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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 important clean fuels, methane plays a significant role in the energy supply system, and its combustion characteristics directly affect the efficiency and emission. In order to avoid the production of a large number of nitrogen oxides, the low temperature combustion associated with cool flame is getting more and more attention. But until now the cool flame characteristics of methane is still unclear, even its existence is controversial. In this work, the property of premixed methane/oxygen cool flame has been experimentally studied in a cylindrical reactor and numerically analyzed using different mechanisms. The effects of pressure, temperature and equivalence ratio on methane cool flame have been investigated experimentally. The experimental results show that the cool flame can be obtained in the range of 0.2–4.8 equivalent ratio. The lower limit of pressure of the cool flame region decreases with the increasing initial temperature. The sensitivity and reaction path have been analyzed through the numerical simulation, which reveals that the bifurcation in CH3 oxidation process has remarkable effects on cool flame formation.

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...

As promising alternatives to fossil fuels, hydrogen (H 2 ) and syngas are playing important roles in the development and control of high-efficiency, low-emission engines. Achieving accurate prediction of H 2 -fueled combustion requires a reliable chemical mechanism which, however, still exists considerable uncertainty. The laminar flame speed (LFS) has been widely employed to validate and optimize chemical mechanisms and to model turbulent premixed combustion. While in the literature there are extensive LFS data measured using the outwardly propagating spherical flame (OPF) method for hydrogen/air and syngas/air mixtures at normal temperature and pressure (NTP), the accuracy of the LFS data is not fully explored. This work aims to (i) review the uncertainty in the LFSs measured by different groups for hydrogen/air and syngas/air mixtures at NTP using the OPF method, and (ii) identify underlying sources of the uncertainty. It is found that there are considerable discrepancies in the LFS measurements, leading to these experimental data being unreliable for restraining the uncertainty of chemical models. The underlying sources of uncertainty are discussed in different flame propagation regimes and their contributions to the discrepancies are assessed individually using 1-D simulations. The results show that the contribution of ignition effects to the uncertainty depends strongly on the equivalence ratio and that the ignition effects could be one of the main sources of uncertainty for the LFSs of fuel-rich mixtures. Furthermore, it is found that the accuracy of measured LFSs is strongly affected by the choice of extrapolation model and flame radius range for extrapolation. The nonlinear extrapolation is less sensitive to the flame radius range than linear extrapolation, implying that using nonlinear extrapolation models can reduce the impact of the flame radius range selected on the uncertainty, especially for fuel-rich and/or fuel-lean mixtures. Nevertheless, strong nonlinear behavior between stretched flame speed and stretch rate still makes a major contribution to the very large discrepancies even when the nonlinear extrapolation models are used. To address the nonlinear stretch behavior, a new nonlinear extrapolation model NQH is proposed and it is shown to be more accurate than other models as pressure increases. Moreover, the recommendations on H 2 and syngas LFS measurements using the OPF method are provided.

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...

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.

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.

High-temperature thermoelectric generation (TEG) system that driven by combustion is an efficient way for simultaneously providing heat and electric power, but its application prospect is restricted by very low ratio of electric power to heat supply. In this work, an annular high-temperature TEG system is extended by adding medium-temperature TEG modules, and correspondingly a novel two-stage annular multi-hole burner with baffle construction is specially designed to replace original single-stage annular multi-hole burner. The optimal geometric dimensions of the new burner are obtained by conducting a simplified 2D model in ANSYS. Experiment results show that, due to the new burner design, the inner-wall temperature of the high-temperature TEG module is averagely increased by 73 K, bringing an electric power rise of 14.8%; in comparison with the single-stage high-temperature TEG system, the total electric power generation of the high- and medium-temperature coupled TEG system is greatly improved by 73.8%. To further improve the electric performance of the high-temperature TEG module, a well-developed multi-physics field coupled model which containing 2D combustion reactions in the new burner and 1D multi-physical thermoelectric (TE) effects is established to reveal the optimal length and number of TE element, and to assess the optimization of cold-side heat sink.

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.

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.

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.

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.

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.

The transitions from ignition to flames as well as the combustion dynamics in stratified n-heptane and toluene mixtures are numerically modeled by a correlated dynamic adaptive chemistry method coupled with a hybrid multi-timescale method (HMTS/CO-DAC) in a one-dimensional constant volume chamber. The study attempts to answer how the kinetic difference between n-alkanes and aromatics leads to different ignition to flame transitions and knocking-like acoustic wave formation at low temperature and engine pressure conditions with fuel stratification. It is found that the low temperature chemistry (LTC) and fuel stratification of n-heptane leads to the formation of multiple ignition fronts. Four different combustion wave fronts, a low temperature ignition (LTI) front followed by a high temperature ignition (HTI) front, a premixed flame (PF) front, and a diffusion flame (DF) front, are demonstrated. The fast LTI and HTI wave front propagation leads to a shock-like strong acoustic wave propagation, thus strongly modifying the dynamics of the subsequent diffusion and premixed flame fronts. On the other hand, for the toluene mixture, due to the lack of LTC, only two combustion wave fronts are formed, a HTI front and a premixed flame front, exhibiting stable flow field and no formation of shock-like acoustic wave. The dynamics of transition from combustion to shock waves is further analyzed by using a modified Burgers’ equation. The analysis for n-heptane/air mixture indicates that both the onset of LTI and the strong dependency of HTI on the equivalence ratio can either promote or attenuate the transition from strong acoustic wave to shock wave. However, the toluene/air mixture exhibits no coupling with acoustic wave, suggesting that the rich LTC reactivity with fuel stratification, specific to the n-alkane chemistry, can lead to knocking and acoustic formation.

This paper presents a flameless catalytic combustion-based thermoelectric power generator that uses commercial thermoelectric modules. The structure of the thermoelectric generator (TEG) is introduced and the power performance is measured based on a designed circuit system. The open circuit voltage of the TEG is about 7.3 V. The maximum power output can reach up to 6.5 W when the load resistance matches the TEG internal resistance. However, the system output is sensitive to load variation. To improve this characteristic, maximum power point tracking technique is used and results in an open circuit voltage of 13.8 V. The improved characteristic makes the TEG system a good charger to keep the lead acid battery fully charged so as to meet the needs of electronic instruments on gas pipelines. In addition, the combustion features have been investigated based on the temperature measurement. Test results show that the uniformity of combustion heat transfer process and the combustion chamber structure play important roles in improving system power output. It can get an optimized TEG system (maximum power output: 8.3 W) by uniformly filling a thermal insulation material (asbestos) to avoid a non-uniform combustion heat transfer process.

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.

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.

To enhance the computational efficiency for the simulation of plasma assisted combustion (PAC) models, three new techniques, on-the-fly adaptive kinetics (OAK), point-implicit stiff ODE solver (ODEPIM), and correlated transport (CoTran), are combined together to generate a new simulation framework. This framework is applied to non-equilibrium plasma assisted oxidation of C2H4/O2/Ar mixtures in a low-temperature flow reactor. The new framework has been extensively verified by both temporal evolution and spatial distribution of several key species and gas temperature. Simulation results show that it accelerates the total CPU time by 3.16 times, accelerates the calculation of kinetics by 80 times, and accelerates the calculation of transport properties by 836 times. The high accuracy and performance of the new framework indicates that it has great application potentials to many different areas in the modeling and simulation of plasma assisted combustion.

The ignition, flame propagation, and transition to detonation of n-heptane-air mixtures in a one-dimensional, cylindrical chamber are numerically modeled at homogeneously charged compression ignition (HCCI) conditions by using a multi-time scale (MTS) method with a comprehensively reduced kinetic mechanism. It is found that depending on the initial temperature and temperature gradient, there exist many new combustion regimes. At low temperatures, it is shown that there is a coupled low temperature flame (LTF) and high temperature flame (HTF) propagation regime. At intermediate temperatures, the results demonstrated that there are six different combustion regimes, an initial single flame front propagation regime, a coupled LTF and HTF double flame regime, a decoupled LTF and HTF double flame regime, a low temperature ignition regime, a single HTF regime, and a hot ignition regime. At high temperatures, only HTF and hot ignition are observed. Furthermore, it is found that the existence negative temperature coefficient (NTC) region dramatically changes the critical temperature for flame acoustic coupling. The rapid increase of the magnitude of critical temperature in the NTC region enhances the occurrence of supersonic ignition regime and suppresses detonation transition. The results show that the low temperature flame chemistry affects dramatically the flame regimes, flame transitions to ignition and detonation, and the temporal histories of pressure and heat releases.