demonstrated successfully.

The effect of the solid-circulation rate and pattern as well as the air-gap size on heat-transfer coefficients between a horizontal, cylindrical heater and vibrated beds of Master Beads (spherical alumina) and glass spheres was studied. Solid piles were observed to form at specific bed locations. Solid-circulation paths were directed from the shallowest toward the deepest region of the vibrated bed. For beds in which the solid pile formed above the heating surface, local solid-circulation loops were observed above and below the heater. Air gaps developed at the top and bottom of the cylindrical heater. Heat-transfer coefficients of 140-350 W/m²K in beds of glass spheres and 180-480 W/m²K in beds of Master Beads were determined for a temperature difference of 30°C between the heater and vibrated bed. The trends in the behavior of the heat-transfer coefficient could be explained in terms of a model that accounted for the air-gap size and particle renewal in the layer closest to the heater. Increased solid-circulation rates improved the heat-transfer performance until larger air-gap sizes eventually compromised any increase in solid circulation. The expansion of the interlayer spacing of H-Magadiite (a layered silicic acid) by the introduction of pillars containing silicon atoms was investigated. A trisiloxane and two trichloroorganosilane compounds were used as the pillaring agents. The interlayer space of H-Magadiite was successfully expanded by pillaring with trichloroorganosilanes. The minimum dimensions of the pores that access the interlayer space of the pillared compounds were determined as being 6.2 Å and 9.5 Å (dimensions at perpendicular directions). Pillaring of H-Magadiite at low pH and temperatures close to 0 °C yielded the highest surface areas, e.g., increasing the surface area from 35 to 130-200 m²/g. The pillared compounds were found to be thermally stable up to temperatures of 650°C.
Ph. D.

An experimental investigation is presented of unsteady N2-H2 jet flames in a co-flow of hot combustion products of lean premixed hydrogen combustion. The unsteady jet flame is characterized by rapid ignition followed by a gradually blowout of the flame. Audio recordings and Schlieren imaging high speed videos are used to investigate the unsteady flame. The frequency of the blowout-re-ignition event is investigated as a function of nitrogen dilution mole fraction (YN2=0.180-0.566), co-flow equivalence ratio (Phi=0.20-0.27) and jet velocity (Vjet=300-500 m/s). The results from the audio recordings and Schlieren imaging high speed videos indicate that re-ignition of the flame occurs as a result of autoignition. The ignition frequency increases with increasing nitrogen dilution mole fraction until a maximum frequency is reached of about 20-27 Hz. After the maximum frequency is reached the frequency decreases with a further increase of the nitrogen dilution mole fraction until the flame is completely blown out. By increasing the co-flow equivalence ratio the flame becomes unsteady and blown out at increasing nitrogen dilution mole fractions. The range of nitrogen dilution mole fractions over which the flame is unsteady is decreasing with increasing co-flow equivalence ratio. By increasing the jet velocity the flame with low co-flow equivalence ratios (Phi=0.20-0.22) becomes unsteady and blown out for decreasing nitrogen dilution mole fractions. For higher co-flow equivalence ratios (Phi=0.24-0.27) the range of nitrogen dilution mole fractions over which the flame is unsteady increases with increasing velocity. An increase in the velocity at higher co-flow equivalence ratios leads to an unsteady flame for lower nitrogen dilution mole fractions and a blown out flame for higher nitrogen dilution mole fractions. These results suggests that the autoignition phenomena of the N2-H2 jet flame issued into a vitiated co-flow is controlled by both chemistry and turbulent mixing. The results from the audio recordings and the Schlieren imaging high speed videos correspond well. This gives confidence to using audio recordings as a method of diagnostics of unsteady hydrogen jet flames.

There are few experimental data of a fundamental nature that clearly demonstrate the similarities and differences in burning rates between single phase and two phase combustion, either in laminar or turbulent conditions. Such data are essential towards a better understanding of the spray combustion phenomena as well as a whole system. In the present study, experimental investigations of combustion of droplet and vapour air mixtures under quiescent and turbulence conditions have been conducted in a fan stirred combustion vessel. Aerosols were generated by expansion of gaseous pre-mixture to produce a homogeneously distributed suspension of fuel droplets. Spherically expanding flames following central ignition were employed to quantify the flame structure and propagation rate. The effect of droplets on flame propagation was investigated by comparing the burning rate of gaseous mixtures at initial pressure and temperature close to those of aerosol mixtures. In quiescent conditions, aerosols of two different fuels, isooctane and ethanol, were investigated at near atmospheric conditions. The effect of fuel droplets, up to 31 J.1m diameter, on laminar flame propagation was examined at a wide range of equivalence ratios. In the early stages of flame development, inertia of fuel droplets leads to local enrichment in equivalence ratio which increases the initial burning rate of lean aerosols but decreases that of rich ones. For the later stages of flame propagation, the presence of liquid droplets causes earlier onset of instabilities and cellularity than for gaseous flames, particularly at rich conditions. This leads to an enhanced burning rate and is probably due to heat loss from the flame and local disturbances due to droplet evaporation and subsequent diffusion processes. In turbulent studies, the effect of isooctane droplets up to 14 J.1m in diameter on flame propagation was examined at various values of root mean square turbulence velocities between zero and 4.0 mls. It is suggested that during early flame development, the turbulence was found to induce droplet motion before flame initiation which dominated over those resulting from the flame, negating the effect of droplet inertia. In the later stages, the presence of droplets in a low turbulent flame resulted in a significant burning rate enhancement. However, this enhancement became progressively less important as turbulent wrinkling became dominant. Between low and high turbulence, there was a transition regime between instability dominated and turbulence dominated regimes. As a consequence, the burning rate enhancement due to droplets under this transition range was rather complex.

This thesis focuses on enhancing knowledge on co-firing oxy-combustion cycles to boost development of this valuable technology towards the aim of it becoming an integral part of the energy mix. For this goal, the present work has addressed the engineering issues with regards to operating a retrofitted multi-fuel combustor pilot plant, as well as the development of a rate-based simulation model designed using Aspen Plus®. This model can estimate the gas composition and adiabatic flame temperatures achieved in the oxy-combustion process using coal, biomass, and coal-biomass blends. The fuels used for this study have been Daw Mill coal, El Cerrejon coal and cereal co-product. A parametric study has been performed using the pilot-scale 100kWth oxy-combustor at Cranfield University and varying the percentage of recycle flue gas, the type of recycle flue gas (wet or dry), and the excess oxygen supplied to the burner under oxy-firing conditions. Experimental trials using co-firing with air were carried out as well in order to establish the reference cases. From these tests, experimental data on gas composition (including SO3 measurement), temperatures along the rig, heat flux in the radiative zone, ash deposits characterisation (using ESEM/EDX and XRD techniques), carbon in fly ash, and acid dew point in the recycle path (using an electrochemical noise probe), were obtained. It was clearly shown during the three experimental campaigns carried out, that a critical parameter was that of minimising the air ingress into the process as it was shown to change markedly the chemistry inside the oxy-combustor. Finally, part of the experimental data collected (related to gas composition and temperatures) has been used to validate the kinetic simulation model developed in Aspen Plus®. For this validation, a parametric study considering the factor that most affect the oxy-combustion process (the above mentioned excess amount of air ingress) was varied. The model was found to be in a very good agreement with the empirical results regarding the gas composition.

In this thesis, two-stage combustion of biomass was experimentally/numerically investigated in a multifuel reactor. The following emissions issues have been the main focus of the work: 1- NOx and N2O 2- Unburnt species (CO and CxHy) 3- Corrosion related emissions The study had a focus on two-stage combustion in order to reduce pollutant emissions (primarily NOx emissions). It is well known that pollutant emissions are very dependent on the process conditions such as temperature, reactant concentrations and residence times. On the other hand, emissions are also dependent on the fuel properties (moisture content, volatiles, alkali content, etc.). A detailed study of the important parameters with suitable biomass fuels in order to optimize the various process conditions was performed. Different experimental studies were carried out on biomass fuels in order to study the effect of fuel properties and combustion parameters on pollutant emissions. Process conditions typical for biomass combustion processes were studied. Advanced experimental equipment was used in these studies. The experiments showed the effects of staged air combustion, compared to non-staged combustion, on the emission levels clearly. A NOx reduction of up to 85% was reached with staged air combustion using demolition wood as fuel. An optimum primary excess air ratio of 0.8−0.95 was found as a minimizing parameter for the NOx emissions for staged air combustion. Air staging had, however, a negative effect on N2O emissions. Even though the trends showed a very small reduction in the NOx level as temperature increased for non-staged combustion, the effect of temperature was not significant for NOx and CxHy, neither in staged air combustion or non-staged combustion, while it had a great influence on the N2O and CO emissions, with decreasing levels with increasing temperature. Furthermore, flue gas recirculation (FGR) was used in combination with staged combustion to obtain an enhanced NOx reduction. The fate of the main corrosive compounds, in particular chlorine, was determined in an experimental campaign using fuel mixtures. The corrosion risk associated with three fuel mixtures was quite different. Grot (Norwegian term used for tree’s tops and branches) was found to be a poor corrosion-reduction additive and could not serve as an alternative fuel for co-firing with straw. Peat was found to reduce the corrosive compounds only at high peat additions (50 wt%). Sewage sludge was the best alternative for corrosion reduction as 10 wt% addition almost eliminated chlorine from the fly ash. Numerical studies were also performed to estimate the emission level in the flue gas using a comprehensive mechanism in a configuration which simulated two-stage combustion of biomass. Furthermore, a reduction of the comprehensive chemical mechanism was performed since the mechanism is still complex and needs very long computational time and powerful hardware resources. The selected detailed mechanism in this study contains 81 species and 703 elementary reactions. Necessity analysis was used to determine which species and reactions that are of less importance for the predictability of the final result and, hence, can be discarded. For validation, numerical results using the derived reduced mechanism were compared with the results obtained with the original detailed mechanism. The reduced mechanism contains 35 species and 198 reactions, corresponding to 72% reduction in the number of reactions and, therefore, improving the computational time considerably. Yet the model based on the reduced mechanism predicts correctly concentrations of NOx and CO that are essentially identical to those of the complete mechanism in the range of reaction conditions of interest. The modeling conditions are selected in a way to mimic values in the different ranges of temperature, excess air ratio and residence time, since these variables are the main affecting parameters on NOx emission.
KRAV
CenBio

Master of Science
Department of Mechanical and Nuclear Engineering
Kirby S. Chapman
The increased emissions monitoring requirements of industrial gas turbines have created a demand for less expensive emissions monitoring systems. Typically, emissions monitoring is performed with a Continuous Emissions Monitoring System (CEMS), which monitors emissions by direct sampling of the exhaust gas. An alternative to a CEMS is a system which predicts emissions using easily measured operating parameters. This system is referred to as a Parametric Emissions Monitoring System (PEMS). A review of the literature indicates there is no globally applicable PEMS. Because of this, a PEMS that is applicable to a variety of gas turbine manufacturers and models is desired. The research presented herein includes a literature review of NOx reduction techniques, NOx production mechanisms, current PEMS research, and combustor modeling. Based on this preliminary research, a combustor model based on first-engineering principles was developed to describe the NOx formation process and relate NOx emissions to combustion turbine operating parameters. A review of available literature indicates that lean-premixed combustion is the most widely-used NOx reduction design strategy, so the model is based on this type of combustion system. A review of the NOx formation processes revealed four well-recognized NOx formation mechanisms: the Zeldovich, prompt, nitrous oxide, and fuel-bound nitrogen mechanisms. In lean-premixed combustion, the Zeldovich and nitrous oxide mechanisms dominate the NOx formation. This research focuses on combustion modeling including the Zeldovich mechanism for NOx formation. The combustor model is based on the Siemens SGT-200 combustion turbine and consists of a series of well-stirred reactors. Results show that the calculated NOx is on the same order of magnitude, but less than the NOx measured in field tests. These results are expected because the NOx calculation was based only on the Zeldovich mechanism, and the literature shows that significant NOx is formed through the nitrous oxide mechanism. The model also shows appropriate trends of NOx with respect to various operating parameters including equivalence ratio, ambient temperature, humidity, and atmospheric pressure. Model refinements are suggested with the ultimate goal being integration of the model into a PEMS.

The thesis reports experimental and theoretical studies of premixed combustion rates at high pressure and temperature. It focuses on measurements of laminar and turbulent burning velocities at high pressures and temperatures approaching those in engines, with emphasis on flame instabilities. To encourage the development of such instabilities, mixtures with negative Markstein numbers were employed. Three different methods were used to measure burning velocities in a spherical bomb. The bomb was fitted with windows for observing flame propagation at the centre of the bomb and a transducer to measure pressure. Four fans at the wall of the bomb were employed for mixing and the generation of turbulence. The first two methods of measuring burning velocities were well established and involved central ignition. The third method was new and involved implosions of two flame kernels that originated at spark plugs mounted near the wall. It enabled the later stages of burning at the high pressures to be observed and burning velocities to be measured. The first method depended on highspeed schlieren photographic measurements of the flame speed, dr / dt , at different radii,r, supplemented by pressure measurements. The second method was employed when the flame front has propagated beyond the boundaries of the window and could no longer be observed. The expression for the burning velocity rested upon the assumption that the flame was spherical and the fractional pressure rise was equal to the fractional mass burned. Two different approaches were employed for the new third method, one was based on geometrical considerations, the other on the fractional pressure rise. A knowledge of the flame area and the appropriate geometrical analysis enabled two expressions to be obtained for the burning velocity. The agreement between the two different approaches for obtaining burning velocities, and the general consistency of the results for both initially laminar and turbulent flames, showed the technique to be accurate and suitable for obtaining burning velocities at high pressure. As a result, burning velocities, initially laminar, were measured for iso-octane - air at equivalence ratios ranging from 0.8 to 1.6 at initial pressures of 0.5 and 1.0 MPa. They were also measured for hydrogen - air mixtures at equivalence ratios of 0.3 to 0.5. Modification of the linear theory of flame instability of Bechtold and Matalon enabled the laminar burning velocity to be obtained from the values of unstable burning velocities. Enhancements of the laminar burning velocity of up to six fold were measured. Turbulent burning velocities were measured over a range of rms turbulent velocities ranging from 0.25 to 3 mls. It was found that these values of burning velocity were higher than those predicted from earlier expressions, derived predominantly from more stable flames close to atmospheric pressure. The possibility that turbulent burning velocities might be enhanced, not only by the effect of flame stretch at negative Markstein numbers, but also by flamelet instabilities was also investigated at high pressures and with mixtures with very low Markstein numbers. Stoichiometric and rich iso-octane-air flames were selected for this study and mixtures were ignited at initial pressures of 0.5 and 1.0 MPa. This enabled burning velocities to be measured up to 6 MPa.

The need for effective disposal of huge volumes of industrial waste is becoming more challenging due to expected imposition of stringent pollution control regulations in the near future. Thermochemical conversion, particularly gasification of organics in the waste is considered the best route from the perspective of volume reduction and prevalent eco-friendly concept of waste-to-energy transformation. It is considered imperative to have adequate understanding of basic combustion features as a part of the thermochemical conversion process, leading to gasification. The aim of this thesis is to understand the fundamental combustion processes associated with one of the top listed hazardous wastes from distilleries (Biochemical Oxygen Demand (BOD) ~ 40,000 - 50,000 mg/L), commonly known as vinasse, stillage or spent wash, through experiments and modeling efforts. Specially designed experiments on distillery effluent combustion and gasification are conducted in laboratory scale reactors. As an essential starting point of the studies on ignition and combustion of distillery effluent containing solids consisting of 62 ± 2 % organics and 38 ± 2 % inorganics (primarily sugarcane derivatives), the roles of solids concentration, drop size and ambient temperature were investigated through experiments on (1) liquid droplets of 65 % and 77 % solids (remaining water) and (2) spheres of dried effluent (100 % solids) of size 0.5 mm to 20 mm diameter combusted at ambient temperatures of 773 to 1273 K. The investigation reveals that the droplets burn with two distinct regimes of combustion, flaming and char glowing. The ignition delay ‘t1’ of the droplets increased with size as is in the case of non-volatile droplets, while that of bone-dry spheres was found to be independent of size. The ‘t1’ decreased with increase in solids concentration. The ignition delay has showed an Arrhenius dependence on temperature. The initial ignition of the droplets and the dry spheres led to either homogeneous (flaming) or heterogeneous (flameless) combustion, depending on the ambient temperature in the case of sphere and on solid concentration and the ambient temperature, in the case of liquid droplets. The weight loss during the flaming combustion was found to be 50 - 80 % while during the char glowing it was 10-20 % depending on the ambient temperature. The flaming time tc is observed as tc~ d2c , as in the case of liquid fuel droplets and wood spheres. The char glowing time tc' is observed as tc ~ d2c as in the case of wood char, though the inert content of effluent char is as large as 50 % compared to 2 - 3 % in wood char. In the case of initial flameless combustion, the char combustion rate is observed to be lower. The heterogeneous char combustion in quiescent air in controlled temperature conditions has been studied and modeled using one-dimensional, spherico-symmetric conservation equations and the model predicts most of the features of char combustion satisfactorily. The measured surface and core temperatures during char glowing typically are in the range of 200 to 400 K and are higher than the controlled temperature of the furnace. Based on the results of single droplet combustion studies, combustion experiments were conducted in a laboratory scale vertical reactor (throughput ranging from 4 to 10 g/s) with the primary aim of obtaining sustained combustion. Spray of effluents with 50 % and 60 % solids (calorific value 6.8 - 8.2 MJ/kg), achieved by an air blast atomizer, was injected into a hot oxidizing environment to determine the parameters (ambient temperature and air-fuel ratio) at which auto-ignition could occur and subsequently studies were continued to investigate pre-ignition, ignition and combustion processes. Effluent with lower solids concentration was considered first from the point of view of the less expensive evaporator required in the field conditions for concentration and a spin-off in terms of better atomization consequently. Three classes of experiments were conducted: 1) Effluent injection from the wall with no auxiliary heat input, 2) Effluent injection with auxiliary heat input and 3) effluent injection within kerosene enveloping flame. Though individual particles in the spray periphery were found to combust, sustained spray combustion was not achieved in any of the three sets of experiments even with fine atomization. While conducting the third class of experiments in an inclined metallic reactor, sustained combustion of the pool resulting of accumulated spray seemed to result in large conversion of carbon. This led to the adoption of a new concept for effluent combustion in which the residence time is controlled by varying reactor inclination and the regenerative heat transfer from the product gases supplies heat for endothermic pre-ignition process occurring on the bed. Combustion and gasification experiments were conducted in an inclined plate reactor with rectangular cross section (80 mm x 160 mm) and 3000 mm long. A support flame was found necessary in the injection zone in addition to the regenerative heat transfer. Effluent with 60% solids was injected as film on the reactor bed. This film disintegrated into fine particles due to induced aerodynamic stretching and shear stripping. Combustion of individual particles provided exothermic heat profile and resulted into high carbon conversion. However, effluent clogging in the cold injection zone hindered system from attaining steady state. Effluent injected directly on the hot zone caused it to remain mobile due to the spheroidal evaporation and thus assuaging this problem. Improved mass distribution was achieved by displacing nozzle laterally in a cycle, actuated by a mechanism. Consistent injection led to sustained effluent combustion with resulting carbon conversion in excess of 98 %. The typical gas fractions obtained during gasification condition (air ratio = 0.3) were CO2 = 14.0 %, CO = 7.0 %, H2 = 12.9 %, CH4 - 1 % H2S = 0.6 - 0.8 % and about 2 % of saturated moisture. This composition varied due to variation in temperature (± 30 K) and is attributed to combined effect of local flow variations, shifting zones of endothermic processes due to flowing of evaporating effluent over a large area. In order to minimize this problem, experiments were conducted by injecting effluent at higher solids (73 % solids is found injectable). The effluent was found to combust close to the injection location-due to the reduced ignition delay and lower endothermic evaporation load helped raising the local temperature. This caused the pyrolysis to occur in this hottest zone of the reactor with higher heating rates resulting in larger yield of devolatilized products and improved char conversion. Effluent combustion was found to sustain temperature in the reactor under sub-stoichiometric conditions without support of auxiliary heat input and achieved high carbon conversion. These results inspired the use of higher concentration effluent, which is also known in the case of wood to have improved gasification efficiency due to reduction in moisture fraction. In addition, the recent studies on the sulfur emission in the case of black liquor combustion in recovery boilers have revealed that with increase in solids concentration, release of sulfur in gas phase is reduces. The required concentration can be carried out using low-grade waste heat from the reactor itself. It was found through experiments that, even though spray ignition occurred at this concentration, the confined reactor space prevented the spray from attaining sustained combustion. This led to the conduct of experiments in a new vertical reactor with adequate thermal inertia, essential to prevent variations in local temperature to reach a steady state gasification and required space to accommodate the spray. The results of the experiments conducted in the vertical reactor in which effluents with 73 % solids, heated close to the boiling point and injected as fine spray in a top-down firing mode are consolidated and reported in the thesis in detail. Single particle combustion with enveloping faint flame was seen unlike stable flame found in coal water slurry spray combustion. Sustained gasification of gas-entrained particles occurred at reactor temperature in the range of 950 K - 1000 K and sub-stoichiometric air ratio 03 - 0.35 without the support of auxiliary fuel. The typical gas fractions obtained during gasification condition (air ratio = 0.3) were CO2 = 10.0 -11.5 %, CO - 10.0 - 12.0 %, H2 - 6.7 - 8.0 %, CH4 = 1.75 % H2S = 0.2 - 0.4 % and about 2 % of saturated moisture. The carbon conversion obtained was in the range of 95 - 96 %. These experiments have provided the conditions for gasification. The extraction of potassium salts (mostly sulfates, carbonate and chloride) from the ash, using a simple water leaching process, was found to recover these chemicals to as high an extent as 70 - 75 % of total ash. In summary it is concluded that increasing the solid concentrations to as high levels as acceptable to the system (~ 75 %) and introducing as a fine spray of heated material (~ 363 K) into furnace with air at sub-stoichiometric conditions in a counter current combustion reactor will provide the frame work for the design of a gasification system for vinasse and similar effluent material. The thesis consists of seven chapters. Chapter 1 introduces the problem and motivation of the work presented in the thesis. Literature review is presented in Chapter 2. The Chapter 3 deals with the single particle combustion studies. The results of effluent spray combustion experiments conducted in a laboratory scale vertical reactor are presented in Chapter 4. The results of combustion and gasification experiments conducted in another variant of a reactor, namely, inclined flat plate rectangular reactor is consolidated in Chapter 5. The results of gas-entrained spray gasification experiment of higher concentration effluent injected as spray in the vertical reactor are presented in Chapter 6. The general conclusions and scope for the future work are presented in the concluding chapter 7.

The need for effective disposal of huge volumes of industrial waste is becoming more challenging due to expected imposition of stringent pollution control regulations in the near future. Thermochemical conversion, particularly gasification of organics in the waste is considered the best route from the perspective of volume reduction and prevalent eco-friendly concept of waste-to-energy transformation. It is considered imperative to have adequate understanding of basic combustion features as a part of the thermochemical conversion process, leading to gasification. The aim of this thesis is to understand the fundamental combustion processes associated with one of the top listed hazardous wastes from distilleries (Biochemical Oxygen Demand (BOD) ~ 40,000 - 50,000 mg/L), commonly known as vinasse, stillage or spent wash, through experiments and modeling efforts. Specially designed experiments on distillery effluent combustion and gasification are conducted in laboratory scale reactors. As an essential starting point of the studies on ignition and combustion of distillery effluent containing solids consisting of 62 ± 2 % organics and 38 ± 2 % inorganics (primarily sugarcane derivatives), the roles of solids concentration, drop size and ambient temperature were investigated through experiments on (1) liquid droplets of 65 % and 77 % solids (remaining water) and (2) spheres of dried effluent (100 % solids) of size 0.5 mm to 20 mm diameter combusted at ambient temperatures of 773 to 1273 K. The investigation reveals that the droplets burn with two distinct regimes of combustion, flaming and char glowing. The ignition delay ‘t1’ of the droplets increased with size as is in the case of non-volatile droplets, while that of bone-dry spheres was found to be independent of size. The ‘t1’ decreased with increase in solids concentration. The ignition delay has showed an Arrhenius dependence on temperature. The initial ignition of the droplets and the dry spheres led to either homogeneous (flaming) or heterogeneous (flameless) combustion, depending on the ambient temperature in the case of sphere and on solid concentration and the ambient temperature, in the case of liquid droplets. The weight loss during the flaming combustion was found to be 50 - 80 % while during the char glowing it was 10-20 % depending on the ambient temperature. The flaming time tc is observed as tc~ d2c , as in the case of liquid fuel droplets and wood spheres. The char glowing time tc' is observed as tc ~ d2c as in the case of wood char, though the inert content of effluent char is as large as 50 % compared to 2 - 3 % in wood char. In the case of initial flameless combustion, the char combustion rate is observed to be lower. The heterogeneous char combustion in quiescent air in controlled temperature conditions has been studied and modeled using one-dimensional, spherico-symmetric conservation equations and the model predicts most of the features of char combustion satisfactorily. The measured surface and core temperatures during char glowing typically are in the range of 200 to 400 K and are higher than the controlled temperature of the furnace. Based on the results of single droplet combustion studies, combustion experiments were conducted in a laboratory scale vertical reactor (throughput ranging from 4 to 10 g/s) with the primary aim of obtaining sustained combustion. Spray of effluents with 50 % and 60 % solids (calorific value 6.8 - 8.2 MJ/kg), achieved by an air blast atomizer, was injected into a hot oxidizing environment to determine the parameters (ambient temperature and air-fuel ratio) at which auto-ignition could occur and subsequently studies were continued to investigate pre-ignition, ignition and combustion processes. Effluent with lower solids concentration was considered first from the point of view of the less expensive evaporator required in the field conditions for concentration and a spin-off in terms of better atomization consequently. Three classes of experiments were conducted: 1) Effluent injection from the wall with no auxiliary heat input, 2) Effluent injection with auxiliary heat input and 3) effluent injection within kerosene enveloping flame. Though individual particles in the spray periphery were found to combust, sustained spray combustion was not achieved in any of the three sets of experiments even with fine atomization. While conducting the third class of experiments in an inclined metallic reactor, sustained combustion of the pool resulting of accumulated spray seemed to result in large conversion of carbon. This led to the adoption of a new concept for effluent combustion in which the residence time is controlled by varying reactor inclination and the regenerative heat transfer from the product gases supplies heat for endothermic pre-ignition process occurring on the bed. Combustion and gasification experiments were conducted in an inclined plate reactor with rectangular cross section (80 mm x 160 mm) and 3000 mm long. A support flame was found necessary in the injection zone in addition to the regenerative heat transfer. Effluent with 60% solids was injected as film on the reactor bed. This film disintegrated into fine particles due to induced aerodynamic stretching and shear stripping. Combustion of individual particles provided exothermic heat profile and resulted into high carbon conversion. However, effluent clogging in the cold injection zone hindered system from attaining steady state. Effluent injected directly on the hot zone caused it to remain mobile due to the spheroidal evaporation and thus assuaging this problem. Improved mass distribution was achieved by displacing nozzle laterally in a cycle, actuated by a mechanism. Consistent injection led to sustained effluent combustion with resulting carbon conversion in excess of 98 %. The typical gas fractions obtained during gasification condition (air ratio = 0.3) were CO2 = 14.0 %, CO = 7.0 %, H2 = 12.9 %, CH4 - 1 % H2S = 0.6 - 0.8 % and about 2 % of saturated moisture. This composition varied due to variation in temperature (± 30 K) and is attributed to combined effect of local flow variations, shifting zones of endothermic processes due to flowing of evaporating effluent over a large area. In order to minimize this problem, experiments were conducted by injecting effluent at higher solids (73 % solids is found injectable). The effluent was found to combust close to the injection location-due to the reduced ignition delay and lower endothermic evaporation load helped raising the local temperature. This caused the pyrolysis to occur in this hottest zone of the reactor with higher heating rates resulting in larger yield of devolatilized products and improved char conversion. Effluent combustion was found to sustain temperature in the reactor under sub-stoichiometric conditions without support of auxiliary heat input and achieved high carbon conversion. These results inspired the use of higher concentration effluent, which is also known in the case of wood to have improved gasification efficiency due to reduction in moisture fraction. In addition, the recent studies on the sulfur emission in the case of black liquor combustion in recovery boilers have revealed that with increase in solids concentration, release of sulfur in gas phase is reduces. The required concentration can be carried out using low-grade waste heat from the reactor itself. It was found through experiments that, even though spray ignition occurred at this concentration, the confined reactor space prevented the spray from attaining sustained combustion. This led to the conduct of experiments in a new vertical reactor with adequate thermal inertia, essential to prevent variations in local temperature to reach a steady state gasification and required space to accommodate the spray. The results of the experiments conducted in the vertical reactor in which effluents with 73 % solids, heated close to the boiling point and injected as fine spray in a top-down firing mode are consolidated and reported in the thesis in detail. Single particle combustion with enveloping faint flame was seen unlike stable flame found in coal water slurry spray combustion. Sustained gasification of gas-entrained particles occurred at reactor temperature in the range of 950 K - 1000 K and sub-stoichiometric air ratio 03 - 0.35 without the support of auxiliary fuel. The typical gas fractions obtained during gasification condition (air ratio = 0.3) were CO2 = 10.0 -11.5 %, CO - 10.0 - 12.0 %, H2 - 6.7 - 8.0 %, CH4 = 1.75 % H2S = 0.2 - 0.4 % and about 2 % of saturated moisture. The carbon conversion obtained was in the range of 95 - 96 %. These experiments have provided the conditions for gasification. The extraction of potassium salts (mostly sulfates, carbonate and chloride) from the ash, using a simple water leaching process, was found to recover these chemicals to as high an extent as 70 - 75 % of total ash. In summary it is concluded that increasing the solid concentrations to as high levels as acceptable to the system (~ 75 %) and introducing as a fine spray of heated material (~ 363 K) into furnace with air at sub-stoichiometric conditions in a counter current combustion reactor will provide the frame work for the design of a gasification system for vinasse and similar effluent material. The thesis consists of seven chapters. Chapter 1 introduces the problem and motivation of the work presented in the thesis. Literature review is presented in Chapter 2. The Chapter 3 deals with the single particle combustion studies. The results of effluent spray combustion experiments conducted in a laboratory scale vertical reactor are presented in Chapter 4. The results of combustion and gasification experiments conducted in another variant of a reactor, namely, inclined flat plate rectangular reactor is consolidated in Chapter 5. The results of gas-entrained spray gasification experiment of higher concentration effluent injected as spray in the vertical reactor are presented in Chapter 6. The general conclusions and scope for the future work are presented in the concluding chapter 7.

Direct numerical simulation is used to study turbulent plane wall-jets. The investigation is aimed at studying dynamics, mixing and reactions in wall bounded flows. The produced mixing statistics can be used to evaluate and develop models for mixing and combustion. An aim has also been to develop a simulation method that can be extended to simulate realistic combustion including significant heat release. The numerical code used in the simulations employs a high order compact finite difference scheme for spatial integration, and a low-storage Runge-Kutta method for the temporal integration. In the simulations the inlet based Reynolds and Mach numbers of the wall-jet are Re = 2000 and M=0.5 respectively, and above the jet a constant coflow of 10% of the inlet jet velocity is applied. The development of an isothermal wall-jet including passive scalar mixing is studied and the characteristics of the wall-jet are compared to observations of other canonical shear flows. In the near-wall region the jet resembles a zero pressure gradient boundary layer, while in the outer layer it resembles a plane jet. The scalar fluxes in the streamwise and wall-normal direction are of comparable magnitude. In order to study effects of density differences, two non-isothermal wall-jets are simulated and compared to the isothermal jet results. In the non-isothermal cases the jet is either warm and propagating in a cold surrounding or vice versa. The turbulence structures and the range of scales are affected by the density variation. The warm jet contains the largest range of scales and the cold the smallest. The differences can be explained by the varying friction Reynolds number. Conventional wall scaling fails due to the varying density. An improved collapse in the inner layer can be achieved by applying a semi-local scaling. The turbulent Schmidt and Prandtl number vary significantly only in the near-wall layer and in a small region below the jet center. A wall-jet including a single reaction between a fuel and an oxidizer is also simulated. The reactants are injected separately at the inlet and the reaction time scale is of the same order as the convection time scale and independent of the temperature. The reaction occurs in thin reaction zones convoluted by high intensity velocity fluctuations.
QC 20100621

This thesis presents the results and analyses of computational studies on certain problems of combustion instability in solid propellants. Specifically, effects of relaxing certain assumptions made in previous models of unsteady burning of solid propellants are investigated. Knowledge of unsteady burning of solid propellants is essential in studying the phenomenon of combustion instability in solid propellant rocket motors. In Chapter 1, an introduction to different types of unsteady combustion investigated in this thesis, such as 1) intrinsic instability, 2) pressure-driven dynamic burning, 3) extinction by depressurization, and 4) L* -instability, is given. Also, a review of previous experimental and theoretical studies of these phenomena is presented. From this review it is concluded that all the previous studies, which investigated the unsteady combustion of solid propellants, made one or more of the following assumptions: 1) quasi-steady gas-phase (QSG), 2) quasi-steady condensed phase reaction zone (QSC), 3) small perturbations, and 4) unity Lewis number. These assumptions limit the validity of the results obtained with such models to: 1) relatively low frequencies (< 1 kHz) of pressure oscillations and 2) small deviations in pressure from its steady state or mean values. The objectives of the present thesis are formulated based on the above conclusions. These are: 1) to develop a nonlinear numerical model of unsteady solid propellant combustion, 2) to relax the assumptions of QSG and QSC, 3) to study the consequent effects on the intrinsic instability and pressure-driven dynamic burning, and 4) to investigate the L* -instability in solid propellant rocket motors. In Chapter 2, a nonlinear numerical model, which relaxes the QSG and QSC assumptions, is set up. The transformation and nondimensionalization of the governing equations are presented. The numerical technique based on the method of operator-splitting, used to solve the governing equations is described. In Chapter 3, the effect of relaxing the QSG assumption on the intrinsic instability is investigated. The stable and unstable solutions are obtained for parameters corresponding to a typical composite propellant. The stability boundary, in terms of the nondimensional parameters identified by Denison and Baum (1961), is predicted using the present model. This is compared with the stability boundary obtained by previous linear stability theories, based on activation energy asymptotics in the gas-phase, which employed QSC and/or QSG assumptions. It is found that in the limit of large activation energy and low frequencies, present result approaches the previous theoretical results. This serves as a validation of the present method of solution. It is confirmed that relaxing the QSG assumption widens the stable region. However, it is found that a distributed reaction in the gas-phase destabilizes the burning. The effect of non-unity Lewis number on the stability boundary is also investigated. It is found that at parametric values corresponding to low pressures and large flame stand-off distances, small amplitude, high frequency (at frequencies near the characteristic frequency of the gas-phase) oscillations in burning rate appear when the Lewis number is greater than one. In Chapter 4, the effect of relaxing the QSG assumption is further investigated with respect to the pressure-driven dynamic burning. Comparison of the pressure-driven frequency response function, Rp, obtained with the present model, both in the QSG and non-QSG framework, with those obtained with previous linear stability theories invoking QSG and QSC assumptions are made. As the frequency of pressure oscillations approaches zero, |RP| predicted using present models approached the value obtained by previous theoretical studies. Also, it is confirmed that the effect of relaxing QSG is to decrease the |Rp| at frequencies near the first resonant frequency. Moreover, relaxing QSG assumption produces a second resonant peak in |Rp| at a frequency near the characteristic frequency of the gas-phase. Further, Rp calculated using the present model is compared with that obtained by a previous linear theory which relaxed the QSG assumption. The two models predicted the same resonant frequencies in the limit of small amplitudes of pressure oscillations. Finally, it is found that the effect of large amplitude of pressure oscillations is to introduce higher harmonics in the burning rate and to reduce the mean burning rate. In Chapter 5, first the present non-QSC model is validated by comparing its results with that of a previous non-QSC model for radiation-driven burning. The model is further validated for steady burning results by comparing with experimental data for a double base propellant (DBP). Then, the effect of relaxing the QSC assumption on steady state solution is investigated. It is found that, even in the presence of a strong gas-phase heat feedback, QSC assumption is valid for moderately large values of condensed phase Zel'dovich number, as far as steady state solution is concerned. However, for pressure-driven dynamic burning, relaxing the QSC assumption is found to increase |RP| at all frequencies. The error due to QSC assumption is found to become significant, either when |Rp| is large or as the driving frequency approaches the characteristic frequency of the condensed phase reaction zone. The predicted real part of the response function is quantitatively compared with experimental data for DBP. The comparison seems to be better with a value of condensed phase activation energy higher than that suggested by Zenin (1992). In Chapter 6, burning rate transients for a DBP during exponential depressurization are computed using non-QSG and non-QSC models. Salient features of extinction and combustion recovery are predicted. The predicted critical initial depressurization rate, (dp/dt)i, is found to decrease markedly when the QSC assumption is relaxed. The effect of initial pressure level on critical (dp/dt)i is studied. It is found that the critical (dp/dt)i decreases with the initial pressure. Also, the overshoot of burning rate during combustion recovery is found to be relatively large with low initial pressures. However as the initial pressure approached the final pressure, the reduction in initial pressure causes a large increase in the critical (dp/dt)i. No extinction is found to occur when the initial pressure is very close to the final pressure. In Chapter 7, a numerical model is developed to simulate the L* -instability in solid propellant motors. This model includes a) the propellant burning model that takes into account nonlinear pressure oscillations and that takes into account an unsteady gas- and condensed phase, and b) a combustor model that allows pressure and temperature oscillations of finite amplitude. Various regimes of L* -burning of a motor, with a typical composite propellant, namely 1) steady burning, 2) oscillatory burning leading to steady state, 3) oscillatory burning leading to extinction, 4) reignition and 5) chuffing are predicted. The predicted dependence of frequency of L* -oscillations on mean pressure is compared with one set of available experimental data. It is found that proper modeling of the radiation heat flux from the chamber walls to the burning surface may be important to predict the re-ignition. In Chapter 8, the main conclusions of the present study are summarized. Certain suggestions for possible future studies to enhance the understanding of dynamic combustion of solid propellants are also given.

This thesis presents the results and analyses of computational studies on certain problems of combustion instability in solid propellants. Specifically, effects of relaxing certain assumptions made in previous models of unsteady burning of solid propellants are investigated. Knowledge of unsteady burning of solid propellants is essential in studying the phenomenon of combustion instability in solid propellant rocket motors. In Chapter 1, an introduction to different types of unsteady combustion investigated in this thesis, such as 1) intrinsic instability, 2) pressure-driven dynamic burning, 3) extinction by depressurization, and 4) L* -instability, is given. Also, a review of previous experimental and theoretical studies of these phenomena is presented. From this review it is concluded that all the previous studies, which investigated the unsteady combustion of solid propellants, made one or more of the following assumptions: 1) quasi-steady gas-phase (QSG), 2) quasi-steady condensed phase reaction zone (QSC), 3) small perturbations, and 4) unity Lewis number. These assumptions limit the validity of the results obtained with such models to: 1) relatively low frequencies (< 1 kHz) of pressure oscillations and 2) small deviations in pressure from its steady state or mean values. The objectives of the present thesis are formulated based on the above conclusions. These are: 1) to develop a nonlinear numerical model of unsteady solid propellant combustion, 2) to relax the assumptions of QSG and QSC, 3) to study the consequent effects on the intrinsic instability and pressure-driven dynamic burning, and 4) to investigate the L* -instability in solid propellant rocket motors. In Chapter 2, a nonlinear numerical model, which relaxes the QSG and QSC assumptions, is set up. The transformation and nondimensionalization of the governing equations are presented. The numerical technique based on the method of operator-splitting, used to solve the governing equations is described. In Chapter 3, the effect of relaxing the QSG assumption on the intrinsic instability is investigated. The stable and unstable solutions are obtained for parameters corresponding to a typical composite propellant. The stability boundary, in terms of the nondimensional parameters identified by Denison and Baum (1961), is predicted using the present model. This is compared with the stability boundary obtained by previous linear stability theories, based on activation energy asymptotics in the gas-phase, which employed QSC and/or QSG assumptions. It is found that in the limit of large activation energy and low frequencies, present result approaches the previous theoretical results. This serves as a validation of the present method of solution. It is confirmed that relaxing the QSG assumption widens the stable region. However, it is found that a distributed reaction in the gas-phase destabilizes the burning. The effect of non-unity Lewis number on the stability boundary is also investigated. It is found that at parametric values corresponding to low pressures and large flame stand-off distances, small amplitude, high frequency (at frequencies near the characteristic frequency of the gas-phase) oscillations in burning rate appear when the Lewis number is greater than one. In Chapter 4, the effect of relaxing the QSG assumption is further investigated with respect to the pressure-driven dynamic burning. Comparison of the pressure-driven frequency response function, Rp, obtained with the present model, both in the QSG and non-QSG framework, with those obtained with previous linear stability theories invoking QSG and QSC assumptions are made. As the frequency of pressure oscillations approaches zero, |RP| predicted using present models approached the value obtained by previous theoretical studies. Also, it is confirmed that the effect of relaxing QSG is to decrease the |Rp| at frequencies near the first resonant frequency. Moreover, relaxing QSG assumption produces a second resonant peak in |Rp| at a frequency near the characteristic frequency of the gas-phase. Further, Rp calculated using the present model is compared with that obtained by a previous linear theory which relaxed the QSG assumption. The two models predicted the same resonant frequencies in the limit of small amplitudes of pressure oscillations. Finally, it is found that the effect of large amplitude of pressure oscillations is to introduce higher harmonics in the burning rate and to reduce the mean burning rate. In Chapter 5, first the present non-QSC model is validated by comparing its results with that of a previous non-QSC model for radiation-driven burning. The model is further validated for steady burning results by comparing with experimental data for a double base propellant (DBP). Then, the effect of relaxing the QSC assumption on steady state solution is investigated. It is found that, even in the presence of a strong gas-phase heat feedback, QSC assumption is valid for moderately large values of condensed phase Zel'dovich number, as far as steady state solution is concerned. However, for pressure-driven dynamic burning, relaxing the QSC assumption is found to increase |RP| at all frequencies. The error due to QSC assumption is found to become significant, either when |Rp| is large or as the driving frequency approaches the characteristic frequency of the condensed phase reaction zone. The predicted real part of the response function is quantitatively compared with experimental data for DBP. The comparison seems to be better with a value of condensed phase activation energy higher than that suggested by Zenin (1992). In Chapter 6, burning rate transients for a DBP during exponential depressurization are computed using non-QSG and non-QSC models. Salient features of extinction and combustion recovery are predicted. The predicted critical initial depressurization rate, (dp/dt)i, is found to decrease markedly when the QSC assumption is relaxed. The effect of initial pressure level on critical (dp/dt)i is studied. It is found that the critical (dp/dt)i decreases with the initial pressure. Also, the overshoot of burning rate during combustion recovery is found to be relatively large with low initial pressures. However as the initial pressure approached the final pressure, the reduction in initial pressure causes a large increase in the critical (dp/dt)i. No extinction is found to occur when the initial pressure is very close to the final pressure. In Chapter 7, a numerical model is developed to simulate the L* -instability in solid propellant motors. This model includes a) the propellant burning model that takes into account nonlinear pressure oscillations and that takes into account an unsteady gas- and condensed phase, and b) a combustor model that allows pressure and temperature oscillations of finite amplitude. Various regimes of L* -burning of a motor, with a typical composite propellant, namely 1) steady burning, 2) oscillatory burning leading to steady state, 3) oscillatory burning leading to extinction, 4) reignition and 5) chuffing are predicted. The predicted dependence of frequency of L* -oscillations on mean pressure is compared with one set of available experimental data. It is found that proper modeling of the radiation heat flux from the chamber walls to the burning surface may be important to predict the re-ignition. In Chapter 8, the main conclusions of the present study are summarized. Certain suggestions for possible future studies to enhance the understanding of dynamic combustion of solid propellants are also given.

Combustion of hydrocarbons in internal combustion engines results in emissions that can be harmful both to human health and to the environment. Although the engine technology is improving, the emissions of NO_x, PM and UHC are still challenging. Besides, the overall consumption of fossil fuel and hence the emissions of CO₂ are increasing because of the increasing number of vehicles. This has lead to a focus on finding alternative fuels and alternative technologies that may result in lower emissions of harmful gases and lower CO₂ emissions. This thesis treats various topics that are relevant when using blends of fuels in different internal combustion engine technologies, with a particular focus on using hydrogen as a fuel additive. The topics addressed are especially the ones that impact the environment, such as emissions of harmful gases and thermal efficiency (fuel consumption). The thesis is based on experimental work performed at four different test rigs:

1. A dynamic combustion rig with optical access to the combustion chamber where spark ignited premixed combustion could be studied by means of a Schlieren optical setup and a high speed video camera.

2. A spark ignition natural gas engine rig with an optional exhaust gas recycling system.

3. A 1-cylinder diesel engine prepared for homogeneous charge compression ignition combustion.

4. A 6-cylinder standard diesel engine

The engine rigs were equipped with cylinder pressure sensors, engine dynamometers, exhaust gas analyzers etc. to enable analyses of the effects of different fuels. The effect of hydrogen blended with methane and natural gas in spark ignited premixed combustion was investigated in the dynamic combustion rig and in a natural gas engine. In the dynamic combustion rig, the effect of hydrogen added to methane on the flame speed and the flame structure was investigated at elevated pressure and temperature. A considerable increase in the flame speed was observed when adding 30 vol% hydrogen to the methane, but 5 vol% hydrogen also resulted in a noticeable increase. The flame structure was also influenced by the hydrogen addition as the flame front had a higher tendency to become wrinkled or cellular. The effect is believed to mainly be caused by a reduction in the effective Lewis number of the mixture. In the gas engine experiments, the effect of adding 25 vol% hydrogen to natural gas was investigated when the engine was run on lean air/fuel mixtures and on stoichiometric mixtures with exhaust gas recirculation. The hydrogen addition was found to extend the lean limit of stable combustion and hence caused lower NOx emissions. The brake thermal efficiency increased with the hydrogen addition, both for the fuel lean and the stoichiometric mixtures with exhaust gas recirculation. This is mainly because of shorter combustion durations when the hydrogen mixture was used, leading to thermodynamically improved cycles.

Two types of experiments were performed in compression ignition engines. First, homogenous charge compression ignition (HCCI) experiments were performed in a single cylinder engine fueled with natural gas and diesel oil. As HCCI engines have high thermal efficiency and low NO_x and PM emissions it may be more favorable to use natural gas in HCCI engines than in spark ignition engines. The mixture of natural gas, diesel oil and air was partly premixed before combustion. The natural gas/diesel ratio was used to control the ignition timing as the fuels have very different ignition properties. The natural gas was also replaced by a 20 vol% hydrogen/natural gas mixture to study the effect of hydrogen on the ignition and combustion process. Also, rape seed methyl ester (RME) was tested instead of the diesel oil. The combustion phasing was found to mainly be controlled by the amount of liquid fuel injected. The presence or absence of hydrogen resulted in only marginal changes on the combustion. Because the diesel oil and RME have much lower autoignition temperatures than both hydrogen and natural gas, the properties of the liquid fuel may overshadow the effect of the hydrogen addition. A large difference however, was found between the RME and the diesel oil with the necessity to inject much more RME than diesel oil to obtain the same combustion phasing.

The last experiments with compression ignition were performed by using a standard Scania diesel engine where the possibilities to reduce particulate matter (PM) and other emissions by introduction of combustible gas to the inlet air (named fumigation) were investigated. Hydrogen, methane and propane were introduced at different rates replacing up to 40% of the total fuel energy. Also, a biodiesel consisting of mainly RME was tested instead of the diesel oil. Because of the low density of hydrogen gas, less of the fuel energy could be replaced by hydrogen than by the two other gases. Higher rates of hydrogen would sacrifice the safety by exceeding the lower flammability limit in the inlet air. Only moderate reductions in PM were achieved at high gas rates, and because of the limitation in the practical achievable hydrogen rate it was not possible to obtain considerable reductions in PM emission by hydrogen addition. The NOx emissions were found to be little influenced by the fumigation, but the THC emissions strongly increased with increased methane and propane rates, especially at a low engine load. Propane fumigation resulted in considerably less THC emissions than methane fumigation. The biodiesel resulted in higher PM emissions than the diesel fuel at low load, but was considerably lower at the higher loads. This is believed to be because of the low volatility of the biodiesel which may lead to emissions of un-burned fuel at low load when the temperature is low. At higher loads this is believed to be less of a problem because the temperature is higher, and the oxygen content of biodiesel is believed to increase the PM oxidation and/or reduce the formation of PM.

An investigation of the combustion of lead sulphide .concentrates under controlled conditions has been carried out. A fast response, two-wavelength radiation pyrometer and a "laminar flow" furnace were constructed to facilitate the measurement of the temperature of individual particles during combustion. Chemical analysis and electron microscopy studies of the reaction products were also performed. The combustion of galena, pyrite, pyrrhotite, sphalerite and two commercial concentrates (Brunswick and Sullivan) at 1130K was investigated. The effects of particle size between 63-125μm and gaseous oxygen concentration between 10 and 100% were examined. For combustion in both air and oxygen a number of different combustion pulses were identified which corresponded to the combustion of different mineral species or to different physical phenomena. An analogous series of pulse classifications was identified for combustion in oxygen however they reflected the greater intensity and temperature of the reactions. The form of combustion was strongly dependent on oxygen concentration. From the results it was not possible to identify the effect of particle size on combustion behaviour. The vaporisation of lead sulphide appears important to the mechanism of galena combustion. In air the temperature of combustion appears limited to 1500-1700K (of boiling point PbS of 1609K); whereas in oxygen, massive vaporisation results in a heating arrest at 1700-2000K and disintegration into droplets which combust at 2000-2400K. Transition from air-type to oxygen-type combustion occurs at oxygen concentrations between 40 and 65% and is thought to be due to the transition from a liquid to gaseous phase PbO reaction product. The initial stage of pyrite reaction is thermal decomposition to porous pyrrhotite. The ignition of this porous pyrrhotite was more rapid than dense pyrrhotite, but once molten, the combustion of the two was indistinguishable and the peak temperature observed was very reproducible. In air the peak combustion temperatures of 2400-2600K appeared to coincide with a sudden expansion of the particle, possibly due to the inflation of thin-walled iron oxide cenospheres. In oxygen the reactions are more intense and disintegration typically occurs on reaching a peak temperature of 3000-3400K, Between ~10 and 35% oxygen the maximum combustion temperature increased linearly, but at higher concentrations remained constant at 3000-3400K. The results suggest the maximum temperature reached is limited by the occurrence of a physical phenomenum possibly associated with the vaporisation of iron. Sphalerite did not ignite at the temperatures considered, but shells of zinc oxide were observed in the reaction products. For the commercial concentrates pulses of intermediate combustion characteristics and a wide range of combustion temperatures (typically intermediate to those of PbS and FeS) were observed, as well as many pulses similar to those of the constituent minerals. The former were considered to be due to the combustion of agglomerations of many smaller individual particles. The effect of the mineral composition was evident in the combustion results, with increased quantities of iron sulphide tending to result in more intense reactions. The results suggest that metallic lead formation occurs during the initial stages of reaction, probably after melting as the result of reaction between the surface oxides/sulphates and unreacted PbS. A simple reaction model for iron sulphide combustion suggests that the reaction of the molten drop is controlled by gas-phase oxygen mass transfer with the measured heating rates consistent with the formation of wustite and sulphur dioxide.
Applied Science, Faculty of
Materials Engineering, Department of
Graduate

In the combustion of fuels and related organic compounds the initial step consists of a free radical forming process occurring either homogeneously or heterogeneously, such as RH + O_{2 → R + HO}_2 (1) The radical R, reacts with oxygen to produce an alkyl or other peroxy radical: R + O_2 ↔ RO₂ (2) One of the controversies involved in the mechanism for the oxidation of hydrocarbons is the route for the unimolecular decomposition of the hydroperoxy alkyl radical (R_-HOOH). This would be produced as a result of the isomerisation of the alkyl peroxy radical (RO₂). There are three possible unimolecular paths for R_-HOOH together with the addition of oxygen to form hydroperoxy alkyl peroxy radical. This study is concerned with the generation of an alkyl peroxy alkyl radical and its decomposition to both epoxide and olefin formation and at lower temperatures predominantly follows the thermochemically more favourable route. No direct information is available about the rate constants of the two decomposition routes of alkyl peroxyalkyl/hydroperoxy alkyl radicals. There are different ways to find out the rate constants for the decomposition of alkyl peroxy alkyl/hydroperoxy alkyl radical to olefin and oxirane. One such way was a study of the gas phase, hydrogen chloride catalysed decomposition of di-t-butyl peroxide. A surrogate hydroperoxy alkyl radical was generated via this study and the most favourable route for the decomposition of dtBP_-H is confirmed. Again, on thermochemical grounds, the formation of isobutene oxide predominates over the formation of isobutene. The modelling of this study assisted considerably in choosing the reaction steps for a probable mechanism and in the assessment of rate parameters for the individual steps. A bonafide hydroperoxy alkyl radical was generated via the study of the sensitized decomposition of t-butyl hydroperoxide in an uncoated, coated reaction vessel and also in the presence of oxygen. The Arrhenius parameters for the ratio of the rate of formation of isobutene to isobutene oxide was observed experimentally, and are in good agreement with the estimated values in the coated reaction vessel but in uncoated and in the presence of oxygen, this ratio is nearly doubled which suggests that isobutene is formed heterogeneously and surface played an important role. In order to observe the effect of surface: volume ratio on product formation, this system was studied in four different coated reaction vessels and it was concluded that the surface effect was negligible on a coated spherical reaction vessel. The bond dissociation energy DH^o(RO-OH) in alkyl hydroperoxides, is important because the value of the rate constant is critical to cool flames production. The pyrolysis of t-butyl hydroperoxide was carried out, in a bath of isobutane in order to isolate the tBuO-OH bond breaking step. Acetone formation constituted a direct measure of the rate of decomposition of t-butyl hydroperoxide. The O-O bond dissociation energy was found experimentally, which is in good agreement with other group workers values.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2010.
This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.
Cataloged from PDF version of thesis.
Includes bibliographical references (p. 121-127).
Hydrogen-rich alternative fuels are likely to play a significant role in future power generation systems. The emergence of the integrated gasification combined cycle (IGCC) as one of the favored technologies for incorporating carbon capture into coal-based power plants increases the need for gas turbine combustors which can operate on a range of fuels, particularly syngas, a hydrogen-rich fuel produced by coal gasification. Lean premixed combustion, the preferred high-efficiency, low-emissions operating mode in these combustors, is susceptible to strong instabilities even in ordinary fuels. Because hydrogen-rich fuels have combustion properties which depend strongly on composition, avoiding the dynamics that energize combustion instability across all operating conditions is a significant challenge. In order to explore the effect of fuel composition on combustion dynamics, a series of experiments were carried out in two optically-accessible laboratory-scale combustors: a planar backward-facing step combustor and an axisymmetric swirlstabilized combustor. Fuels consisting of carbon monoxide and hydrogen, or propane and hydrogen were tested over a range of equivalence ratios and at various inlet temperatures. Dynamic pressure and chemiluminescence measurements were taken for each case. High-speed video and stereographic particle imaging velocimetry were used to explore the dynamic interactions between the flame and the flow field of the combustor. Stable, quasi-stable, and unstable operating modes were identified in each combustor, with each mode characterized by a distinct dynamic flame shape and acoustic response which is dependent on the composition of the reactants and the inlet temperature. In both combustors, the quasi-stable and unstable modes are associated with acoustically driven flame-vortex interactions in the combustion-anchoring region. In the planar combustor, the flame is convoluted around a large wake vortex, which is periodically shed from the step. In the swirl-stabilized combustor, the flame shape is controlled by the dynamics of the inner recirculation zone formed as a result of vortex breakdown. In both cases, the unstable mode is associated with velocity oscillation amplitudes that exceed the mean flow velocity. The apparent similarity between the response curves and flame dynamics in the two combustors indicate that the intrinsic local dynamics--instead of global acoustics--govern the flame response. Analysis shows that for each combustor, the pressure response curves across a range of operating conditions can be collapsed onto a single curve by introducing an appropriate similarity parameter that captures the flame response to the vortex. Computations are performed for stretched flames in hydrogen-rich fuels and the results are used to explain the observed similarity and to define the form of the similarity parameter. This similarity parameter works equally well for both experiments across fuel compositions and different inlet conditions, demonstrating that it fundamentally embodies the reciprocity between the flow and the combustion process that drives the instability. A linear model of the combustor's acoustics shows that the onset of combustion instability at a particular frequency can be related to a time delay between the velocity and the exothermic response of the flame that is inversely proportional to the local burning velocity. This analysis captures the impact of the fuel composition and operating temperature on the mode selection through an appropriately-weighted strained flame consumption speed, further emphasizing the influence of local transport-chemistry interactions on the system response. This new result confirms the role of turbulent combustion dynamics in driving thermoacoustic instabilities.
by Raymond Levi Speth.
Ph.D.