Dissertations / Theses on the topic 'Turbulent combustion'

A study of the squish-jet design concept in spark ignition engines, with central ignition, was conducted in a constant volume chamber. The effects of jet size, jet number and jet orientation in generating turbulence and jet enhanced turbulent combustion were investigated. Three sets of configurations with three port sizes were used in this study. The research was carried out in three stages: 1.Qualitative information was obtained from flow visualization experiments via schlieren photography at 1000 frames per second. The flow medium was air. A sequence of frames at specific time intervals were selected to study the results from the respective configurations and jet sizes. The swirling nature of the flow is vivid in the offset arrangement. 2.Pre-ignition pressure and combustion pressure traces were measured with a piezoelectric pressure transducer from which characterising parameters such as maximum pressure, ignition advance and mass burn rate were analysed. Mass fraction curves were calculated using the simple model of fractional pressure rise. A maximum pressure increase of 66% over the reference quiescent combustion case, and combustion duration reduction of 77% were obtained for the offset arrangement with 2 mm diameter port. Comparisons of the times required for 10%, 50% and 90% mass burned are identified and confirmed that it took the 2 mm jet the shortest time to burn 90% of the mixture in the chamber. 3.Two-component velocity measurements were made using an LDV system. Measurements were taken in the central vertical plane of the chamber at specified locations. The data collected were window ensemble- averaged for the mean and fluctuating velocities over a number of cycles. Data intermittency and low data rate precluded, however, cycle-by-cycle analysis. Mean tangential velocities were calculated for each case and the data were used to construct a movie of the tangential velocity as a function of time, suitable for quantitative flow visualization. The vortical nature of the flow was recorded, the distribution being neither solid body rotation nor free vortex, but some complex fluid motion. The jet scale and orientation influence the in generation of turbulence flow field in the chamber, affecting the rate of combustion and the ensuing maximum pressure rise. The offset jet arrangement gives the best results, whereas radially opposed jets have a reduced effect. Increasing the number of jets in opposed arrangement does not enhance turbulent flow. Turbulent flow in the spark region during the onset of ignition was found to be important.
Applied Science, Faculty of
Mechanical Engineering, Department of
Graduate

A turbulent combustion noise model based on first principles is developed in this thesis. The model predicts (1) the pressure time series, (2) Sound Pressure Level (SPL) spectrum, (3) Over-All Sound Pressure Level (OASPL), (4) the thermoacoustic efficiency, (5) the peak frequency, and (6) the sound power of combustion generated noise. In addition, a correlation for sound power is developed based on fundamental burner and fuel variables known to affect the acoustic characteristics of turbulent combustion. The predicted pressure time series exhibits consistency with reality in that it has no steady component. It also confirms speculation in the literature that the predominant noise mechanism in open turbulent flames results from a "transition burning" phenomenon at the flame front. The predicted Sound Pressure Level spectrum, Over-All Sound Pressure Level, and the thermoacoustic efficiency are in excellent agreement with the results available in the literature. The shifts in the peak frequency with basic burner and fuel parameters are consistent with experimental observations from the literature. The disagreements between the predicted and the observed exponents of fuel and burner parameters for sound power are shown to be well within the standard deviation of the experimental observations. Certain areas for further analytical research on the combustion noise mechanism are identified.
Master of Science

Implicit Large Eddy Simulation (ILES) with high-resolution and high-order computational modelling has been applied to flows with turbulent mixing and combustion. Due to the turbulent nature, mixing of fuel and air and the subsequent combustion still remain challenging for computational fluid dynamics. However, recently ILES, an advanced numerical approach in Large Eddy Simulation methods, has shown encouraging results in prediction of turbulent flows. In this thesis the governing equations for single phase compressible flow were solved with an ILES approach using a finite volume Godunov-type method without explicit modelling of the subgrid scales. Up to ninth-order limiters were used to achieve high order spatial accuracy. When simulating non chemical reactive flows, the mean flow of a fuel burner was compared with the experimental results and showed good agreement in regions of strong turbulence and recirculation. The one dimensional kinetic energy spectrum was also examined and an ideal k−5/ 3 decay of energy could be seen in a certain range, which increased with grid resolution and order of the limiter. The cut-off wavenumbers are larger than the estimated maximum wavenumbers on the grid, therefore, the numerical dissipation sufficiently accounted for the energy transportation between large and small eddies. The effect of density differences between fuel and air was investigated for a wide range of Atwood number. The mean flow showed that when fuel momentum fluxes are identical the flow structure and the velocity fields were unchanged by Atwood number except for near fuel jet regions. The results also show that the effects of Atwood number on the flow structure can be described with a mixing parameter. In combustion flows simulation, a non filtered Arrhenius model was applied for the chemical source term, which corresponds to the case of the large chemical time scale compared to the turbulent time scale. A methane and air shear flow simulation was performed and the methane reaction rate showed non zero values against all temperature ranges. Small reaction rates were observed in the low temperature range due to the lack of subgrid scale modelling of the chemical source term. Simulation was also performed with fast chemistry approach representing the case of the large turbulent time scale compared to the chemical time scale. The mean flow of burner flames were compared with experimental data and a fair agreement was observed.

The thesis comprises of a thorough assessment of turbulent non-premixed combustion modelling techniques, emphasising the fundamental issue of turbulence-chemistry interaction. The combustion models studied are the flame-sheet, equilibrium, eddy breakup and laminar flamelet models. An in-house CFD code is developed and all the combustion models are implemented. Fundamental numerical issues involving the discretisation schemes are addressed by employing three discretisation schemes namely, hybrid, power law and TVD. The combustion models are evaluated for a number of fuels ranging from simple H2/CO and CO/H2/N2 to more complex Cl4/H2 burning in bluff body stabilised burners at different inlet fuel velocities. The bluff body burner with its complex recirculation zone provides a suitable model problem for industrial flows. The initial and boundary conditions are simple and well-defined. The bluff body burner also provides a controlled environment for the study of turbulence-chemistry interaction at the neck zone. The high quality experimental database available from the University of Sydney and other reported measurements are used for the validation and evaluation of combustion models. The present calculations show that all the combustion models provide good predictions for near equilibrium flames for temperature and major species. Although the equilibrium chemistry model is capable of predicting minor species, the predictive accuracy is found to be inadequate when compared to the experimental data. The laminae flamelet model is the only model which has yielded good predictions for the minor species. For flames at higher velocities. the laminar flamelet model again has provided better predictions compared to predictions of other models considered. With different fuels, the laminar flamelet model predictions for CO/H2/N2 fuel are better than those for CH4/H2 fuel. The reasons for this discrepancy are discussed in detail. The effects of differential diffusion are studied in the laminar flamelet modelling strategy. The flamelet with unity Lewis number is found to give a better representation of the transport of species. The laminar flamelet model has yielded reasonably good predictions for NO mass fraction. The predictions of NO mass fraction are found to be very sensitive to differential diffusion effects. This study has also considered the issue of inclusion of radiative heat transfer in the laminar flamelet model. The radiation effects are found to be important only where the temperature is very high. The study undertaken and reported in this thesis shows that the presently available laminar flamelet modelling concepts are capable of predicting species concentrations and temperature fields with an adequate degree of accuracy. The flamelet model is also well suited for the prediction of NO emissions. The inclusion of radiation heat transfer has enhanced the predictive capability of the laminar flamelet model.

L'enrichissement des hydrocarbures par l'hydrogène permet d'améliorer les performances de la combustion pauvre (augmentation de la réactivité, résistance à l'étirement, stabilité, réduction des polluants, ...). Il est primordial de connaitre les caractéristiques de la combustion de ces combustibles hybrides dans différentes conditions, afin de pouvoir les utiliser d'une manière sûre et efficace dans les installations pratiques. L'approche expérimentale reste coûteuse et limitée à certaines conditions opératoires. Cependant, le calcul numérique peut constituer la solution la plus adaptée, compte tenu du progrès réalisé dans le domaine de l'informatique et de la modélisation. Dans ce contexte, ce travail que nous avons effectué à l'ICARE (Institut de Combustion, Aérothermique et Réactivité, CNRS Orléans) vise à compléter les résultats des essais expérimentaux. Les effets de la richesse du mélange et l'ajout de l'hydrogène sur la structure et la formation des polluants sont étudiés dans ce travail. L'augmentation de la richesse du combustible permet de stabiliser la flamme, mais augmente la température et produit plus de CO, CO2 et NOx. Par contre, l'addition de H2 augmente l'efficacité du mélange, stabilise la flamme avec une légère élévation de la température maximale et une diminution des fractions massiques de CO, CO2 et NOx. Le remplacement d'une fraction de 10% où même 20% du gaz principal par l'hydrogène améliore les performances des installations et ne nécessite aucune modification sur les systèmes de combustion.

Moderate or Intense Low-oxygen Dilution (MILD) combustion is one of combustion technologies which can improve efficiency and reduce emissions simultaneously. This combustion type is characterised by the highly preheated reactant temperature and the relatively small temperature rise during combustion due to the intense dilution of the reactant mixture. These unique combustion conditions give MILD combustion very attractive features such as high combustion efficiency, reduction of pollutant emissions, attenuation of combustion instabilities and flexibility of the flow field. However, our understanding of MILD combustion is not enough to employ the MILD combustion technology further for modern combustion devices. In this thesis, Direct Numerical Simulation (DNS) has been carried out for turbulent MILD combustion under four MILD and classical premixed conditions. A two-phase strategy is employed in the DNS to include the effect of imperfect mixing between fresh and exhaust gases before intense chemical reactions start. In the simulated instantaneous MILD reaction rate fields, both thin and distributed reaction zones are observed. Thin reaction zones having flamelet like characteristics propagate until colliding with other thin reaction zones to produce distributed reaction zones. Also, the effect of such interacting reaction zones on scalar gradient has to be taken into account in flamelet approaches. Morphological features of MILD reaction zones are investigated by employing Minkowski functionals and shapefinders. Although a few local reaction zones are classified as thin shape, the majority of local reaction zones have pancake or tube-like shapes. The representative scales computed by the shapefinders also show a typical volume where intense reactions appear. Given high temperature and existence of radicals in the diluted reactants, both reaction dominated and flame-propagation dominated regions are locally observed. These two phenomena are closely entangled under a high dilution condition. The favourable conditions for these phenomena are investigated by focusing on scalar fluxes and reaction rate. A conditional Probability Density Function (PDF) is proposed to investigate flamelet/non-flamelet characteristics of MILD combustion. The PDF can be obtained by both numerically and experimentally. The PDF shows that MILD combustion still has the direct relationship between reaction rate and scalar gradient, although the tendency is statistically weak due to the distributed nature of MILD reaction zones. Finally, based on the physical aspects of MILD combustion explained in this work, a representative model reactor for MILD combustion is developed. The model reactor is also used in conjunction with the presumed PDF for a mean and filtered reaction rate closure. The results show a good agreement between the modelled reaction rate and the DNS results.

In this study, RANS based axisymmetric simulations of the jet flames, bluff-body flames and swirling flames have been attempted by employing steady and unsteady flamelet models. The jet flames have been studied for pure hydrogen and diluted hydrogen (CO/H2/N2) fuels. The bluff-body flames have been studied for three different fuels CH4/H2, H2/CO and CH3OH. The swirling flame has been investigated for CH4/H2 fuel. The importance of unsteady effects is thoroughly assessed for combustion predictions. The transient effects are considered in a post-processing manner employing the Lagrangian Flamelet Model (LFM) for jet flames and the Eulerian Particle Flamelet Model (EPFM) for recirculating bluff-body and swirling flames.

The goal of the present doctoral thesis was to develop a turbulence-chemistry interaction model for turbulent combustion simulations for high Reynolds number flows of practical interest. Present simulations were carried out using the unsteady Reynolds-averaged Navier-Stokes (URANS) and the Large Eddy Simulation (LES) formulations. The compressible flow treatment was used for both cases in order to model the coupling between hydromechanics and thermodynamics. The transient Navier-Stokes equations were applied to catch up unsteady combustion physics properly. The interaction between turbulence and chemistry was modeled based on the Eddy Dissipation Concept (EDC) with a detailed chemistry treatment. The open source OpenFOAM toolbox was used as the main frame for mathematical modeling and numerical simulations. First, non-reacting turbulent bluff-body flows were investigated with the goal of validation, verification and understanding of the capabilities of the numerical method using the conventional URANS approach. These results were analyzed in detail and agreed fairly well with experimental data. Then, the validation of the URANS approach (based on the standard k-ε model) was extended for reacting turbulent flows: the Sandia Flame D, the Sandia Flame CHNa and the Sydney Bluff-Body Flame HM1E. The chemistry was described by the full GRI-3.0 mechanism. There was relatively good agreement between simulations and measurements and it is believed that one of the main reasons for the observed discrepancies was the round-jet anomaly of the standard k-ε model. Furthermore, the numerical method was extended to a large-eddy simulation model. A sub-critical circular cylinder flow at a Reynolds number Re = 3:9 103 and a Mach number M = 0:2 was simulated to evaluate the applicability of the implemented LES approach. In general, the LES results agreed fairly well with the available experimental and numerical data and gave an indication of the adequacy and the accuracy of the implemented method. As a next step, LES validation was extended for a modest sub-critical circular cylinder flow at Re = 2x104. Both an incompressible and a compressible (M = 0:2) flow treatment was used. The predicted results revealed significant inaccuracies like spurious oscillations of the compressible flow solution. The incompressible flow results were found to be consistent with the existing LES studies as well as with measurements. Testing of the non-reflecting boundary conditions was performed for the Aeolian tones aeroacoustic predictions. The laminar flow over a circular cylinder at Re = 140 and M = 0:2 was calculated by direct solution of the unsteady compressible Navier-Stokes equations. The sound generated by a circular cylinder at Re = 2:2 x 104 and M = 0:2 was predicted using LES. The calculated acoustic fields showed a dipole directivity, similar to a natural vortex shedding. The impact of the Doppler effect was investigated and discussed as well. In general, (in spite of spurious oscillations in the near field) the computed aerodynamic and far-field acoustic results were found to be in satisfactory agreement with measurements and analytical relationships. Finally, the method was extended for the turbulent reactive flow predictions using LES. The LES formulation of the Eddy-Dissipation Concept was proposed. The validation was performed for the Sandia Flame D, where reasonable agreement between predicted and measured data was achieved. It is believed, that the observed discrepancies were related with the lack of grid resolution and inaccurate boundary conditions for the turbulence at inlet boundaries.

The propagation of premixed flames in turbulent flows is a problem of wide physical and technological interest, with a significant literature on their propagation speed and front topology. While certain scalings and parametric dependencies are well understood, a variety of problems remain. One major challenge, and focus of this thesis, is to model the influence of fuel/oxidizer composition on turbulent burning rates. Classical explanations for augmentation of turbulent burning rates by turbulent velocity fluctuations rely on global arguments - i.e., the turbulent burning velocity increase is directly proportional to the increase in flame surface area and mean local burning rate along the flame. However, the development of such global approaches is complicated by the abundance of phenomena influencing the propagation of turbulent premixed flames. Emphasizing key governing processes and cutting-off interesting but marginal phenomena appears to be necessary to make further progress in understanding the subject. An alternative approach to understand turbulent augmentation of burning rates is based upon so-called "leading points", which are intrinsically local properties of the turbulent flame. Leading points concepts suggest that the key physical mechanism controlling turbulent burning velocities of premixed flames is the velocity of the points on the flame that propagate farthest out into the reactants. It is postulated that modifications in the overall turbulent combustion speed depend solely on modifications of the burning rate at the leading points since an increase (decrease) in the average propagation speed of these points causes more (less) flame area to be produced behind them. In this framework, modeling of turbulent burning rates can be thought as consisting of two sub-problems: the modeling of (1) burning rates at the leading points and of (2) the dynamics/statistics of the leading points in the turbulent flame. The main objective of this thesis is to critically address both aspects, providing validation and development of the physical description put forward by leading point concepts. To address the first sub-problem, a comparison between numerical simulations of one-dimensional laminar flames in different geometrical configurations and statistics from a database of direct numerical simulations (DNS) is detailed. In this thesis, it is shown that the leading portions of the turbulent flame front display a structure that on average can be reproduced reasonably well by results obtained from model geometries with the same curvature. However, the comparison between model laminar flame computations and highly curved flamelets is complicated by the presence of negative (i.e., compressive) strain rates, due to gas expansion. For the highest turbulent intensity investigated, local consumption speeds, curvatures, strain rates and flame thicknesses approach the maximum values obtained by the laminar model geometries, while other cases display substantially lower values. To address the second sub-problem, the dynamics of flame propagation in simplified flow geometries is studied theoretically. Utilizing results for Hamilton-Jacobi equations from the Aubry-Mather theory, it is shown how the overall flame front progation under certain conditions is controlled only by discrete points on the flame. Based on these results, definitions of leading points are proposed and their dynamics is studied. These results validate some basic ideas from leading points arguments, but also modify them appreciably. For the simple case of a front propagating in a one-dimensional shear flow, these results clearly show that the front displacement speed is controlled by velocity field characteristics at discrete points on the flame only when the amplitude of the shear flow is sufficiently large and does not vary too rapidly in time. However, these points do not generally lie on the farthest forward point of the front. On the contrary, for sufficiently weak or unsteady flow perturbations, the front displacement speed is not controlled by discrete points, but rather by the entire spatial distribution of the velocity field. For these conditions, the leading points do not have any dynamical significance in controlling the front displacement speed. Finally, these results clearly show that the effects of flame curvature sensitivity in modifying the front displacement speed can be successfully interpreted in term of leading point concepts.

This thesis is concerned with the development and implementation of computational fluid dynamics (CFD) based prediction methodologies for turbulent reacting flows with principal application to turbulent diffusion flame combustors. Numerical simulation of combustion problems involve strong coupling between chemistry, transport and fluid dynamics. The works accomplished in this study can be separated mainly into three distinct areas: i) assessment of the performance of turbulent combustion models and to implement suitable submodels for combustion and flame behaviour into CFD code; ii) Conducting CFD modelling of turbulent diffusion flames, radiation heat loss from combustion and flame zones; and iii) modelling of pollutants like NOx (oxides of nitrogen), identification of the effect of radiation heat loss on NOx formation. The combustion models studied are the flame-sheet, equilibrium, eddy break-up and laminar flamelet models. An in-house CFD code is developed and combustion models are implemented. The basic numerical issues involving the discretisation schemes are addressed by employing three discretisation schemes namely, hybrid, power law and TVD (total variation diminishing) schemes. The combustion of different fuels ranging from simple H2/N2 and CO/H2/N2 to complex CH4/H2 are investigated for different inlet velocities and boundary conditions. The performances of the combustion models are analysed for these fuels. The configurations used for the validation and assessment of the combustion models are co-flowing jet flames and bluff body burner stabilized flames. The high quality experimental databases available from Sandia national laboratories, the University of Sydney and other reported measurements are used for the purpose of evaluating the combustion models. The predicted results demonstrate the effects of turbulent mixing and the effects of chemical reactions on the combustion models. The calculations show that all the combustion models like flame-sheet and equilibrium models are found to be inadequate even for the near equilibrium flames. Although the equilibrium chemistry model is capable of predicting the mixture fraction, temperature and concentrations of major and minor species, the predictive accuracy is found to be inadequate specially, when compared to the experimental data. In situations, where finite rate chemistry effects are important the laminar flamelet model is a good choice. The key contributions of this thesis are as follows: 1) Modification of in-house CFD code for turbulent reacting flow and development of CFD based iterative scheme for the turbulent diffusion flames to account for radiation heat loss from combustion and flame zones. 2) Thorough assessment of turbulent combustion modelling techniques for different cases of diffusion flames, demonstration of the importance of differential diffusion in the flamelet modelling of combustion and comprehensive validation 3) Demonstration of the importance of radiation heat loss in the modelling of turbulent combustion, implementation of radiation modelling in the three cases of diffusion flames and comprehensive validation of CFD based combustion radiation results. 4) Development of modelling strategy for the pollutants like oxides of nitrogen (NOx), implementation of NOx modelling in the different flames cases and identified the effect of radiation heat loss on NOx formation. The works addressed in this thesis are presented with the applications to turbulent diffusion flame combustors. However, these works can easily be extended to the industrial applications and applied to a large variety of other challenging domains.

In recent times significant public attention has been drawn to the topic of combustion. This has been due to the fact that combustion is the underlying mechanism of several key challenges to modern society: climate change, energy security (finite reserves of fossil fuels) and air pollution. The further development of combustion science is undoubtedly necessary to find improved solutions to manage these combustion science related challenges in the near and long term future. Combustion is essentially an exothermic process, this exothermicity or heat release essentially occurs at small scales, by small scales it meant these scales are small relative to the fluid length scales, for example heat release layer thicknesses in flames are typically much less than the fluid integral length scales. As heat release occurs at small scales this means that in turbulent combustion the small scales of the turbulence (which can be of the order of the heat release layer thickness) can possibly interact and influence the heat release and thus chemistry of the flame reaction zone. Premixed combustion is a combustion mode where the fuel and oxidiser are completely premixed prior to the flame reaction zone, this mode of combustion has been shown to be a promising method to maximise combustion efficiency and minimise pollutant formation. The continued and further application of premixed combustion to practical applications is limited by the current understanding of turbulent premixed combustion, these limitations in understanding are linked to the specific flame phenomena that can significantly influence premixed combustion in a combustion device, examples of such phenomena are: flame flashback, flame extinction and fuel consumption rate – all phenomena that are influenced by the interaction of the small scales of turbulence and chemistry. It is the study and investigation of the interaction of turbulence and chemistry at the small scales (termed finite-rate chemistry) in turbulent premixed flames that is the aim of this thesis which is titled “Finite-rate chemistry effects in turbulent premixed combustion”. Two very closely related experimental burner geometries have been developed in this thesis: the Piloted Premixed Jet Burner (PPJB) and the Premixed Jet Burner (PJB). Both feature an axisymmetric geometry and exhibit a parabolic like flow field. The PPJB and PJB feature a small 4mm diameter central jet from which a high velocity lean-premixed methane-air mixture issues. Surrounding the central jet in the PPJB is a 23.5mm diameter pilot of stoichiometric methane-air products, the major difference between the PPJB and the PJB is that the PJB does not feature a stoichiometric pilot. The pilot in the PPJB provides a rich source of combustion intermediates and enthalpy which promotes initial ignition of the central jet mixture. Surrounding both the central jet and pilot is a large diameter hot coflow of combustion products. It is possible to set the temperature of the hot coflow to the adiabatic flame temperature of the central jet mixture to simulate straining and mixing against and with combustion products without introducing complexities such as quenching and dilution from cold air. By parametrically increasing the central jet velocity in the PPJB it is possible to show that there is a transition from a thin conical flame brush to a flame that exhibits extinction and re-ignition effects. The flames that exhibit extinction and re-ignition effects have a luminous region near the jet exit termed the initial ignition region. This is followed by a region of reduced luminosity further downstream termed the extinction region. Further downstream the flame luminosity increases this region is termed the re-ignition region. For the flames that exhibit extinction and re-ignition it is proposed that intense turbulent mixing and high scalar dissipation rates drives the initial extinction process after the influence of the pilot has ceased (x/D>10). Re-ignition is proposed to occur downstream where turbulent mixing and scalar dissipation rates have decreased allowing robust combustion to continue. As the PJB does not feature a pilot, the flame stabilisation structure is quite different to the PPJB. The flame structure in the PJB is essentially a lifted purely premixed flame, which is an experimental configuration that is also quite unique. A suite of laser diagnostic measurements has been parametrically applied to flames in the PPJB and PJB. Laser Doppler Velocimetry (LDV) has been utilised to measure the mean and fluctuating radial and axial components of velocity at a point, with relevant time and length scale information being extracted from these measurements. One of the most interesting results from the LDV measurements is that in the PPJB the pilot delays the generation of high turbulence intensities, for flames that exhibit extinction the rapid increase of turbulence intensity after the pilot corresponds to the start of the extinction region. Using the LDV derived turbulence characteristics and laminar flame properties and plotting these flames on a traditional turbulent regime diagram indicates that all of the flames examined should fall in the so call distributed reaction regime. Planar imaging experiments have been conducted for flames using the PPJB and PJB to investigate the spatial structure of the temperature and selected minor species fields. Results from two different simultaneous 2D Rayleigh and OH PLIF experiments and a simultaneous 2D Rayleigh, OH PLIF and CH2O PLIF experiment are reported. For all of the flames examined in the PPJB and PJB a general trend of decreasing conditional mean temperature gradient with increasing turbulence intensity is observed. This indicates that a trend of so called flame front thickening with increased turbulence levels occurs for the flames examined. It is proposed that the mechanism for this flame front thickening is due to eddies penetrating and embedding in the instantaneous flame front. In the extinction region it is found that the OH concentration is significantly reduced compared to the initial ignition region. In the re-ignition region it is found that the OH level increases again indicating that an increase in the local reaction rate is occurring. In laminar premixed flames CH2O occurs in a thin layer in the reaction zone, it is found for all of the flames examined that the CH2O layer is significantly thicker than the laminar flame. For the high velocity flames beyond x/D=15, CH2O no longer exist in a distinct layer but rather in a near uniform field for the intermediate temperature regions. Examination of the product of CH2O and OH reveals that the heat release in the initial ignition region is high and rapidly decreases in the extinction region, an increase in the heat release further downstream is observed corresponding to the re-ignition region. This finding corresponds well with the initial hypothesis of an extinction region followed by a re-ignition region that was based on the mean chemiluminescence images. Detailed simultaneous measurement of major and minor species has been conducted using the line Raman-Rayleigh-LIF technique with CO LIF and crossed plane-OH PLIF at Sandia National Laboratories. By measuring all major species it is also possible to define a mixture fraction for all three streams of the PPJB. Using these three mixture fractions it was found that the influence of the pilot in the PPJB decays very rapidly for all but the lowest velocity flames. It was also found that for the high velocity flames exhibiting extinction, a significant proportion of the coflow fluid is entrained into the central jet combustion process at both the extinction region and re-ignition regions. The product of CO and OH conditional on temperature is shown to be proportion to the net production rate of CO2 for certain temperature ranges. By examining the product of CO and OH the hypothesis of an initial ignition region followed by an extinction region then a re-ignition region for certain PPJB flames has been further validated complementing the [CH2O][OH] imaging results. Numerical modelling results using the transported composition probability density function (TPDF) method coupled to a conventional Reynolds averaged Naiver Stokes (RANS) solver are shown in this thesis to successfully predict the occurrence of finite-rate chemistry effects for the PM1 PPJB flame series. To calculate the scalar variance and the degree of finite-rate chemistry effects correctly, it is found that a value of the mixing constant ( ) of approximately 8.0 is required. This value of is much larger than the standard excepted range of 1.5-2.3 for that has been established for non-premixed combustion. By examining the results of the RANS turbulence model in a non-reacting variable density jet, it is shown that the primary limitation of the predictive capability of the TPDF-RANS method is the RANS turbulence model when applied to variable density flows.

Doctor of Philosophy (PhD)
In recent times significant public attention has been drawn to the topic of combustion. This has been due to the fact that combustion is the underlying mechanism of several key challenges to modern society: climate change, energy security (finite reserves of fossil fuels) and air pollution. The further development of combustion science is undoubtedly necessary to find improved solutions to manage these combustion science related challenges in the near and long term future. Combustion is essentially an exothermic process, this exothermicity or heat release essentially occurs at small scales, by small scales it meant these scales are small relative to the fluid length scales, for example heat release layer thicknesses in flames are typically much less than the fluid integral length scales. As heat release occurs at small scales this means that in turbulent combustion the small scales of the turbulence (which can be of the order of the heat release layer thickness) can possibly interact and influence the heat release and thus chemistry of the flame reaction zone. Premixed combustion is a combustion mode where the fuel and oxidiser are completely premixed prior to the flame reaction zone, this mode of combustion has been shown to be a promising method to maximise combustion efficiency and minimise pollutant formation. The continued and further application of premixed combustion to practical applications is limited by the current understanding of turbulent premixed combustion, these limitations in understanding are linked to the specific flame phenomena that can significantly influence premixed combustion in a combustion device, examples of such phenomena are: flame flashback, flame extinction and fuel consumption rate – all phenomena that are influenced by the interaction of the small scales of turbulence and chemistry. It is the study and investigation of the interaction of turbulence and chemistry at the small scales (termed finite-rate chemistry) in turbulent premixed flames that is the aim of this thesis which is titled “Finite-rate chemistry effects in turbulent premixed combustion”. Two very closely related experimental burner geometries have been developed in this thesis: the Piloted Premixed Jet Burner (PPJB) and the Premixed Jet Burner (PJB). Both feature an axisymmetric geometry and exhibit a parabolic like flow field. The PPJB and PJB feature a small 4mm diameter central jet from which a high velocity lean-premixed methane-air mixture issues. Surrounding the central jet in the PPJB is a 23.5mm diameter pilot of stoichiometric methane-air products, the major difference between the PPJB and the PJB is that the PJB does not feature a stoichiometric pilot. The pilot in the PPJB provides a rich source of combustion intermediates and enthalpy which promotes initial ignition of the central jet mixture. Surrounding both the central jet and pilot is a large diameter hot coflow of combustion products. It is possible to set the temperature of the hot coflow to the adiabatic flame temperature of the central jet mixture to simulate straining and mixing against and with combustion products without introducing complexities such as quenching and dilution from cold air. By parametrically increasing the central jet velocity in the PPJB it is possible to show that there is a transition from a thin conical flame brush to a flame that exhibits extinction and re-ignition effects. The flames that exhibit extinction and re-ignition effects have a luminous region near the jet exit termed the initial ignition region. This is followed by a region of reduced luminosity further downstream termed the extinction region. Further downstream the flame luminosity increases this region is termed the re-ignition region. For the flames that exhibit extinction and re-ignition it is proposed that intense turbulent mixing and high scalar dissipation rates drives the initial extinction process after the influence of the pilot has ceased (x/D>10). Re-ignition is proposed to occur downstream where turbulent mixing and scalar dissipation rates have decreased allowing robust combustion to continue. As the PJB does not feature a pilot, the flame stabilisation structure is quite different to the PPJB. The flame structure in the PJB is essentially a lifted purely premixed flame, which is an experimental configuration that is also quite unique. A suite of laser diagnostic measurements has been parametrically applied to flames in the PPJB and PJB. Laser Doppler Velocimetry (LDV) has been utilised to measure the mean and fluctuating radial and axial components of velocity at a point, with relevant time and length scale information being extracted from these measurements. One of the most interesting results from the LDV measurements is that in the PPJB the pilot delays the generation of high turbulence intensities, for flames that exhibit extinction the rapid increase of turbulence intensity after the pilot corresponds to the start of the extinction region. Using the LDV derived turbulence characteristics and laminar flame properties and plotting these flames on a traditional turbulent regime diagram indicates that all of the flames examined should fall in the so call distributed reaction regime. Planar imaging experiments have been conducted for flames using the PPJB and PJB to investigate the spatial structure of the temperature and selected minor species fields. Results from two different simultaneous 2D Rayleigh and OH PLIF experiments and a simultaneous 2D Rayleigh, OH PLIF and CH2O PLIF experiment are reported. For all of the flames examined in the PPJB and PJB a general trend of decreasing conditional mean temperature gradient with increasing turbulence intensity is observed. This indicates that a trend of so called flame front thickening with increased turbulence levels occurs for the flames examined. It is proposed that the mechanism for this flame front thickening is due to eddies penetrating and embedding in the instantaneous flame front. In the extinction region it is found that the OH concentration is significantly reduced compared to the initial ignition region. In the re-ignition region it is found that the OH level increases again indicating that an increase in the local reaction rate is occurring. In laminar premixed flames CH2O occurs in a thin layer in the reaction zone, it is found for all of the flames examined that the CH2O layer is significantly thicker than the laminar flame. For the high velocity flames beyond x/D=15, CH2O no longer exist in a distinct layer but rather in a near uniform field for the intermediate temperature regions. Examination of the product of CH2O and OH reveals that the heat release in the initial ignition region is high and rapidly decreases in the extinction region, an increase in the heat release further downstream is observed corresponding to the re-ignition region. This finding corresponds well with the initial hypothesis of an extinction region followed by a re-ignition region that was based on the mean chemiluminescence images. Detailed simultaneous measurement of major and minor species has been conducted using the line Raman-Rayleigh-LIF technique with CO LIF and crossed plane-OH PLIF at Sandia National Laboratories. By measuring all major species it is also possible to define a mixture fraction for all three streams of the PPJB. Using these three mixture fractions it was found that the influence of the pilot in the PPJB decays very rapidly for all but the lowest velocity flames. It was also found that for the high velocity flames exhibiting extinction, a significant proportion of the coflow fluid is entrained into the central jet combustion process at both the extinction region and re-ignition regions. The product of CO and OH conditional on temperature is shown to be proportion to the net production rate of CO2 for certain temperature ranges. By examining the product of CO and OH the hypothesis of an initial ignition region followed by an extinction region then a re-ignition region for certain PPJB flames has been further validated complementing the [CH2O][OH] imaging results. Numerical modelling results using the transported composition probability density function (TPDF) method coupled to a conventional Reynolds averaged Naiver Stokes (RANS) solver are shown in this thesis to successfully predict the occurrence of finite-rate chemistry effects for the PM1 PPJB flame series. To calculate the scalar variance and the degree of finite-rate chemistry effects correctly, it is found that a value of the mixing constant ( ) of approximately 8.0 is required. This value of is much larger than the standard excepted range of 1.5-2.3 for that has been established for non-premixed combustion. By examining the results of the RANS turbulence model in a non-reacting variable density jet, it is shown that the primary limitation of the predictive capability of the TPDF-RANS method is the RANS turbulence model when applied to variable density flows.

The work presented in this thesis addresses issues involving the accurate and efficient numerical modelling of turbulence combustion with an emphasis on an industrially representative Tay model combustor. This combustor retained all essential features of a modern aero-engine rich burn combustor and thus the turbulence combustion within this combustor is much more complicated than those observed in the combustor-like burners typically considered in laboratory experiments. A comparative study of two combustion models based on a non-premixed assumption or a partially premixed assumption using the previously proposed models Zimont Turbulent Flame Speed Closure (ZTFSC) and Extended Coherent Flamelet Method (ECFM)) is presented in a first step. Comprehensive chemical reactions containing 244 reactions and 50 species are taken into account using a tabulated detailed chemistry approach and an assumed shape PDF to account for turbulence effects. The purpose of this study is to validate and compare the effectiveness of these models in predicting complex combustion and to improve upon for the defects observed in previous predictions of the same combustor. It is concluded that the use of models invoking the partially premixed combustion assumption can provide much more accurate results than models using a non-premixed combustion assumption especially in the primary zone of the combustor where turbulence combustion interaction is strong. In addition, certain shortcomings of steady RANS type models are identified as a result of strong unsteady effects and their inability to resolve the turbulence spectrum. Following this, two URANS models and the scale resolving simulation (SRS) approach such as a shear stress transport, K-omega, scale adaptive simulation (SSTKWSAS) combined with the partially premixed method identified in the first step are employed in a second step to further improve the accuracy achieved and to provide evidence and guidance in terms of the trade-off between accuracy and computational cost for complex turbulent combustion simulations. The second generation SRS model (SSTKWSAS) is applied to the complicated flow environment of a realistic combustor for the first time. The present work highlights the superiority of the combination of the SSTKWSAS approach and a partially premixed combustion model in terms of both accuracy and efficiency for predicting such combustion problems.

Les premières simulations numériques d'écoulements réactifs sur des configurationscomplexes ont été réalisées à l'aide d'approches RANS (Reynolds Averaged Navier Stokes). Ces dernières, bien adaptées aux écoulements de type couches limites attachées et relativement peu coûteuses en temps de calcul, ne donnent accès qu'à des résultats stationnaires qui s'éloignent parfois de la réalité. Pour réaliser des simulations instationnaires d'écoulements, les méthodes de type LES (Large Eddy Simulation) -- plus précises mais plus coûteuses -- sont de plus en plus utilisées. Cependant, ces méthodes sont mal adaptées à la simulation de la dynamique pariétale, car elles nécessitent un effort de maillage souvent prohibitif près de la paroi. Cette thèse est consacrée au développement dans le code CEDRE (code de simulation d'écoulements réactifs complexes de l'Onera) d'une méthode hybride RANS/LES, appelée Delayed Detached Eddy Simulation (DDES), et à son application à des écoulements réactifs massivement décollés. Après une étape de validation sur des couches limites attachées, la DDES a été appliquée à la simulation des écoulements inerte et réactif dans une chambre de combustion en forme de marche descendante (A3C) et comparée aux résultats des approches RANS et LES classiques, ainsi qu'aux résultats expérimentaux. Cette méthode a ensuite permis de réaliser l'étude de la dynamique de l'écoulement réactif décollé dans la tuyère ATAC montée sur le banc cryotechnique MASCOTTE de l'Onera.

Due to concerns about pollutant emission combustion systems are increasingly being designed to operate in a lean premixed mode. However, the reduction in emissions offered by lean premixed combustion can be offset by its susceptibility to instabilities and ignition and extinction problems. These instabilities, caused by the coupling of unsteady heat release and pressure fluctuations can cause significant damage to combustion devices. One method of avoiding these problems whilst still operating a globally lean system is to employ a stratified premixed mode where areas of richer mixture are used to enhance the stability of the flame. In this thesis a computational modelling methodology for the simulation of stratified premixed flames is developed. Firstly, several sub-models for the dissipation rate of a reacting scalar are evaluated by the simulation of two laboratory scale flames, a turbulent stratified V-flame and a dump combustor fed by two streams of different mixture strength. This work highlights the importance of this quantity and its influence on the simulation results. Any model for stratified combustion requires at least two variables to describe the thermochemical state of the gas: one to represent the mixing field and another to capture the progress of reaction. In turbulent stratified flames the joint probability density function (pdf) of these variables can be used to recover the mean reaction rates. A new formulation for this pdf based on copula methods is presented and evaluated alongside two alternative forms. The new method gives improved results in the simulation of the two test cases above. As it is likely that practical stratified combustion devices will have some unsteadiness to the flow the final part of this work applies the modelling methodology to an unsteady test case. The influence of the unsteady velocity forcing on the pollutant emissions is investigated. Finally the methodology is used to simulate a developmental, liquid fuelled, lean burn aero-engine combustor. Here the model gives reasonable predictions of the measured pollutant emissions for a relatively small computational cost. As such it is hoped that the modelling methodology presented can be useful in the iterative industrial design process of stratified combustion systems.

A numerical study of turbulent combustion systems was pursued to examine different computational modeling techniques, namely computational fluid dynamics (CFD) and chemical reactor network (CRN) methods. Both methods have been studied and analyzed as individual techniques as well as a coupled approach to pursue better understandings of the mechanisms and interactions between turbulent flow and mixing, ignition behavior and pollutant formation. A thorough analysis and comparison of both turbulence models and chemistry representation methods was executed and simulations were compared and validated with experimental works. An extensive study of turbulence modeling methods, and the optimization of modeling techniques including turbulence intensity and computational domain size have been conducted. The final CFD model has demonstrated good predictive performance for different turbulent bluff-body flames. The NOx formation and the effects of fuel mixtures indicated that the addition of hydrogen to the fuel and non-flammable diluents like CO2 and H2O contribute to the reduction of NOx. The second part of the study focused on developing chemical models and methods that include the detailed gaseous reaction mechanism of GRI-Mech 3.0 but cost less computational time. A new chemical reactor network has been created based on the CFD results of combustion characteristics and flow fields. The proposed CRN has been validated with the temperature and species emission for different bluff-body flames and has shown the capability of being applied to general bluff-body systems. Specifically, the rate of production of NOx and the sensitivity analysis based on the CRN results helped to summarize the reduced reaction mechanism, which not only provided a promising method to generate representative reactions from hundreds of species and reactions in gaseous mechanism but also presented valuable information of the combustion mechanisms and NOx formation. Finally, the proposed reduced reaction mechanism from the sensitivity analysis was applied to the CFD simulations, which created a fully coupled process between CFD and CRN, and the results from the reduced reaction mechanism have shown good predictions compared with the probability density function method.
Ph. D.

En physique des explosifs, la postcombustion désigne la phase de combustion qui intervient après la fin de la détonation lorsque l’explosif considéré est initialement déficient en oxydant. Les produits de détonation, qui apparaissent sous la forme d’une boule de feu, peuvent alors à leur tour être oxydés, ce qui permet de libérer une quantité supplémentaire d’énergie dans l’écoulement et d’augmenter le souffle. Ce phénomène complexe est piloté par l’interaction entre des ondes de chocs, une zone de mélange turbulente créée par des instabilités hydrodynamiques de type Rayleigh-Taylor et Richtmyer-Meshkov, et une flamme de diffusion. Compte tenu de son effet significatif sur la performance d’une explosif, une bonne compréhension de la postcombustion est nécessaire afin de pouvoir la modéliser et déterminer avec précision les effets d’une charge donnée. A cette fin, des travaux, à la fois numériques et expérimentaux, ont été menés afin de mieux comprendre le processus de mélange intervenant dans les boules de feu puis le phénomène dans son ensemble. Afin de contourner les difficultés liées à la caractérisation des produits de détonation, cette étude s’est concentrée sur l’explosion de capacités sphériques sous pression qui permet de produire un écoulement similaire à celui provoqué par une détonation sphérique. Les résultats obtenus sont semblables à ceux de la littérature sur la postcombustion des explosifs et apportent un éclairage nouveau sur l’influence de certains paramètres tels que la masse de l’explosif ou les propriétés des perturbations initiant les instabilités.

La complexité de la chimie joue un rôle majeur dans la simulation numérique de la plupart des écoulements réactifs industriels. L'utilisation de schémas cinétiques chimiques détaillés avec les outils de simulation actuels reste toutefois trop coûteuse du fait des faibles pas de temps et d'espaces associés à la résolution d'une flamme, bien souvent inférieurs de plusieurs ordres de grandeur à ceux nécessaires pour capturer les effets de la turbulence. Une solution est proposée pour s'affranchir de cette limite. Un outil automatisé de réduction de schémas cinétiques est développé sur la base d'un ensemble de trajectoires de références construites dans l'espace des compositions pour être représentatives du système à simuler. Ces trajectoires sont calculées à partir de l'évolution de particules stochastiques soumises à différentes conditions de mélange, de réaction et d'évaporation dans le cas de combustible liquide. L'ensemble est couplé à un algorithme génétique pour l'optimisation des taux de réaction du schéma réduit, permettant ainsi une forte réduction du coût calcul. L'approche a été validée et utilisée pour la réduction de divers mécanismes réactionnels sur des applications académiques et industrielles, pour des hydrocarbures simples comme le méthane jusqu'à des hydrocarbures plus complexes, comme le kérosène en incluant une étape optimisée de regroupement des isomères
Complex chemistry is an essential ingredient in advanced numerical simulation of combustion systems. However, introducing detailed chemistry in Computational Fluid Dynamics (CFD) softwares is a non trivial task since the time and space resolutions necessary to capture and solve for a flame are very often smaller than the turbulent characteristic scales by several orders of magnitude. A solution based on the reduction of chemical mechanisms is proposed to tackle this issue. An automated reduction and optimisation strategy is suggested relying on the construction of reference trajectories computed with the evolution of stochastic particles that face mixing, evaporation and chemical reactions. The methodology, which offers strong reduction in CPU cost, is applied to the derivation of several mechanisms for canonical and industrial applications, for simple fuel such as methane up to more complex hydrocarbon fuels, as kerosene, including an optimised lumping procedure for isomers

Increasingly stringent regulation of pollutant emission has motivated the search for cleaner and more efficient combustion devices, which remain the primary means of power generation and propulsion for all kinds of transport. Fuel-lean premixed combustion technology has been identified to be a promising approach, despite many difficulties involve, notably issues concerning flame stability and ignitability. A partially premixed system has been introduced to remedy these problems, however, our understanding on this combustion mode needs to be greatly improved to realise its full potential. This thesis aims to further the understanding of various fundamental physical processes in turbulent partially premixed flames. DNS data of a laboratory-scale hydrogen turbulent jet lifted flame is analysed in this study. The partially premixed nature of this flame is established by examining the instantaneous and averaged reaction rates and the "Flame Index", which indicate premixed and diffusion burning modes coexisting. The behaviour of turbulent flame stretch and its relation to other physical processes, in particular the scalar-turbulence interaction, the effects of partial premixing on the displacement speed of iso-scalar surface and its correlation with the surface curvature are explored using DNS data. The scalar gradient alignment characteristics change from aligning with the most compressive strain to aligning with the most extensive one in regions of intensive heat release. This alignment change creates negative normal strain rate which can result in negative surface averaged tangential strain rate. The partial premixing affects the flame surface displacement speed through the mixture fraction dissipation rate and a second derivative in the mixture fraction space. The correlation of curvature and displacement speed is found to be negative in general and the effects of partial premixing act to reduce this negative correlation. The combined effects of the normal strain rate and the displacement speed/curvature correlation contribute to the negative mean flame stretch observed in the flame brush. Scalar dissipation rates (SDR) of the mixture fraction ẼZZ, progress variable Ẽcc and their cross dissipation rates (CDR) ẼcZ are identified as important quantities in the modelling of partially premixed flames. Their behaviours in the lifted flame stabilisation region are examined in a unified framework. It is found that SDR of mixture fraction is well below the quenching value in this region while SDR of progress variable is smaller than that in laminar flames. The CDR changes from weakly positive to negative at the flame leading edge due to the change in scalar gradient alignment characteristics. Axial and radial variation of these quantities are analysed and it is found that Ẽcc is an order of magnitude bigger than ẼZZ. ẼcZ is two orders of magnitude smaller than Ẽcc and it can be either positive or negative depending on local flow and flame conditions. Simple algebraic models show reasonable agreement compared to DNS when a suitable definition of c is used. Further statistics of the scalar gradients are presented and a presumed lognormal distribution is found to give reasonable results for their marginal PDFs and a bivariate lognormal distribution is a good approximation for their joint PDF. Four mean reaction rate closures based on presumed PDF and flamelets are assessed a priori using DNS data. The turbulent flame front structure is first compared with unstrained and strained laminar premixed and dif fusion flamelets. It is found that unstrained premixed flamelets give overall reasonable approximation in most parts of this flame. A joint PDF model which includes the correlation between mixture fraction and progress variable using a "copula" method shows excellent agreement with DNS results while their statistical independence does not hold in the burning regions of this partially premixed flame. The unstrained premixed flamelet with the correlated joint PDF method is identified to be the most appropriate model for the lifted jet flame calculation. This model is then used in the RANS simulation of turbulent jet lifted flames. A new model to include the contribution from diffusion burning and the effects of partial premixing due to SDR of mixture fraction is also identified and included in the calculation. These models are implemented in a commercial CFD code "Fluent" with user defined scalars and functions. It is found that both the correlated joint PDF model and the model accounting for the diffusive burning in partial premixing are important in order to accurately predict flame lift-off height compared to the experiments.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2018.
Cataloged from PDF version of thesis.
Includes bibliographical references (pages 203-216).
Carbon capture and storage (CCS) is an important strategy for reducing CO₂ emissions, with oxy-fuel combustion being one of the most promising technologies because of it is high efficiency and low cost. In oxy-combustion, CH₄/O₂/CO₂ mixtures burn at low temperatures (~~1700 K), high pressures (~~40 bar), where laminar burning velocities are about 7 times lower than in traditional CH₄ /Air mixtures. Thus oxy-fuel combustors are more prone to blowoff and dynamic instabilities. In this thesis we examine turbulent oxy-combustion flame stabilization physics at the large and small scales using experimental studies and numerical simulations. Experimental measurements are used to establish the stability characteristics of flame macrostructures in a swirl stabilized combustor. We show that the transition in the flame macrostructure to a flame stabilized along both the inner and outer shear layers (Flame IV), scales according to the extinction strain rate, similar to air flames. To achieve accurate scaling, extinction strain rates must be computed at the thermal conditions of the outer shear layer, emphasizing the role of heat interactions with the wall boundary layer. Care must be exercised while modeling the chemical structure of oxy-flames. We show that the kinetics of CO₂ (used as a diluent in oxy-combustion) is important in determining the consumption speed and flame extinction strain rate. Specifically, the extinction strain rate was found to be heavily impacted by the reaction CO₂+ H -->/<-- CO + OH. Large Eddy Simulations (LES) models, first validated for various combustor geometries, fuels and oxidizers, are used to examine the stabilization mechanisms of these flames. First, we demonstrate the importance of choosing the correct global chemical kinetics mechanism in predicting the flow structures in multi-dimensional simulations and develop a priori criterion of selecting a reduced mechanism based on the extinction strain rate. Besides flame macrostructures, recirculation zone lengths are found to linearly scale with extinction strain rates. This scaling holds regardless of fuel or oxidizer type, Reynold's number, inlet temperature, or combustor geometry. It is thus very important that a chemical mechanism is able to correctly predict extinction strain rates if it is to be used in CFD simulations. We use the validated LES framework to model the transition to Flame IV in the swirl combustor for methane oxy-combustion mixtures. The 3D turbulent flame structure strongly resembles a ID strained adiabatic laminar flame structure in the combustor interior, and non-adiabatic flames near the combustor wall. The results support the earlier conclusions regarding the use of the extinction strain rate and the wall thermal boundary condition in scaling and modeling turbulent combustion dynamics.
by Nadim Walid Chakroun.
Ph. D.