Academic literature on the topic 'Chemical autoignition delay'

A three-dimensional direct numerical simulation is conducted for a temporally evolving planar jet of n-heptane at a pressure of 40 atmospheres and in a coflow of air at 1100 K. At these conditions, n-heptane exhibits a two-stage ignition due to low- and high-temperature chemistry, which is reproduced by the global chemical model used in this study. The results show that ignition occurs in several overlapping stages and multiple modes of combustion are present. Low-temperature chemistry precedes the formation of multiple spatially localised high-temperature chemistry autoignition events, referred to as ‘kernels’. These kernels form within the shear layer and core of the jet at compositions with short homogeneous ignition delay times and in locations experiencing low scalar dissipation rates. An analysis of the kernel histories shows that the ignition delay time is correlated with the mixing rate history and that the ignition kernels tend to form in vortically dominated regions of the domain, as corroborated by an analysis of the topology of the velocity gradient tensor. Once ignited, the kernels grow rapidly and establish edge flames where they envelop the stoichiometric isosurface. A combination of kernel formation (autoignition) and the growth of existing burning surface (via edge-flame propagation) contributes to the overall ignition process. An analysis of propagation speeds evaluated on the burning surface suggests that although the edge-flame speed is promoted by the autoignitive conditions due to an increase in the local laminar flame speed, edge-flame propagation of existing burning surfaces (triggered initially by isolated autoignition kernels) is the dominant ignition mode in the present configuration.

The effect of ethane on combustion and natural gas autoignition was studied in the present paper. Two fuel mixture of natural gas with high ethane content were considered, 75% CH4 – 25% C2H6 (mixture 1), and 50% CH4 – 50% C2H6 (mixture 2). Natural gas combustion incidence was analyzed through the calculation of energy properties and the ignition delay time numerical calculations along with an ignition mode analysis. Specifically, the strong ignition limit was calculated to determine the effect of ethane on natural gas autoignition. According to the results, ignition delay time decreases for both mixtures in comparison with pure methane. The strong ignition limit shifts to lower temperatures when ethane is present in natural gas chemical composition.

This study deals with the development of controlled-ignition technology for high-performance compression ignition alcohol engines. Among the alcohol fuels, we focus on ethanol as it is a promising candidate of alternative fuels replacing petroleum. The objective of this study is to reveal the physical and chemical phenomena in the mixture formation process up to autoignition of an ethanol spray. In our previous numerical study, we showed the mixture formation process for gas oil and ethanol sprays in the form of spatial excess air ratio and temperature distributions inside a spray and their temporal histories from fuel injection. The results showed a good agreement with those of theoretical analysis based on the momentum theory of spray penetration. Calculation was also confirmed as reasonable by comparing to the experimental results. Through the series of our experimental and numerical studies, the reason for poor autoignition quality of an ethanol spray was revealed, that is, difficulty in simultaneous attainments of autoignition-suitable concentration and temperature in the spray mixture formation due to its fuel and thermal properties of smaller stoichiometric air-fuel ratio and much greater heat of evaporation compared to conventional diesel fuels. However, autoignition of an ethanol spray has not been obtained yet in either experiments or numerical analysis. As the next step, we numerically examined several surrounding gas pressure and temperature conditions to make clear the surrounding gas conditions enough to obtain stable autoignition. One of the commercial CFD codes CONVERGE was used in the computational calculation with the considerations of turbulence, atomization, evaporation, and detailed chemical reaction. Required surrounding gas pressure and temperature for stable autoignition with acceptable ignition delay of an ethanol spray and feasibility of the development of high-performance compression ignition alcohol engines are discussed in this paper.

In the range of 800–1200 K, both experiments and kinetic modeling demonstrate a significant difference in the dependence of the ignition delay time of methane and hydrogen on pressure and temperature, with the complex influence of these parameters on the autoignition delay time of methane–hydrogen–air mixtures. In connection with the prospects for the widespread use of methane–hydrogen mixtures in energy production and transport, a detailed analysis of their ignition at temperatures below 1000 K, the most important region from the point of view of their practical application, is carried out. It is shown that such a complex behavior is associated with the transition in this temperature range from low-temperature mechanisms of oxidation of both methane and hydrogen, in which peroxide radicals and molecules play a decisive role, to high-temperature mechanisms of their oxidation, in which simpler radicals dominate. A kinetic interpretation of the processes occurring in this case is proposed.

In order to research the high-concentration CO2 effects on ignition delay time (IDT) of diesel surrogate fuel (70% n-heptane/30% toluene), a carbon dioxide effect (CDE) model is established, which considers fuel and ambient gas concentration, density, and temperature influence on autoignition under CO2/O2 atmosphere. Firstly, a chemical model of n-heptane/toluene is established, and the coupling, reduction, and simulation processes are carried out in chemical kinetic software with the IDT as the target parameter. Secondly, a constant volume combustion chamber (CVCC) visualization platform is built by incorporating a high-speed camera system and different working conditions are set in the CO2 volume fraction range (40%-70%) at 3.0 MPa and 850 K for an autoignition experiment. Thirdly, experiment and simulation results are discussed in air, 60% CO2/40% O2, 50% CO2/50% O2, and 40% CO2/60% O2 atmospheres, including the IDT, CO2 effects, temperature sensitivity, and OH radical rate of production (ROP). The results show that the CDE model well predicts the 70% n-heptane/30% toluene IDT under the CO2/O2 atmosphere and the average error in 60% CO2/40% O2 atmosphere is 5.29%. Besides, when the CO2 volume fraction increases from 40% to 60%, the CO2 thermal effect plays a leading role in the IDT prolongation and the OH radical ROP peak of R4 (O+H2O⟶2OH) decreases by 180%.

The adaptation of auto-ignition tabulation for effective use of complex chemical mechanisms will be presented in this paper. Taking cool flame ignition phenomenon into account could improve numerical simulations of combustion in compression ignition engines. Current approaches of successful simulation of this phenomenon are based on the extraction of ignition delay times, heat releases and also reaction rates from tabulated data dependant on four parameters: temperature, pressure, equivalence ratio and exhaust gasses mass fraction. The methods described here were used to create lookup tables including cool flame using a comprehensive chemical mechanism without including reaction rates data (as used by other authors). The method proved to be stable for creating tables and these results will be shown, as well as initial implementation results using the tables in computational fluid dynamics software.

The dynamics of a homogeneous adiabatic autoignition of an ammonia/air mixture at constant volume was studied, using the algorithmic tools of Computational Singular Perturbation. Since ammonia combustion is characterized by both unrealistically long ignition delays and elevated NO x emissions, the time frame of action of the modes that are responsible for ignition was analyzed by calculating the developing time scales throughout the process and by studying their possible relation to NO x emissions. The reactions that support or oppose the explosive time scale were identified, along with the variables that are related the most to the dynamics that drive the system to an explosion. It is shown that reaction H 2 O 2 (+M) → OH + OH (+M) is the one contributing the most to the time scale that characterizes ignition and that its reactant H 2 O 2 is the species related the most to this time scale. These findings suggested that addition of H 2 O 2 in the initial mixture will influence strongly the evolution of the process. It was shown that ignition of pure ammonia advanced as a slow thermal explosion with very limited chemical runaway. The ignition delay could be reduced by more than two orders of magnitude through H 2 O 2 addition, which causes only a minor increase in NO x emissions.

NO formation, which is one of the main disadvantages of ammonia combustion, was studied during the isochoric, adiabatic autoignition of ammonia/air mixtures using the algorithm of Computational Singular Perturbation (CSP). The chemical reactions supporting the action of the mode relating the most to NO were shown to be essentially the ones of the extended Zeldovich mechanism, thus indicating that NO formation is mainly thermal and not due to fuel-bound nitrogen. Because of this, addition of water vapor reduced NO formation, because of its action as a thermal buffer, but increased ignition delay, thus exacerbating the second important caveat of ammonia combustion, which is unrealistically long ignition delay. However, it was also shown that further addition of just 2% molar of H2O2 does not only reduce the ignition delay by a factor of 30, but also reverses the way water vapor affects ignition delay. Specifically, in the ternary mixture NH3/H2O/H2O2, addition of water vapor does not prolong but rather shortens ignition delay because it increases OH radicals. At the same time, the presence of H2O2 does not affect the influence of H2O in suppressing NO generation. In this manner, we were able to show that NH3/H2O/H2O2 mixtures offer a way to use ammonia as carbon-less fuel with acceptable NOx emissions and realistic ignition delay.

The article presents the test results of a four-stroke, four-cylinder, naturally aspirated, DI 60 kW diesel engine operating on diesel fuel (DF) and its 5 vol% (E5), 10 vol% (E10), and 15 vol% (E15) blends with anhydrous (99.8%) ethanol (E). An additional ethanol–diesel–biodiesel blend E15B was prepared by adding the 15 vol% of ethanol and 5 vol% of biodiesel (B) to diesel fuel (80 vol%). The purpose of the research was to examine the influence of the ethanol and RME addition to diesel fuel on the start of injection and autoignition delay. The widely differing physical and chemical properties of the biofuel blends along with engine load and speed modes were taken into account to provide sound analysis of the experimental test results. Studies showed that the density of biofuel blends E5, E10, E15 and E15B was 0.33%, 0.65%, 0.95% and 0.56% lower at the temperature of 40 °C than the corresponding value (0.828 kg/m3) of diesel fuel. Kinematic viscosity of biofuel blends E5, E10, E15 and E15B also decreased by 7.8%, 11.0%, 13.0% and 10.8% at the temperature of 40 °C and the cetane number was 3%, 9%, 14% and 12% lower, respectively, compared to commercial diesel fuel. The use of biofuel blends E15 and E15B the autoignition delay increased by 4.4% and 9.5% compared to normal diesel operation at full pe = 0.67 MPa (100%) load and 1400 rpm speed at which maximum torque occurs.

The main objective of this Thesis is the study of the autoignition phenomenon of reactive mixtures from a theoretical and experimental point of view. A wide parametric study has been carried out in a Rapid Compression-Expansion Machine (RCEM) for different initial temperatures, compression ratios, equivalence ratios and molar fractions of oxygen (by using synthetic EGR) for different fuels. The ignition delay referred to cool flames (if it can be identified), as well as the ignition delay referred to the high-temperature stage of the ignition, have been experimentally obtained and their trends have been explained regarding the chemical kinetics of each fuel. The different effects of the species that compose the synthetic EGR on the ignition delay have been studied, decoupling the thermodynamic effects from the chemical ones. Different compositions have been taken into account to generate the synthetic EGR, and validation limits have been obtained for each mixture. The thermodynamic and the chemical effects have shown to be opposed, while the dominant one is different depending on the working temperature. Several chemical kinetic mechanisms have been validated by comparison to the experimental results. A detailed mechanism for iso-octane and n-heptane blends and a reduced mechanisms for n-dodecane have been analyzed. Moreover, a sub-model for the generation and decay of excited OH* has been validated by comparison to chemiluminescence and spectroscopy results. The different radiation sources have been studied for iso-octane and n-heptane by means of spectroscopy techniques. Besides, chemiluminescence measurements filtered at 310nm (OH* emission wavelength) have been performed in order to analyze the generalization and propagation velocity of the autoignition front. The ignition propagation has shown to depend on the thermodynamic conditions reached in the combustion chamber when the first ignition spot occurs and not on the global reactivity of the mixture. Furthermore, two different radiation sources have been found at 310nm in the spectroscopic analysis depending on the ignition intensity: the decay of the OH* radical from excited to ground state and the oxidation of CO to CO2 (CO continuum). However, these optical techniques have been applied only in the experiments carried out with iso-octane and n-heptane due to technical limitations. Finally, a new predictive model has been theoretically developed starting from the Glassman's model for autoignition. This method is based on modeling the accumulation rate of chain carriers up to reach their critical concentration (obtaining the ignition delay referred to cool flames) and, afterwards, modeling the disappearance rate of such chain carriers up to their consumption (when the maximum heat release rate is reached, obtaining the ignition delay referred to the high-temperature stage of the process). The predictive capability of the model has been compared to the ability of other methods that can be found in the literature, such as the Livengood & Wu integral method. The validity of each method has been tested, defining a working methodology to obtain reasonable predictions for the ignition delay.
El objetivo de esta Tesis Doctoral es el estudio del fenómeno de autoencendido de mezclas reactivas desde un punto de vista teórico y experimental. Se ha realizado un amplio estudio paramétrico en una Máquina de Compresión-Expansión Rápida (RCEM por sus siglas en inglés) barriendo diversas temperaturas iniciales, relaciones de compresión, dosados relativos y fracciones molares de oxígeno (mediante el uso de EGR sintético) para distintos combustibles. El tiempo de retraso del fenómeno de llamas frías (en el caso de existir), así como el tiempo de retraso de la etapa de alta temperatura, han sido obtenidos experimentalmente y sus tendencias explicadas mediante cinética química. Se han estudiado los diferentes efectos de las distintas especies involucradas en el EGR sintético sobre el tiempo de retraso, desligando aquellos de carácter termodinámico de los efectos puramente químicos. Se han tenido en cuenta distintas composiciones para definir dicho EGR, estableciendo límites de validez para cada una de las mezclas propuestas. Los efectos termodinámicos y químicos resultaron ser opuestos, siendo dominante uno u otro a distintos rangos de temperatura de trabajo. Varios mecanismos de cinética química han sido validados gracias a los resultados experimentales obtenidos. Además de un mecanismo detallado para mezclas PRF de iso-octano y n-heptano, se ha llevado a cabo la validación de otro mecanismo simplificado para el n-dodecano. Por otro lado, un submodelo de formación y decaimiento de OH* excitado ha sido validado contra resultados de quimioluminiscencia y espectroscopía. Se han estudiado las diferentes fuentes de radiación del proceso de autoencendido para el iso-octano y el n-heptano mediante técnicas de espectroscopía. Además, se han realizado medidas de quimioluminiscencia filtrada a 310nm (longitud de onda de emisión del radical OH*) para el análisis de la generalización y velocidad de propagación del frente de autoencendido. La propagación del encendido ha mostrado ser dependiente de las condiciones termodinámicas alcanzadas en la cámara de combustión en el instante de ignición más que de la reactividad de la mezcla. Se han encontrado dos fuentes de radiación distintas a 310nm mediante espectroscopía, dependiendo de la intensidad del encendido: el decaimiento del radical OH* de estado excitado a estado natural y la oxidación del CO a CO2 (continuo del CO). No obstante, estas técnicas han sido utilizadas solamente para los dos combustibles de referencia de la escala de octanaje debido a limitaciones técnicas. Finalmente, se ha desarrollado un nuevo modelo predictivo de manera teórica partiendo del modelo de Glassman para el autoencendido. Este método se basa en modelar primero la tasa de acumulación de portadores de cadena hasta su concentración crítica (obteniendo así el tiempo de retraso referido a la etapa de llamas frías) y, tras dicho instante, modelar la tasa de consumo de dichos portadores de cadena hasta su completa desaparición (instante en el cual se produce la máxima exotermia del proceso, prediciendo el tiempo de retraso referido a la etapa de alta temperatura del encendido). La capacidad predictiva del modelo ha sido comprobada para cada uno de los seis combustibles ensayados. Además, dicha capacidad predictiva ha sido comparada con la de otros métodos existentes en la literatura, como la integral de Livengood & Wu. La validez de cada uno de los métodos ha sido analizada, definiendo una metodología de uso para obtener predicciones razonables del tiempo de retraso.
L'objectiu d'aquesta Tesi Doctoral és l'estudi del fenomen d'autoencesa de mescles reactives des d'un punt de vista teòric i experimental. S'ha realitzat un ampli estudi paramètric en una Màquina de Compressió-Expansió Ràpida (RCEM per les seues sigles en anglès) cobrint diverses temperatures inicials, relacions de compressió, dosatges relatius i fraccions molars d'oxigen (mitjançant l'ús de EGR sintètic) per a diferents combustibles. El temps de retard del fenomen de flames fredes (en el cas d'existir), així com el temps de retard de l'etapa d'alta temperatura, han sigut obtinguts experimentalment i les seues tendències explicades mitjançant cinètica química. S'han estudiat els diferents efectes de les diferents espècies involucrades en l'EGR sintètic sobre el temps de retard, deslligant aquells de caràcter termodinàmic dels efectes purament químics. S'han tingut en compte diferents composicions per a definir aquest EGR, establint límits de validesa per a cadascuna de les mescles proposades. Els efectes termodinàmics i químics van resultar ser oposats, sent dominant un o un altre a diferents rangs de temperatura de treball. Diversos mecanismes de cinètica química han sigut validats gràcies als resultats experimentals obtinguts. A més d'un mecanisme detallat per a mescles PRF d'iso-octà i n-heptà, s'ha dut a terme la validació d'un altre mecanisme simplificat per al n-dodecà. D'altra banda, un submodel de formació i decaïment d'OH* excitat ha sigut validat contra resultats de quimioluminescència i espectroscopía. S'han estudiat les diferents fonts de radiació del procés d'autoencesa per a l'iso-octà i l'n-heptà mitjançant tècniques d'espectroscopía. A més, s'han realitzat mesures de quimioluminescència filtrada a 310nm (longitud d'ona d'emissió del radical OH*) per a l'anàlisi de la generalització i velocitat de propagació del front d'autoencesa. La propagació de l'encesa ha mostrat ser depenent de les condicions termodinàmiques aconseguides en la cambra de combustió en l'instant d'ignició més que de la reactivitat de la mescla. S'han trobat dues fonts de radiació diferents a 310nm mitjançant espectroscopía, depenent de la intensitat de l'encesa: el decaïment del radical OH* d'estat excitat a estat natural i l'oxidació del CO a CO2 (continu del CO). No obstant açò, aquestes tècniques han sigut utilitzades solament per als dos combustibles de referència de l'escala de octanaje a causa de limitacions tècniques. Finalment, s'ha desenvolupat un nou model predictiu de manera teòrica partint del model de Glassman per a l'autoencesa. Aquest mètode es basa a modelar primer la taxa d'acumulació de portadors de cadena fins a la seua concentració crítica (obtenint així el temps de retard referit a l'etapa de flames fredes) i, després d'aquest instant, modelar la taxa de consum d'aquests portadors de cadena fins a la seua completa desaparició (instant en el qual es produeix la màxima exotermia del procés, predient el temps de retard referit a l'etapa d'alta temperatura de l'encesa). La capacitat predictiva del model ha sigut comprovada per a cadascun dels sis combustibles assajats. A més, aquesta capacitat predictiva ha sigut comparada amb la d'altres mètodes existents en la literatura, com la integral de Livengood & Wu. La validesa de cadascun dels mètodes ha sigut analitzada, definint una metodologia d'ús per a obtenir prediccions raonables del temps de retard.
López Pintor, D. (2017). Theoretical and experimental study on the autoignition phenomena of homogeneous reactive mixtures [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/90642
TESIS

Detailed investigation of the vaporization of an isolated of ethanol and 1-propanol droplet was carried out in this experimental study. The experimental set-up consists of a heated chamber with a cross quartz fibers configuration as droplet support. An alcohol droplet is located at the intersection of the cross quartz fibre with a controlled initial diameter (300-600µm). Ambient temperature is varied from 298 to 973K at atmospheric pressure. The quasi-steady theory has been used to compare and to explain all experimental results. The real impact of the water concentration on the vaporization rate of an ethanol droplet is also examined, where two 'quasi-steady' periods are observed on the d2-curves, clearly showing that the vaporization of an ethanol droplet is accompanied by the simultaneous condensation of water vapour on the droplet surface and thus the temporal evolution of the droplet squared diameter exhibits an unsteady behavior. The histories of the instantaneous vaporization rates of both 1-propanol and ethanol droplets confirm this phenomenon. The autoignition experimental study of ethanol, 1-propanol and blends of ethanol and water have been carried out in a rapid compression machine at a compressed pressure of 30bar over a temperature range of 750-860K for stoichiometric mixture of fuel and air. The ignition delay times recorded show a significant decrease with increasing temperature. 1-propanol is more reactive than ethanol, which results in shorter ignition delay times. However, water addition to ethanol increases the reactivity of the mixture and results in a shorter ignition delay times than 1-propanol

Biodiesel is an alternative to diesel fuel that can reduce life-cycle greenhouse emissions. It is made from plant oils or waste animal fat. Changing an engine's fuel from #2 diesel to biodiesel does not require modifications to the engine, and has typically reduced emissions of particulate matter (PM) but increased nitrogen oxides (NOₓ). However, government regulations tightly limit exhaust pollutants, hence the increase is problematic. In order to investigate the relationship between biodiesel fuel, ignition delay and emissions, experiments with a compression ignition, direct injection (CIDI) engine were performed. The emissions, ignition delays and apparent heat release rates from four locally produced biodiesel fuels and #2 diesel were measured. Emissions of NOₓ were greater on average from the biodiesel fuels than the #2 diesel. Emissions of PM were not consistent, and measurements of ignition delay were the opposite of expectations. It was postulated that the fuel was undergoing atypical combustion, but this could not be confirmed with the limited data available from this investigation technique. The engine experiments exemplified the need for alternative investigation techniques and numerical simulation of CIDI combustion was the leading choice. This technique was already well developed, however chemical ignition delay models for #2 diesel had received little validation and no data existed to generate a biodiesel chemical ignition delay model. The novel drop method was conceived to measure the chemical ignition delay of heavy fuels at CIDI combustion relevant conditions and a shock tube was designed and built to use this method. It combusted fuel in a heterogeneous environment and minimised pre-experiment reactions. It was validated with n-dodecane, which indicated an assumed equivalence ratio at the ignition point was required. It then produced the first measurements of the chemical ignition delay of methyl oleate (a biodiesel surrogate) and #2 diesel at CIDI relevant conditions without pre-experimental reactions. Methyl oleate generated shorter chemical ignition delays than diesel and displayed NTC behaviour. The drop method shows strong potential for future measurements of the chemical ignition delay of many different heavy fuels.
Thesis (Ph.D.) -- University of Adelaide, School of Mechanical Engineering, 2013

Population growth and increasing standards of living have resulted in a rapid demand for energy. Our primary energy production is still dominated by fossil fuels. This extensive usage of fossil fuels has led to global warming, environmental pollution, as well as the depletion of hydrocarbon resources. The prevailing difficult situation offers not only a challenge but also an opportunity to search for alternatives to fossil fuels. Hence, there is an urgent need to explore environmentally friendly and cost-effective renewable energy sources. Oxygenates (alcohols, ethers) and ammonia are among the potential renewable alternative fuels of the future. This thesis investigates the combustion characteristics of promising alternative fuels and their blends using a combination of experimental and modelling methodologies. The studied fuels include ethanol, diethyl ether, dimethyl ether, dimethoxy methane, γ-valerolactone, cyclopentanone, and ammonia. For the results presented in this thesis, the studies may be classified into three main categories: 1. Ignition delay time measurements of ethanol and its blends by using a rapid compression machine and a shock tube. The blends studied include binary mixtures of ethanol/diethyl ether and ternary mixtures of ethanol/diethyl ether/ethyl levulinate. A chemical kinetic model has been constructed and validated over a wide range of experimental conditions. The results showed that a high-reactivity fuel, diethyl ether, may be blended with a low-reactivity fuel, ethanol, in varying concentrations to achieve the desired combustion characteristics. A ternary blend of ethanol/diethyl ether/ethyl levulinate may be formulated from a single production stream, and this blend is shown to behave similarly to a conventional gasoline. 2. Ignition delay time and flame speed measurements of ammonia blended with combustion promoters by utilizing a rapid compression machine and a constant volume spherical reactor. The extremely low reactivity of ammonia makes it unsuitable for direct use in many combustion systems. One of the potential strategies to utilize ammonia is to blend it with a combustion promoter. In this work, dimethyl ether, diethyl ether, and dimethoxy methane are explored as potential promoters of ammonia combustion. Chemical kinetic models were developed and validated in the high temperature regime by using flame speed data and in the low-to-intermediate temperature regime by using ignition delay time data. The results showed that even a small addition (~ 5 – 10%) of combustion promoters can significantly alter ammonia combustion, and diethyl ether was found to have the highest propensity to enhance ammonia ignition and flame propagation. Blends of combustion promoters with ammonia can thus be utilized in modern downsized turbo-charged engines. 3. Octane boosting and emissions minimization effects of next generation oxygenated biofuels. These studies were carried out using a cooperative fuel research engine operating in a homogenous charge compression ignition (HCCI) mode. The oxygenated fuels considered here include γ-valerolactone and cyclopentanone. The results showed that γ-valerolactone and cyclopentanone can be effective additives for octane boosting and emission reduction of conventional fuels. Overall, the results and outcomes of this thesis will be highly useful in choosing and optimizing alternative fuels for future transportation systems.

The autoignition properties of gas turbine fuels have been studied for many years and by numerous researchers. The advent of ultra low emission industrial gas turbines using lean premixed technologies has given rise to premixer designs with longer residence times. This, in conjunction with the ever-increasing pressure ratios of aeroderivative machines, leads to the potential for autoignition within premix ducts, and has therefore renewed the interest in this field. Although much has been published, data in the region of interest to high pressure ratio gas turbines is extremely sparse. Similarly, modelled autoignition delay times are not very accurate, as most reaction mechanisms were not generated to cover this range of conditions. Hence the uncertainties of autoignition delay times at gas turbine conditions are significant, thereby either imposing over-stringent design limitations or introducing risks of ignition occurrence in the early design process. A series of experiments have been carried out for methane and simulated natural gas fuels in the region of interest, using shock tubes as the test vehicle. The experimental technique was chosen to isolate only the chemical kinetic component of the autoignition delay time, without any additional delays due to mixing and heating of the test gases. Predictive correlations and a chemical kinetic model (the GRI mechanism) have also been used to predict autoignition delay times at the same conditions. The correlation between experiment and prediction has been shown to be poor at representative temperatures. This paper discusses some of the possible explanations for this poor agreement.

The influence of fuel autoignition chemistry is known to be relevant when approaching the limits of lean blowout and lean ignition in a continuous combustion environment. This was investigated by employing four reference fuels having very different autoignition delay profiles but similar boiling points to interrogate various test environments and thereby to assess the relevance of the differences in autoignition chemistry. A combustion bomb apparatus was used to characterize the reference fuels together with a sample of commercial Jet A-1 for comparison. The measurements were cross-checked using a chemical kinetic simulation model. A continuous combustion rig was used to study the threshold ignition and blowout performance of the pre-vaporized reference fuels and a laminar flame speed bomb was used to study the influence of autoignition chemistry on normal, stoichiometric combustion and normal ignition conditions. In all the experiments, the results reflected the distinctive differences of the test fuels in terms of their autoignition delay timescales. The findings were interpreted against the background of the commercial jet fuel autoignition chemistry and the relevance of traditional autoignition delay metrics such as Octane or Cetane rating. Notwithstanding the influence of fuel evaporation and mixing timescales which can exert an overriding influence in a practical, gas turbine application, it was concluded that the fuel’s autoignition delay timescale also plays a very significant role in threshold operational situations.

Progress on development and validation of detailed chemical kinetic mechanisms for the U.S. Air Force JP-8 and JP-7 fuels [1] is reported in this article. Two JP-8 surrogate fuel blends were considered. The first JP-8 surrogate blend contained 12 pure hydrocarbon components, which were 15% n-C10H22, 20% n-C12H26, 15% n-C14H30, 10% n-C16H34, 5% i-C8H18, 5% C7H14, 5% C8H16, 5% C8H10, 5% C10H14, 5% C9H12, 5% C10H12 and 5% C11H10 by weight. The second JP-8 surrogate blend contained 4 components, which were 45% n-C12H26, 20% n-C10H22, 25% C10H14, and 10% C7H14 by weight. A five-component surrogate blend for JP-7 was also considered. The JP-7 surrogate blend components were 30% n-C10H22, 30% n-C12H26, 30% C10H20, 5% i-C8H18, and 5% C7H8 by weight. The current status of the JP-8 and JP-7 mechanisms is that they consist of 221 species and 1483 reactions and 205 species and 1438 reactions respectively. Both JP-8 and JP-7 mechanisms were evaluated using a lean fuel-air mixture, over a temperature range of 900–1050 K and for atmospheric pressure conditions by predicting autoignition delay times and comparing them to the available experimental data for Jet-A fuel. The comparisons demonstrated the ability of the 12-component JP-8 surrogate fuel blend to predict the autoignition delay times over a wider range of temperatures than the 4-component JP-8 surrogate fuel blend. The 5-component JP-7 surrogate blend predicted autoignition delay times lower than those of JP-8 blends and Jet-A fuel. The JP-8 and JP-7 mechanisms predictions, however, showed less agreement with the measurements towards the lower end of the temperature range (i.e., less than 900 K). Therefore, low temperature oxidation reactions and the sensitivities of the autoignition delays to reaction rate constants are still needed.

Abstract In recent years, hydrogen-carrying compounds have accrued interest as an alternative to traditional fossil fuels due to their function as zero-emission fuels. As such, there is interest in investigating hydrogen-carrying compounds to improve understanding of the fuels’ characteristics for use in high-pressure systems. In the current study, the oxidation of ammonia/natural gas/hydrogen mixtures was carried out to study CO formation profiles as well as the ignition delay times behind reflected shock waves in order to refine chemical kinetic models. Experiments were carried out in the University of Central Florida’s shock tube facility by utilizing chemiluminescence to obtain OH* emission and laser absorption spectroscopy to obtain CO profiles. Experimental results were then compared with the GRI 3.0 mechanism, as well as the proprietary UCF 2022 mechanism utilizing CHEMKIN-Pro software. In general, both models were able to capture the trend in autoignition delay times and CO time histories for natural gas and ammonia mixtures. However, for ammonia-hydrogen mixtures, GRI 3.0 failed to predict ignition delay times whereas the UCF 2022 mechanism was able to capture the IDTs within the uncertainty limits of the experiments. A sensitivity analysis was conducted for different mixtures to understand the important reactions at the experimental conditions. Finally, a reaction pathway analysis was carried out to understand important ammonia decomposition pathways in the presence of hydrogen and natural gas.

Abstract To evaluate the effect of exhaust gas recirculation (EGR) and variable fuel reactivity on knock and misfire in spark ignited national gas engines, experiments were conducted in a rapid compression machine to measure homogeneous ignition delay, flame propagation rate, and end-gas autoignition fraction for stoichiometric natural gas/oxidizer/EGR blends. Natural gas with a range of chemical reactivity was simulated using mixtures of CH4, C2H6, and C3H8. Reactive exhaust gas recirculation (R-EGR) gases were simulated with mixtures of Ar, CO2, CO, and NO and non-reactive exhaust gas recirculation gases (NR-EGR) were simulated with mixtures of AR and CO2. Homogeneous ignition delay period, flame propagation rate and end-gas autoignition fraction were measured at compressed pressures and temperatures of 30.2 to 34.0 bar and 667 to 980 K, respectively. Flame propagation rate decreased with both R-EGR and NR-EGR substitution. The substitution of R-EGR increased the end-gas autoignition fraction, whereas NR-EGR substitution decreased the end-gas autoignition fraction. The results indicate that the presence of the reactive species NO in the R-EGR has a strong impact on end-gas autoignition fraction. An 82-species reduced chemical kinetic mechanism was also developed that reproduces measured homogeneous ignition delay period with a total average relative error of 11.0%.

Abstract The minimization of autoignition risk is critical to the design of premixers of high power aeroderivative gas turbines as an increased use of highly reactive future fuels (for example, hydrogen or higher hydrocarbons) is anticipated. Safety factors based on ignition delays of homogeneous mixtures, are generally used to guide the choice of a residence time for a given premixer. However, autoignition chemistry at aeroderivative conditions is fast (0.5–2 milliseconds) and can be initiated within typical premixer residence times. The analysis of what takes place in this short period necessarily involves the study of low-temperature autoignition precursor chemistry, but precursors can change with fuel and local reactivity. Chemical Explosive Modes are a natural alternative to study this as they can provide a measure of autoignition risk by considering the whole thermochemical state in the framework of an eigenvalue problem. When transport effects are included by coupling the evolution of the Chemical Explosive Modes to turbulence, it is possible to obtain a measure of spatial autoignition risk where both chemical (e.g. ignition delay) and aerodynamic (e.g. local residence time) influences are unified. In this article, we describe a method that couples Large Eddy Simulation to newly developed, reduced autoignition chemical kinetics to study autoignition precursors in an example pre-mixer representative of real life geometric complexity. A blend of pure methane and dimethyl ether (DME), a common fuel used for experimental autoignition studies, was transported using the reduced mechanism (38 species / 238 reactions) at engine conditions at increasing levels of DME concentration until exothermic autoignition kernels were formed. The resolution of species profiles was ensured by using a thickened flame model where dynamic thickening was carried out with a flame sensor modified to work with multi-stage heat release. The paper is outlined as follows: First, a reduced mechanism is constructed and validated for modeling methane as well as di-methyl ether (DME) autoignition. Second, sensitivity analysis is used to show the need for Chemical Explosive Modes. Third, the thickened flame model modifications are described and then applied to an example premixer at 25 bar / 890K preheat. The Chemical Explosive Mode analysis closely follows the large thermochemical changes in the premixer as a function of DME concentration and identifies where the premixer is sensitive and flame anchoring is likely to occur.

The objective of this work was the improvement of methods for predicting autoignition in turbulent flows of different natural gas mixtures and air. Measurements were performed in a mixing duct where fuel was laterally injected into a turbulent flow of preheated and pressurized air. To study the influence of higher order hydrocarbons on autoignition, natural gas was mixed with propane up to 20% by volume at pressures up to 15 bar. During a measurement cycle, the air temperature was increased until autoignition occurred. The ignition process was observed by high-speed imaging of the flame chemiluminescence. In order to attribute a residence time (ignition delay time) to the locations where autoignition was detected the flow field and its turbulent fluctuations were simulated by numerical codes. These residence times were compared to calculated ignition delay times using detailed chemical simulations. The measurement system and data evaluation procedure are described and preliminary results are presented. An increase in pressure and in fraction of propane in the natural gas both reduced the ignition delay time. The measured ignition delay times were systematically longer than the predicted ones for temperatures above 950 K. The results are important for the design process of gas turbine combustors and the studies also demonstrate a procedure for the validation of design tools under relevant conditions.

Abstract Liquid fueled combustors are commonly used in the gas turbine industry in situations such as high temperature fuel mixing ducts, liquid fueled reheat combustors, and other high temperature liquid fueled combustors. Modern combustors operate at high inlet temperatures, increasing the likelihood of autoignition events. Autoignition is primarily characterized using a single-step Arrhenius rate equation. Generally, this method is ideal for modeling the chemical processes involved in simplistic settings such as for analyzing ignition delays with premixed reactive mixtures in shocktubes, however it may not fully encapsulate the underlying physio-chemical processes involved in the presence of a multi-phase flow which can significantly affect the chemical processes such as autoignition. These conditions are often encountered in reality, for example, in a gas turbine combustor using fuel sprays where interactive phenomena such as fuel droplet evaporation, mixing, and chemical reactions may occur simultaneously and non-homogeneously. The results presented in this report begin to elucidate the role of droplets in determining the behavior of autoignition kernels with an attempt to improve our capability to predict autoignition phenomena in liquid fuel injector application in gas turbine industry. To investigate the autoignition phenomena in a multi-phase flow inside a gas turbine combustor, a simplified co-flow type geometry is considered at atmospheric pressure where a single Jet-A fuel spray enters the co-flowing high temperature vitiated products of a pilot burner. Fuel is injected using an aerodynamically shaped pressure-swirl atomizing injector installed co-axially with the flow inside an optically accessible quartz test section. The air temperatures and oxygen content of the flow can range from 950–1300K and 9–11%, respectively. It has previously been found that while average ignition delay times agree or nearly agree with prior theoretical and experimental studies (eg. for prevaporized fuel, electrically heated), high speed imaging experiments illustrate that the spatial location of the formed kernels can be broadly scattered. Also, this variation in autoignition kernel location is higher at lower temperatures. Simultaneous high speed CH and OH chemiluminescence also suggest that the kernels are formed at lower equivalence ratios at lower preheat temperatures and then proceed to increase in equivalence ratio. While at higher preheat temperatures, kernels form at a higher equivalence ratio and stay at the ratio as they propagate downstream. In the current study, a 5000fps, 283nm laser sheet is introduced along the center axis of the test section. Two synchronized, intensified, high-speed cameras simultaneously captured the fluorescence of Jet-A and OH chemical reaction at 308nm and the Mie scattering of droplets at 283nm. Autoignition kernels and that droplets are visualized at flow velocities ranging from 40–50 m/s and temperatures ranging from 1100–1300K. This technique allows the fuel and reaction fluorescence to be differentiated and from this image, information is obtained on the proximity of fuel droplets and autoignition kernels during their formation and subsequent propagation.

Abstract Characterisation of autoignition risk is crucial for designing and optimising low-emission combustion systems as there is an increased demand for highly reactive and novel fuel mixtures. Achieving a residence time to prevent autoignition and obtaining an adequate mixing quality is a challenging trade-off for these fuels in lean-premixed combustion systems. The level of complexity increases further due to low-temperature chemical pathways and pressure-dependent reactions that strongly influence ignition delay at engine operating conditions. Detailed chemical kinetic mechanisms with hundreds of species and thousands of reactions are developed and employed to address this complexity and predict ignition delay accurately, especially for heavier hydrocarbons. However, direct implementation of these kinetic mechanisms is computationally prohibitive in high-fidelity CFD approaches such as large eddy simulation (LES) and stochastic simulation tools that require a large number of evaluations. Advanced stochastic methods are essential tools to quantify uncertainties due to the inherent variabilities in ambient, operating conditions and fuel composition on ignition delay time calculation for practical applications. This study introduces and implements a computationally efficient method based on metamodellig to predict ignition delay time over a wide range of operating conditions and fuel compositions for gas turbine combustion systems. A metamodel or surrogate model is an accurate and quick approximation of the original computational model based on a detailed chemical kinetic mechanism. Polynomial chaos expansion (PCE) as an advanced method is employed to build metamodels using a limited set of runs of the original ignition delay time model based on NUIGMech1.0 chemical kinetic mechanism as the most detailed and state-of-the-art chemical kinetic mechanism for natural gas. Developed metamodels for ignition delay time are valid over operating conditions of P = 20–40 bar and T = 700–900 K for natural gas containing C1 to C7 hydrocarbons at stoichiometric condition. These metamodels provide a fast, robust, and considerably accurate framework instead of a detailed chemical kinetic model that facilitates (a) characterising ignition delay time at different operating conditions and fuel compositions, (b) designing and optimising premixers and burners and (c) conducting uncertainty quantification and stochastic modelling studies.

In any theoretical investigation of ignition processes in natural gas reciprocating engines, physical and chemical mechanisms must be adequately modeled and validated in an independent manner. The Rapid Compression Machine (RCM) has been used in the past as a tool to validate both autoignition models as well as turbulent mixing effects. In this study, two experimental cases were examined. In the first experimental case, the experimental measurements of Lee and Hochgreb (1998a) were chosen to validate the simulation results. In their experiments, hydrogen/oxygen/argon mixtures were used as reactants. In the simulations, a reduced chemical kinetic mechanism consisting of 10 species and 19 elementary reactions coupled to a CFD software, Fluent 6, was used to simulate the autoignition. The ignition delay from the simulation agreed very well with that from the experimental data of Lee and Hochgreb, (1998b). In the second case, experimental data derived from an RCM with two opposed, pneumatically driven pistons (Brett et al., 2001) were used to study the autoignition of methane/oxygen/argon mixtures. The reduced chemical kinetic mechanism DRM22, derived from the GRI-Mech reaction scheme coupled to Fluent 6, was applied in the simulations. The DRM22 scheme included 22 species and 104 reactions. When methane/oxygen/argon mixture were simulated for the RCM, the ignition delay deviated about 15% from the experimental results. The simulation approaches as well as the validation results are discussed in detail in this paper. The paper also discusses an evaluation of reduced reaction models available in the literature for subsequent Fluent modeling.