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It is well known that the high spontaneous ignition risk of pressurized hydrogen poses a serious threat to hydrogen storage safety. Aiming to reveal the inhibition mechanism of the fuel-blending st...

Due to the increasing attention of low temperature combustion, more precise and convenient low temperature chemical reaction mechanism is needed in the combustion reaction flow modelling. However, ...

As a novel alternative fuel and surrogate component, 2,6,10-trimethyl dodecane has received extensive attention. In order to provide the promise of designing and optimizing the internal combustion engines and propulsion systems by CFD, and provide more choice for branched surrogate component to develop more accurate surrogate model, a comprehensive detailed chemical kinetic model for 2,6,10-trimethyl dodecane has been developed based on 35 reaction classes to numerically describe its experimental observations. The proposed detailed mechanism for 2,6,10-trimethyl dodecane has been validated against with a wide range of experimental data which including ignition delay time, flow reactor and laminar flame speeds. The good agreement between the numerical and the experimental data is observed. Using the kinetic model reduction scheme, a high-temperature and a low-temperature chemical mechanisms were eventually obtained and validated against the detailed mechanism. The successful implementation of kinetic mecha...

As one of the most promising alternative fuel to diesel engines, dimethyl ether plays a significant role in improving combustion efficiency and decreasing emissions, and an in-depth understanding o...

Hydrogen addition is widely used to improve the combustion performance of single-component fuel. In this study, the effects of hydrogen addition on non-premixed ignition of iso-octane by hot air in a diffusion layer were examined and interpreted numerically. Detailed chemistry and transport were considered in simulation. The non-premixed ignition delay times at different hydrogen blending levels were obtained and analyzed. It was found that hydrogen addition greatly reduces the ignition delay. This is mainly due to the fact that the preferential mass diffusion of hydrogen over iso-octane significantly increases the local hydrogen blending level at the ignition kernel. Besides, for the non-premixed ignition process, two modes of reaction front propagation were identified through the analysis based on Damkohler number and consumption speeds. One is the reaction-driven mode characterized by local or sequential homogeneous autoignition; and the other is the diffusion-driven mode, which depends on the balance of mass diffusion, heat transfer and chemical reaction. These two modes lead to different ignition behaviors. For pure iso-octane with low mass diffusivity, ignition is mainly caused by local homogeneous reaction occurring at the most reactive position. With the increase of diffusion layer thickness, the local temperature at the most reactive position increases and therefore the non-premixed ignition delay time of pure iso-octane decreases. However, when hydrogen with high mass diffusivity is added into iso-octane, the non-premixed ignition is controlled by fuel diffusion. With the increase of diffusion layer thickness, the concentration gradient becomes smaller and thereby less hydrogen diffuses into the ignition kernel. Consequently, unlike pure iso-octane, the non-premixed ignition delay time of hydrogen/iso-octane blends increases with the diffusion layer thickness.

A surrogate formulation methodology is proposed by directly using functional groups CH3, CH2, CH,C and phenyl to build the surrogate models for the FACE (fuels for advanced combustion engines) A an...

A comprehensive three-component surrogate for emulating the physical and combustion characteristics of three types of jet fuel, namely, S-8, Jet-A, and RP-3, has been developed by the methodology o...

Thermoelectric generator (TEG) is a promising technology for waste heat recovery. Typical TEG module has a multi-material-layers sandwiched structure, in which the inherent inner-connection-and-contact (ICC) effects and side-surface heat loss may significantly degrade TEG performance. Therefore, quantitative assessment and analysis of the ICC effects and side-surface heat loss simultaneously are crucial for the optimization design of a TEG module. However, existing researches usually emphasize on the two factors separately and qualitatively. In this work, the whole ICC effects of interconnectors, solders and contact surfaces are equivalent to extra assumed ICC layers at both ends of thermoelement for the first time, and based on that a three-dimensional numerical model is established in COMSOL Multiphysics software with simultaneously considering the side-surface heat transfer coefficient ha of thermoelement. The ICC effects of a TEG module are identified by using BOBYQA optimization algorithm in COMSOL to match experimental data. It shows that, as ha increases, the recognized ICC thermal and electric resistances present linear opposite change; when ha > 12 Wm−2K−1, non-physical negative ICC electric resistance emerges. Then, based on the identified ICC effects and reasonable ha (≈5 Wm−2K−1), the results between present model and analytical theory are compared and discussed under the condition of fixed fill factor. Furthermore, multi-parameters optimization of the TEG is carried out for maximum output power, efficiency, power/cost ratio and their equally weighted multi-objective, respectively. The results show that (1) the optimal parameters such as thermoelement length Lopt and fill factor Fopt vary significantly depending on different optimization objectives; (2) the ICC effects can approximately double the Lopt and Fopt corresponding to maximum efficiency or power/cost ratio, while the ICC effects show negligible influence on the Fopt corresponding to maximum output power or equally weighted multi-objective.

Cool flame due to low-temperature chemistry (LTC) has received great attention recently. However, previous studies mainly focused on cool flames in homogenous systems without transport or non-premixed cool flames in droplet combustion or counterflow configuration. There are only a few studies on premixed cool flames, and the transient initiation and propagation of premixed cool flames are still not well understood. In this study, the initiation, propagation and disappearance of one-dimensional premixed cool flames in dimethyl ether (DME)/air mixture is investigated through transient simulation considering detailed chemistry and transport. The premixed cool flame governed by LTC can be initiated by a hot spot. When the hot spot temperature is not high enough to directly trigger the high-temperature chemistry (HTC), only the LTC reactions take place initially and thereby a cool flame is first initiated. During the cool flame propagation, HTC autoignition occurs at the hot spot and it induces a hot flame propagating behind the cool flame. Therefore, double-flame structure for the coexistance of premixed cool and hot flames is observed. Since the hot flame propagates much faster than the cool flame, it eventually catches up and merges with the leading cool flame. A well-defined cool flame speed is found in this study. We inverstigate different factors affecting the cool flame speed and the appearance of hot flame. It is found that at higher equivalence ratio, higher initial temperature or higher oxygen concentration, the premixed cool flame propagates faster and the hot flame appears earlier. Three chemical mechanisms for DME oxidation are considered. Though these three mechanisms have nearly the same prediction of hot flame propagation speed, there are very large discrepancy in the prediction of cool flame propagation speed. Therefore, experimental data of premixed cool flame speed are useful for developing LTC.

Flexible thermoelectric generator became an attractive technology for its wide use especially for curved surfaces applications. This study proposes design of a flexible thermoelectric generator, which is part of a sensor and supplies required electrical power for human body application. The thermoelectric generator module has ink-based thermoelements which are made of nano-carbon bismuth telluride materials. Flexible fins conduct the body heat to the thermoelectric uni-couples, extended fins exchange the heat from the cold side of the thermoelectric generator to the ambient. A fully developed one-dimensional steady-state numerical model including temperature-dependent thermoelectric properties of different materials is built to reveal basic characteristics and optimal design criteria of the thermoelectric generator. Results show that ambient temperature presents significant influence on thermoelectric generator performance, but thermal resistance from blood to skin surface and contact thermal resistance at skin surface has negligible influence. A very low air velocity, e.g. 0.01 m/s is enough to ensure a considerable temperature difference through the thermoelectric element. Increasing thermoelectric elements thickness and thermoelectric module row number in a proper range can significantly enhance thermoelectric generator performance. The maximum output power can reach 0.2 μW/cm2, which indicates the proposed design is promising for supplying human body sensors. In addition, the basic optimal design criteria of the flexible thermoelectric generator and its relative merits are discussed and presented.

Compressed air energy storage is a promising large-scale energy storage technology. Integrating ejectors in the energy-release stage of compressed air energy storage systems is widely recognized as an effective way to improving system efficiency; however, there is a lack of detailed modelling and analysis regarding the optimal working parameters of ejectors. In this study, the thermodynamic models of a 10 MW thermal-storage compressed air energy storage system with or without an ejector (system I and system II, respectively) are established under constant-pressure operation. A one-dimensional semi-empirical model is used to determine the maximum entrainment ratio of the ejector under specific conditions of motive air pressure, a low-pressure air source, and constant-pressure operation. The results show that (1) in system I, the maximum entrainment ratio is positively correlated with motive air pressure, while the total energy-release time, the total amount of entrained low-pressure air, and the total energy-release work present parabolic-like variations as motive air pressure increases; (2) different low-pressure air sources change these performance parameters significantly; (3) compared between systems I and II, the rise amplitude of round-trip efficiency and the profit are positively correlated with motive air pressure for most constant-pressure operations; (4) the optimal rise amplitude of round-trip efficiency and profit of the ejector can reach around 10% and 275$, respectively. In addition, two methods to determine the optimal low-pressure air source for the ejector are proposed for real and design-stage compressed air energy storage systems, while three principles to determine the optimal motive air pressure of the ejector are proposed through a comprehensive analysis of performance parameters.

The kinetics and soot and NO x emission characteristics of the CH 4 /dimethyl ether (DME) jet diffusion flames (JDFs) are studied by experiments and simulations with a detailed chemical mechanism. The results showed that decomposition of DME in the pyrolysis zone generated massive CH 3 , which changed the local flame structure and soot-correlated chemistry to some extent. Due to reductions of the incipient species concentrations including benzene (A1), pyrene (A4), C 3 H 3 , and C 2 H 2 , soot loading of the CH 4 JDF decreased by reducing margins with DME addition. A1 and thus soot formation rates due to DME addition were most sensitive to the recombination reaction of C 3 H 3 (C 3 H 3 + C 3 H 3 = A1). With respect to the CH 4 /DME JDFs, NO x was emitted mainly through the thermal and prompt pathways. The thermally-generated EI NOx increased exponentially with DME addition because of the increasing enhancement of OH concentration in the radical pool. By contrast, the promptly-generated EI NOx decreased in reducing margins with DME addition because of the reducing decrease in CH concentration. The synergistic effect of DME addition on the total NO x emission, i.e. the overall EI NOx decreased firstly and then increased with DME addition, was examined in this paper. Additionally, it is reported that the 40%CH 4 /60%DME case was comprehensively optimal in terms of soot and NO x emission reductions.

Water vapor dilution has great impact on fundamental combustion processes such as ignition, flame propagation and extinction. In the literature, there are many studies on how water vapor addition affects flame propagation and extinction limit. However, the influence of water vapor addition on ignition receives little attention. In this study, numerical simulations considering detailed chemical mechanisms are conducted for the ignition of methane, n -butane and n -decane/air/water vapor mixtures. The emphasis is spent on examining the effects of water vapor dilution on the ignition of these fuels at normal and reduced pressures. The minimum ignition energies (MIE) at different dilution ratios and initial pressures are obtained. It is found that at normal and reduced pressures, the MIE is proportional to the inverse of pressure and it increases exponentially with water vapor dilution ratio. A general correlation among the MIE, pressure and dilution ratio is proposed for each fuel. Furthermore, for stoichiometric methane/air/water vapor mixtures, the chemical and radiation effects of water vapor dilution are isolated and quantified. It is found that the three-body recombination reaction greatly increases the MIE and reduces the dilution limit.

A novel mechanism reduction method that integrates path flux analysis with multi generations and sensitivity analysis (MPFASA) is proposed to reduce the complex detailed chemistry and to generate skeletal mechanisms. At first, the path flux analysis with multi generations (MPFA) method is used to efficiently reduce detailed mechanisms; then the sensitivity analysis (SA) method is applied to further eliminate the redundant species and their related reactions on the basis of the skeletal chemistry obtained by the MPFA method. Detailed mechanisms of methane and n-heptane are reduced by the MPFASA method, which are validated in the context of autoignition and perfectly stirred reactor for methane/air and n-heptane/air mixtures, over a wide range of operating conditions. The comparison shows that the skeletal mechanisms generated by the MPFASA method can well reproduce the results of detailed mechanisms and contain a much smaller number of species and reactions than those obtained by using the MFPA met...

The laminar flame speed is one of the most important combustion properties of a combustible mixture. It is an important target for chemical mechanism validation and development, especially at fuel-lean and high pressure conditions. In this study, the laminar flame speeds of two types of lean high-hydrogen syngas/oxygen/helium mixtures were measured at normal and evaluated pressures up to 10 atm using a dual-chambered high pressure combustion facility. Similar to experiments, numerical simulations of outwardly spherical flame propagation were conducted. Three chemical mechanisms for syngas available in the literature were considered in simulation and their performance in terms of predicting the stretched flame speeds, laminar flame speeds and burned Markstein lengths was examined through comparison between experimental and simulation results. It was found that at both normal and elevated pressures, the present experimental results agree well with those predicted by simulations using these three chemical mechanisms. Therefore, these chemical mechanisms for syngas can well predict the laminar flame properties of lean high-hydrogen syngas. Besides, the laminar flame speeds measured in the present work were compared with those measured from the heat flux method and large difference was observed.

In this work, the spontaneous ignition of high-pressure accidentally released hydrogen in a one-dimensional tube was numerically studied using the high-order WENO reconstruction method, multi-component diffusion model and detailed kinetics mechanism. The result shows that the spontaneous ignition of high-pressure hydrogen jet is essentially a non-premixed ignition process between compressed hot air and expanded low-temperature fuel. It is found that increasing the molecular weight of the fuel can greatly reduce the air temperature and thereby improve the storage safety. Further analysis of the reacting mixing layer reveals that the autoignition occurs in a fuel-lean condition where the fuel mass fraction is less than 0.02. During the reaction front propagation, two types of flames are observed in the H2/air diffusion layer, which are a diffusion flame near the stoichiometric position and a premixed flame in the fuel-rich space. The reaction pathway analysis demonstrates that the two types of flames are controlled by the low temperature radical destruction reaction (R1: H + O2(+M) HO2(+M)) and the high temperature radical formation reaction (R9: H + O2 O + OH), respectively. Moreover, the sensitivity evaluation of different reactions on the ignition delay indicates that the two reactions also play a dominate role on the overall combustion rate. In the end, the flame front displacement speed calculation shows that the contribution of diffusion to the reaction front evolution is always slightly greater than that of chemistry except the ignition timing.

The early CO2 formation is a characteristic for the methyl esters group of biodiesel. A new reaction pathway was added to skeletal methyl esters mechanism for improving the prediction of early CO2 formation. The methyl decanoate, methyl 9-decenoate, methyl 5-decenoate and methyl stearate sub-mechanisms with added reaction pathway were optimized by adjusting reaction rate constants for more accurate prediction. Based on decoupling methodology, a new skeletal mechanism for methyl butanoate was constructed by integrating detailed H2/CO/C1 sub-mechanism, reduced C2-C3 sub-mechanism and methyl butanoate sub-mechanism. These improved mechanisms were validated well in a shock tube for ignition delay times and in a jet-stirred reactor for major species concentrations over wide operating conditions, respectively. When compared to available mechanism in the literature, the present mechanism has good improvement for the prediction of early CO2 formation. Moreover, the effect of newly added reactions on ignition delay times was analyzed by sensitivity analysis method. Added reaction of Fuel Radical = ME2J + Short Chain Hydrocarbon mainly causes the influence on ignition delay time at high temperature, and decrease the reactivity of oxidation of fuel radical. The reaction of OCHO + M H + CO2 + M dominates the early CO2 formation, and makes less contribution to production of CO2 with higher temperature. The improved mechanisms, which consist of methyl esters from a relatively short to long carbon chain, have a good performance for the prediction of early CO2 formation.

An efficient surrogate fuel formulation methodology, which directly uses the chemical structure information from nuclear magnetic resonance (NMR) spectroscopy analysis, has been proposed. Five functional groups, paraffinic CH2, paraffinic CH3, aromatic C-CH, olefinic CH-CH2, and cycloparaffin CH2, have been selected to show the basic molecular structure of the fuels for the advanced combustion engines (FACE) fuels. A palette that contains six candidate components, n-heptane, iso-octane, toluene, 2,5-dimethylhexane, methylcyclohexane, and 1-hexene, is chosen for different FACE fuels, based on the consideration that surrogate mixtures should provide the representative functional groups and comparable molecular sizes. The kinetic mechanisms of these six candidate components are chosen to assemble a detailed mechanism of each surrogate fuel for FACE gasoline. Whereafter, the accuracy of FACE A and F surrogate models was demonstrated by comparing the model predictions against experimental data in homogeneous ignition, jet stirred reactor oxidation, and premixed flame.

An improved path flux analysis with a multi generations (IMPFA) method is proposed to eliminate unimportant species and reactions, and to generate skeletal mechanisms. The production and consumption path fluxes of each species at multiple reaction paths are calculated and analysed to identify the importance of the species and of the elementary reactions. On the basis of the indexes of each reaction path of the first, second, and third generations, the improved path flux analysis with two generations (IMPFA2) and improved path flux analysis with three generations (IMPFA3) are used to generate skeletal mechanisms that contain different numbers of species. The skeletal mechanisms are validated in the case of homogeneous autoignition and perfectly stirred reactor of methane and n-decane/air mixtures. Simulation results of the skeletal mechanisms generated by IMPFA2 and IMPFA3 are compared with those obtained by path flux analysis (PFA) with two and three generations, respectively. The comparisons of ignition ...

A methodology of surrogate fuel formulation by directly using molecular structure and functional groups for both oxygenated and hydrocarbon fuels is proposed and investigated. The novelty of this method is to construct surrogate fuel mixtures by directly matching the molecular structure and the key functional groups instead of using the combustion property targets explicitly. This method is tested by using two different classes of fuels, biodiesel and jet fuel. For biodiesel, by using four functional groups such as CH3 , CH2 , CH2 CH CH , and COO CH3, a surrogate mixture of methyl-9-decenoate, 1,4-hexadiene and n-dodecane is formulated to demonstrate the efficacy of this method by comparing the resulting gas phase combustion targets between the formulated surrogate and biodiesel. For jet fuels, five functional groups such as CH3, CH2, CH, C, and phenyl were used to construct the Princeton 1st and 2nd generation surrogate jet fuel mixtures. The simulated results are compared with the experimental data and the results predicted by other surrogate fuel formulation methods. The comparisons show that the present method can formulate surrogated mixtures of both oxygenated and hydrocarbon real fuels and reproduce the combustion characteristics. Therefore, this method can be used not only for biodiesel and jet fuels, but also for other alternative fuels.

Behaviors of premixed CH4/air flame in mesoscale channels with and without cavities were experimentally investigated. No stable symmetric flame was observed in the channel without cavities and flame is prone to inclining and pulsating. In contrast, flame can be effectively anchored in the presence of cavities. When the inlet velocity is increased sufficiently high, curved fluctuating flame front appears. Blow-off limits of the channel with cavities are several times larger than the corresponding burning velocity of incoming CH4/air mixture, while the flashback limits are almost the same as the straight channel counterparts. These indicate that the cavities have a strong ability to extend the operational range of inlet velocity. Numerical simulation demonstrates that combined effects, i.e., the formation of recirculation zone and low velocity zone in the cavities, preferential diffusion effect, as well as the preheating effect of upstream inner walls, are major mechanisms responsible for flame stabilization. Furthermore, numerical result reveals that large strain rate and heat loss rate exist at the transition point between the ramped cavity wall and the downstream inner wall, which results in flame splitting at high inlet velocity due to local extinction, and eventually leads to flame blow-off. In summary, the combustion behaviors in the mesoscale channel with cavities strongly depend on the interactions between the reaction zone, conjugate heat exchange and flow field. (C) 2014 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

A new correlated dynamic adaptive chemistry (CO-DAC) method is developed and integrated with the hybrid multi-timescale (HMTS) method for computationally efficient modeling of ignition and unsteady flame propagation of real jet fuel surrogate mixtures with a detailed and comprehensively reduced kinetic mechanism. A concept of correlated dynamic adaptive chemistry (CO-DAC) method in both time and space coordinates is proposed by using a few key phase parameters which govern the low, intermediate, and high temperature chemistry, respectively. Correlated reduced mechanisms in time and space are generated dynamically on the fly from the detailed kinetic mechanism by specifying thresholds of phase parameters of correlation and using the multi-generation path flux analysis (PFA) method. The advantages of the CO-DAC methods are that it not only provides the flexibility and accuracy of kinetic model and chemistry integration but also avoids redundant model reduction in time and space when the chemistry is frequently correlated in phase space. To further increase the computational efficiency in chemistry integration, the hybrid multi-timescale (HMTS) method is integrated with the CO-DAC method to solve the stiff ordinary differential equations (ODEs) of the reduced chemistry generated on the fly by CO-DAC. The present algorithm is compared and validated against the conventional VODE solver, DAC and HMTS/DAC methods for simulating ignition and unsteady flame propagation of real jet fuel surrogate mixtures consisting of four component fuels, n-dodecane, iso-octane, n-propyl benzene, and 1,3,5-trimethyl benzene. The results show the present HMTS/CO-DAC algorithm is not only computationally efficient but also robust and accurate. Moreover, it is shown that compared to the DAC and HMTS/DAC methods, the computation time of model reduction in CO-DAC is almost negligible even for a large kinetic mechanism involving hundreds of species. In addition, the results show that computation efficiency of CO-DAC increases from homogeneous ignition to one-dimensional flame propagation for both the first and second generation PFA reduction. Therefore, the present HMTS/CO-DAC method can enable high-order model reduction and achieve higher computation efficiency for multi-dimensional numerical modeling.

An automated hierarchical framework, Global Pathway Analysis (GPA), is presented to understand complex chemical kinetics. The behaviour of the reacting system at macro level is bridged to the elementary reaction level by Global Pathways, which are the chemical pathways from an initial reactant species to a final product species. For each Global Pathway, its dominancy and effect on the system, such as those on the production or consumption of radicals, are quantified to understand its contribution to the system. Four examples are presented as demonstration: First, the classical second explosion limit of hydrogen is found to be resulted from the change of dominancy of a pressure-dependent Global Pathway, which consumes radical via H + O2 + M = HO2 + M reaction. Next, it is found that the negative temperature coefficient (NTC) regime of n-heptane is resulted from the competition between a low-temperature Global Pathway and a high-temperature Global Pathway. Third, a non-monotonic relation between autoignition delays and toluene ratio in toluene/n-decane mixture is analysed. This automated framework has been placed in public domain. Reduced kinetic models can be generated based on Global Pathways too. Finally, this methodology is demonstrated using DNS simulation results of the extinction and re-ignition of a turbulent non-premixed flame. The differences between simulation results are investigated using two different kinetics models via the analysis of global pathways.

Laminar flame speeds measured using the propagating spherical flame method are inherently affected by radiation. Under certain conditions, a substantial uncertainty in laminar flame speed measurement is caused by radiation, which results in a great concern for kinetic mechanism validation and development. In this study, numerical simulations with detailed chemistry and different radiation models are con- ducted to examine the effects of radiation on spherical flame propagation. The emphasis is placed on quantifying the uncertainty and corrections associated with radiation in laminar flame speed measure- ments using propagating spherical flames. The radiation effects on flame speeds at normal and elevated temperatures and pressures are examined for different fuel/air mixtures including methane, propane, iso- octane, syngas, hydrogen, dimethyl ether, and n-heptane. The radiative effects are conservatively evalu- ated without considering radation reflection on the wall. It is found that radiation-induced uncertainty in laminar flame speeds is affected in the opposite ways by the initial temperature and pressure. An empir- ical correlation quantifying the uncertainty associated with radiation is obtained. This correlation is shown to work for different fuels at normal and elevated temperatures and pressures. Therefore, it can be directly used in spherical flame experiments measuring the laminar flame speed. Furthermore, a method to obtain the radiation-corrected flame speed (RCFS) is presented and it can be used for laminar flame speed measurement using the propagating spherical flame method.

We recently confirmed the “tip open phenomenon” of lean H2/air flame in a microchannel with cavities. The critical inlet velocity when fuel conversion ratio drops to 80% was defined as “flame splitting limit”. In the present work, we numerically studied the impact of channel gap distance. Results showed that corresponding limits for 1.0-mm, 0.8-mm and 0.6-mm channels are 26 m/s, 33 m/s and 16 m/s respectively, exhibiting a non-monotonic dependence. The analysis reveals that when the gap distance is decreased from 1.0 mm to 0.8 mm, the proportion of fuel that involved into the cavities is increased, flame length is reduced simultaneously, and better preheating of the fresh mixture is attained. These positive effects lead to an increase in flame splitting limit. As the gap distance is further reduced to 0.6 mm, the excessive stretch effect results in complete extinction of downstream flame, causing a decrease of the splitting limit.

A new error controlled dynamic adaptive chemistry (EC-DAC) scheme is developed and validated for ignition and combustion modeling with large, detailed, and comprehensively reduced n-heptane and n-decane mechanisms. A fuel oxidation progress variable is introduced to determine the local model reduction threshold by using the mass fraction of oxygen. An initial threshold database for error control is created according to the progress variable in a homogeneous ignition system using a detailed mechanism. The threshold database tabulated by the fuel oxidation progress variable is used to generate a dynamically reduced mechanism with a specified error bound by using the Path Flux Analysis (PFA) method. The method leads to an error-controlled kinetic model reduction according to the local mixture reactivity and improves the computation efficiency. Numerical simulations of the homogeneous ignition of n-heptane/air and n-decane/air mixtures at different initial conditions are conducted with one detailed and one comprehensively reduced mechanism involving 1034 and 121 species, respectively. The results show that the present algorithm of error-controlled adaptive chemistry scheme is accurate. The computation efficiency is improved by more than one-order for both mechanisms. Moreover, unsteady simulations of outwardly propagating spherical n-heptane/air premixed flames demonstrate that the method is rigorous even when transport is included. The successful validation in both ignition and unsteady flame propagation for both detailed and reduced mechanisms demonstrates that this method can be efficiently used in the direct numerical simulation of reactive flow for large kinetic mechanisms.

A direct path flux analysis (PFA) method for kinetic mechanism reduction is proposed and validated by using high temperature ignition, perfect stirred reactors, and steady and unsteady flame propagations of n-heptane and n-decane/air mixtures. The formation and consumption fluxes of each species at multiple reaction path generations are analyzed and used to identify the important reaction pathways and the associated species. The formation and consumption path fluxes used in this method retain flux conservation information and are used to define the path indexes for the first and the second generation reaction paths related to a targeted species. Based on the indexes of each reaction path for the first and second generations, different sized reduced chemical mechanisms which contain different number of species are generated. The reduced mechanisms of n-heptane and n-decane obtained by using the present method are compared to those generated by the direct relation graph (DRG) method. The reaction path analysis for n-decane is conducted to demonstrate the validity of the present method. The comparisons of the ignition delay times, flame propagation speeds, flame structures, and unsteady spherical flame propagation processes showed that with either the same or significantly less number of species, the reduced mechanisms generated by the present PFA are more accurate than that of DRG in a broad range of initial pressures and temperatures. The method is also integrated with the dynamic multi-timescale method and a further increase of computation efficiency is achieved.

A new on-grid dynamic multi-timescale (MTS) method is presented to increase significantly the computation efficiency involving multi-physical and chemical processes using detailed and reduced kinetic mechanisms. The methodology of the MTS method using the instantaneous timescales of different species is introduced. The definition of the characteristic time for species is examined and compared with that of the computational singular perturbation (CSP) and frozen reaction rate methods by using a simple reaction system. A hybrid multi-timescale (HMTS) algorithm is constructed by integrating the MTS method with an implicit Euler scheme, respectively, for species with and without the requirement of accurate time histories at sub-base timescales. The efficiency and the robustness of the MTS and HMTS methods are demonstrated by comparing with the Euler and VODE solvers for homogenous ignition and unsteady flame propagation of hydrogen, methane, and n-decane–air mixtures. The results show that both MTS and HMTS reproduce well the species and temperature histories and are able to decrease computation time by about one-order with the same kinetic mechanism. Compared to MTS, HMTS has slightly better computation efficiency but scarifies the stability at large base time steps. The results also show that with the increase of mechanism size and the decrease of time step, the computation efficiency of multi-timescale method increases compared to the VODE solver. In addition, it is shown that the integration of the multi-timescale method with the path flux analysis based mechanism reduction approach can further increase the computation efficiency. Unsteady simulations of outwardly propagating spherical n-decane–air premixed flames demonstrate that the multi-timescale method is rigorous for direct numerical simulations with both detailed and reduced chemistry and can dramatically improve the computation efficiency.

Outwardly and inwardly propagating spherical flames (OPF and IPF) with radiative loss are studied analytically and numerically. Emphasis is placed on investigating the effects of radiation on flame propagating speed, Markstein length, and flame extinction, as well as on examining whether the reactant can be completely consumed via an IPF. A general correlation between flame propagating speed and flame radius for OPF and IPF of large flame radii is derived and utilized to study the effects of radiative loss and Lewis number on flame propagation and extinction. A correlation for Markstein length at different Lewis numbers and radiative loss is also presented. It is shown that the Markstein length is strongly affected by radiative loss as well as Lewis number, and that only for mixtures not close to their flammability limits and without CO2 dilution is the effect of radiation on the Markstein length measured from expanding spherical flames negligible. Furthermore, the theoretical results are validated by num...

For emulation of the chemical kinetic combustion phenomena and physical properties of S-8 POSF 4734 and Jet-A POSF 4658, two surrogate fuels were formulated by directly matching their molecular structure and functional groups. The same functional groups, CH3, CH2, CH, C, and phenyl, were chosen to formulate the S-8 and Jet-A surrogates with n-dodecane/2,5-dimethylhexane (0.581/0.419 mole fraction) and n-dodecane/2,5-dimethylhexane/toluene (0.509/0.219/0.272 mole fraction), respectively. The numerical results using the surrogate fuels were compared with the experimental data and the results predicted by other surrogate fuel formulation methods. The results show that the present method can formulate surrogate mixtures of both jet fuels and Fischer–Tropsch real fuels and reproduce the combustion characteristics in homogeneous ignition and the flow reactor oxidation process. The idea presented here could be extended to other real fuels with the appropriate choice of surrogate fuel components.