摘要 :
Direct numerical simulations, for a stratified flow in an HCCI engine-like conditions, are performed to investigate the exhaust gas recirculation (EGR) and temperature/mixture stratification effects on autoignition of synthetic di...
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Direct numerical simulations, for a stratified flow in an HCCI engine-like conditions, are performed to investigate the exhaust gas recirculation (EGR) and temperature/mixture stratification effects on autoignition of synthetic dimethyl ether (DME) in the negative temperature combustion (NTC) region. Detailed chemistry for a DME/air mixture is employed and solved by a hybrid multi-time scale (HMTS) algorithm to reduce the computational cost. Three ignition stages are observed. The effect of NO to mimic the EGR effect on autoignition are studied. The results show that adding NO enhances autoignition by the rapid OH radical pool formation (one to two orders of magnitude more OH radicals results in 13%-25% reduction in ignition delay times for 1000 ppm initial NO from EGR) and increases the low temperature ignition heat release rate (Q_(ltc)) with approximately similar ignition heat release rates at the second and third ignition stages. Sensitivity analysis is performed and the important reactions pathways are specified. The DNS results show that the scales introduced by the mixture and thermal stratifications have a strong effect after the low temperature chemistry (LTC) ignition. Compared to homogenous ignition, stratified ignitions show similar first autoignition delay times, but about 19% reduction in the second and third ignition delay times. Stratification, however, reveals lower space averaged LTC ignition heat release rate and higher averaged hot ignition heat release rate compared to homogenous ignitions. The results also show that molecular transport plays an important role in stratified low temperature ignition, and that the scalar mixing time scale is strongly affected by local ignition. Two ignition-kernel propagation modes are observed: a wave-like, low-speed, deflagrative mode and a spontaneous, high-speed, kinetically driven mode. Three criteria are introduced to distinguish these modes by different characteristic time scales and Damkhoeler number using a progress variable conditioned by a proper ignition kernel indicator (IKI). The spontaneous ignition mode is characterized by low scalar dissipation rate, high displacement speed flame front, and high mixing Damkhoeler number. The proposed criteria are applied successfully at the different ignition stages.
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摘要 :
Direct numerical simulations, for a stratified flow in an HCCI engine-like conditions, are performed to investigate the exhaust gas recirculation (EGR) and temperature/mixture stratification effects on autoignition of synthetic di...
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Direct numerical simulations, for a stratified flow in an HCCI engine-like conditions, are performed to investigate the exhaust gas recirculation (EGR) and temperature/mixture stratification effects on autoignition of synthetic dimethyl ether (DME) in the negative temperature combustion (NTC) region. Detailed chemistry for a DME/air mixture is employed and solved by a hybrid multi-time scale (HMTS) algorithm to reduce the computational cost. Three ignition stages are observed. The effect of NO to mimic the EGR effect on autoignition are studied. The results show that adding NO enhances autoignition by the rapid OH radical pool formation (one to two orders of magnitude more OH radicals results in 13%-25% reduction in ignition delay times for 1000 ppm initial NO from EGR) and increases the low temperature ignition heat release rate (Q_(ltc)) with approximately similar ignition heat release rates at the second and third ignition stages. Sensitivity analysis is performed and the important reactions pathways are specified. The DNS results show that the scales introduced by the mixture and thermal stratifications have a strong effect after the low temperature chemistry (LTC) ignition. Compared to homogenous ignition, stratified ignitions show similar first autoignition delay times, but about 19% reduction in the second and third ignition delay times. Stratification, however, reveals lower space averaged LTC ignition heat release rate and higher averaged hot ignition heat release rate compared to homogenous ignitions. The results also show that molecular transport plays an important role in stratified low temperature ignition, and that the scalar mixing time scale is strongly affected by local ignition. Two ignition-kernel propagation modes are observed: a wave-like, low-speed, deflagrative mode and a spontaneous, high-speed, kinetically driven mode. Three criteria are introduced to distinguish these modes by different characteristic time scales and Damkhoeler number using a progress variable conditioned by a proper ignition kernel indicator (IKI). The spontaneous ignition mode is characterized by low scalar dissipation rate, high displacement speed flame front, and high mixing Damkhoeler number. The proposed criteria are applied successfully at the different ignition stages.
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The surrogate fuel concept to replicate the detailed gas phase combustion behaviors of conventional and alternative jet aviation fuels in numerical combustion models is extended and tested in specific examples of synthetic jet fue...
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The surrogate fuel concept to replicate the detailed gas phase combustion behaviors of conventional and alternative jet aviation fuels in numerical combustion models is extended and tested in specific examples of synthetic jet fuels derived from coal and natural gas, and also to the pressure and equivalence ratio dependences of the combustion responses of conventional Jet-A fuel. The formulation of surrogate fuels for Syntroleum S-8, Shell SPK and Sasol IPK, is described. Assuming these compositions, a detailed chemical kinetic model construction previously elaborated upon is extended and tested against reference data sets of shock tube ignition delay and laminar burning velocity. Calculations with the detailed kinetic model, containing 3147 species correctly represent the experimentally measured reactivity of the target fuels for shock tube ignition delay. The model also captures trends in the ignition delay for a reference Jet-A as a function of pressure and equivalence ratio. The earlier reported detailed model is expanded to encompass a range of n-alkane carbon numbers up to C_(16) and iso-cetane. The expanded model is validated against available shock tube ignition delay in detailed form and against laminar burning velocity datasets using a series of numerically reduced models of decreasing dimension for n-hexadecane, iso-cetane, and their mixtures. Though the detailed model reproduces the general kinetic behavior for the ignition delays of each jet fuel, the predicted values are generally longer than experimental results. A series of reduced models of the order of 100 species in size, are produced for simulation of flame environments. Calculations for laminar premixed flames for each jet fuel are similar with burning velocities for IPK flames marginally lower than those for the conventional Jet-A which in turn are marginally lower than those for S-8. The requirement for severely reduced, but high fidelity chemical kinetic numerical schemes that retain predictive capacities for the combustion behaviors of real liquid transportation fuels is addressed through the introduction of a strategy to produce "compact" models of the order of 35 species. The strategy utilizes calculations of the detailed model construct as a fundamental and scientific standard, to which engineering approximations achieved through adjusting reaction rates and omitting or diverting the fate of select reaction pathways at high carbon numbers are applied. The strategy is tested for the exemplar real fuel test case of the S-8 ignition delay and laminar burning velocity data sets. The calculations of a compact model comprising only 36 species is demonstrated to retain quantitative fidelity to the calculations produced using the benchmark detailed model. Thus, a procedure to couple the real fuel description through combustion property target characterization, to the production of compact models for use in computational fluid dynamic platforms is demonstrated.
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摘要 :
The surrogate fuel concept to replicate the detailed gas phase combustion behaviors of conventional and alternative jet aviation fuels in numerical combustion models is extended and tested in specific examples of synthetic jet fue...
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The surrogate fuel concept to replicate the detailed gas phase combustion behaviors of conventional and alternative jet aviation fuels in numerical combustion models is extended and tested in specific examples of synthetic jet fuels derived from coal and natural gas, and also to the pressure and equivalence ratio dependences of the combustion responses of conventional Jet-A fuel. The formulation of surrogate fuels for Syntroleum S-8, Shell SPK and Sasol IPK, is described. Assuming these compositions, a detailed chemical kinetic model construction previously elaborated upon is extended and tested against reference data sets of shock tube ignition delay and laminar burning velocity. Calculations with the detailed kinetic model, containing 3147 species correctly represent the experimentally measured reactivity of the target fuels for shock tube ignition delay. The model also captures trends in the ignition delay for a reference Jet-A as a function of pressure and equivalence ratio. The earlier reported detailed model is expanded to encompass a range of n-alkane carbon numbers up to C_(16) and iso-cetane. The expanded model is validated against available shock tube ignition delay in detailed form and against laminar burning velocity datasets using a series of numerically reduced models of decreasing dimension for n-hexadecane, iso-cetane, and their mixtures. Though the detailed model reproduces the general kinetic behavior for the ignition delays of each jet fuel, the predicted values are generally longer than experimental results. A series of reduced models of the order of 100 species in size, are produced for simulation of flame environments. Calculations for laminar premixed flames for each jet fuel are similar with burning velocities for IPK flames marginally lower than those for the conventional Jet-A which in turn are marginally lower than those for S-8. The requirement for severely reduced, but high fidelity chemical kinetic numerical schemes that retain predictive capacities for the combustion behaviors of real liquid transportation fuels is addressed through the introduction of a strategy to produce "compact" models of the order of 35 species. The strategy utilizes calculations of the detailed model construct as a fundamental and scientific standard, to which engineering approximations achieved through adjusting reaction rates and omitting or diverting the fate of select reaction pathways at high carbon numbers are applied. The strategy is tested for the exemplar real fuel test case of the S-8 ignition delay and laminar burning velocity data sets. The calculations of a compact model comprising only 36 species is demonstrated to retain quantitative fidelity to the calculations produced using the benchmark detailed model. Thus, a procedure to couple the real fuel description through combustion property target characterization, to the production of compact models for use in computational fluid dynamic platforms is demonstrated.
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An experimental investigation of the effectiveness of a nanosecond duration repetitively-pulsed plasma discharge device for ignition of a pulsed detonation engine was carried out. Ignition of C_2H_4/air mixtures and aviation gasol...
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An experimental investigation of the effectiveness of a nanosecond duration repetitively-pulsed plasma discharge device for ignition of a pulsed detonation engine was carried out. Ignition of C_2H_4/air mixtures and aviation gasoline/air mixtures at atmospheric pressure produced a maximum reduction in ignition time of 17% and 25%, respectively, as compared to an automotive aftermarket multiple capacitive-discharge spark ignition system. It was found that the ignition time is reduced as total energy input and pulse repetition frequency is increased. Further investigation of ignition events by Schlieren imaging revealed that at low pulse-repetition frequency (0-5 kHz), individual ignition kernels formed by the discharge do not immediately interact, while at higher pulse-repetition frequencies (≥ 10 kHz) ignition kernels combine and result in a faster transition to a self-propagating flame front.
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An experimental investigation of the effectiveness of a nanosecond duration repetitively-pulsed plasma discharge device for ignition of a pulsed detonation engine was carried out. Ignition of C_2H_4/air mixtures and aviation gasol...
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An experimental investigation of the effectiveness of a nanosecond duration repetitively-pulsed plasma discharge device for ignition of a pulsed detonation engine was carried out. Ignition of C_2H_4/air mixtures and aviation gasoline/air mixtures at atmospheric pressure produced a maximum reduction in ignition time of 17% and 25%, respectively, as compared to an automotive aftermarket multiple capacitive-discharge spark ignition system. It was found that the ignition time is reduced as total energy input and pulse repetition frequency is increased. Further investigation of ignition events by Schlieren imaging revealed that at low pulse-repetition frequency (0-5 kHz), individual ignition kernels formed by the discharge do not immediately interact, while at higher pulse-repetition frequencies (≥ 10 kHz) ignition kernels combine and result in a faster transition to a self-propagating flame front.
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The present work combines numerical and experimental efforts together to investigate the effect of low temperature, nano-second pulsed plasma discharges on the oxidation of C_2H_4/O_2/Ar mixtures at 60 Torr pressure. The non-equil...
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The present work combines numerical and experimental efforts together to investigate the effect of low temperature, nano-second pulsed plasma discharges on the oxidation of C_2H_4/O_2/Ar mixtures at 60 Torr pressure. The non-equilibrium plasma discharge is modeled by a two-temperature framework with detailed chemistry-plasma mechanism. The model shows that 75%～77% of input pulse energy was consumed in electron impact dissociation, excitation and ionization reactions, which efficiently produces significant amount of important radical species, fuel fragments and several excited species. The trends of numerical and experimental results agree well. The results from ID model are compared with 0D model and it show that 1D model in general agrees better with experiments than 0D model. The modeling results reveal that reactions between O(~1D) and hydrocarbons are importantly affecting the formation of C_2H_6, CH_2CO, CH_2O, CO, CO_2, H_2O_2, H_2O, O_2(a~1Δ_g) and O_2(b~1Σ~+_g). Due to the persistent relatively high level of O_2(a~1Δ_g) and O_2(b~1Σ~+_g), C_2H_2 converts into HCO directly without the need of going through the intermediate species of HCCO, CH_2~* and CH_2 in the case without plasma. Owing to the long lifetime of O_2(a~1Δ_9), this effect can last to 3.1 sec after the finish of all 150 pulses.
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摘要 :
The present work combines numerical and experimental efforts together to investigate the effect of low temperature, nano-second pulsed plasma discharges on the oxidation of C_2H_4/O_2/Ar mixtures at 60 Torr pressure. The non-equil...
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The present work combines numerical and experimental efforts together to investigate the effect of low temperature, nano-second pulsed plasma discharges on the oxidation of C_2H_4/O_2/Ar mixtures at 60 Torr pressure. The non-equilibrium plasma discharge is modeled by a two-temperature framework with detailed chemistry-plasma mechanism. The model shows that 75%～77% of input pulse energy was consumed in electron impact dissociation, excitation and ionization reactions, which efficiently produces significant amount of important radical species, fuel fragments and several excited species. The trends of numerical and experimental results agree well. The results from ID model are compared with 0D model and it show that 1D model in general agrees better with experiments than 0D model. The modeling results reveal that reactions between O(~1D) and hydrocarbons are importantly affecting the formation of C_2H_6, CH_2CO, CH_2O, CO, CO_2, H_2O_2, H_2O, O_2(a~1Δ_g) and O_2(b~1Σ~+_g). Due to the persistent relatively high level of O_2(a~1Δ_g) and O_2(b~1Σ~+_g), C_2H_2 converts into HCO directly without the need of going through the intermediate species of HCCO, CH_2~* and CH_2 in the case without plasma. Owing to the long lifetime of O_2(a~1Δ_9), this effect can last to 3.1 sec after the finish of all 150 pulses.
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An implicit methodology based on chemical group theory to formulate a jet aviation fuel surrogate by the measurements of several combustion related fuel properties is tested. The empirical formula and derived cetane number of an a...
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An implicit methodology based on chemical group theory to formulate a jet aviation fuel surrogate by the measurements of several combustion related fuel properties is tested. The empirical formula and derived cetane number of an actual aviation fuel, POSF 4658, have been determined. A three component surrogate fuel for POSF 4658, has been formulated by constraining a mixture of n-decane, iso-octane and toluene to reproduce the hydrogen/carbon ratio and derived cetane number of the target fuel. The validity of the proposed surrogate is evaluated by experimental measurement of select combustion properties of POSF 4658, and the POSF 4658 surrogate; 1) A variable pressure flow reactor has been used to chart the chemical reactivity of stoichiometric mixtures of POSF 4658/O_2/N_2 and POSF 4658 surrogate/O_2/N_2 at 12.5 atm and 500-1000K, fixing the carbon content at 0.3% for both mixtures. 2) The high temperature chemical reactivity and chemical kinetic-molecular diffusion coupling of POSF 4658 and POSF 4658 surrogate have been evaluated by measurement of the extinction limits of diffusion flames. 3) The autoignition behavior of POSF 4658 and POSF 4658 surrogate has been measured with a shock tube at 674-1222 K and with a rapid compression machine at 645-714K for stoichiometric mixtures of fuel in air at pressures close to 20 atm. The flow reactor study shows that the character and extent of chemical reactivity of both fuels at low temperature (500-675K) and high temperature (900K+) is extremely similar but differences in the end of the negative temperature coefficient regime between each fuel are observed. The diffusion flame extinction limits of both fuels are observed to be indistinguishable on a molar basis. Ignition delay measurements also show that POSF 4658 exhibits NTC behavior. Moreover, the ignition delays of both fuels are also extremely similar over the temperature range studied between both apparatuses. Chemical kinetic modeling is utilized to interpret these observations in the case of the POSF 4658 surrogate and provides a rationale as to why the two fuels share such similar reactivity.
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Global combustion characteristics of one conventional jet fuel (JP-8) and four non-petroleum alternative jet fuels (Shell Synthetic Paraffinic Kerosene (SPK), Sasol Iso-Paraffinic Kerosene (IPK), Hydrotreated Renewable Jet (HRJ Ca...
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Global combustion characteristics of one conventional jet fuel (JP-8) and four non-petroleum alternative jet fuels (Shell Synthetic Paraffinic Kerosene (SPK), Sasol Iso-Paraffinic Kerosene (IPK), Hydrotreated Renewable Jet (HRJ Camelina and HRJ Tallow)) are experimentally examined. The ranking of the fully pre-vaporized global combustion characteristics of these five fuels has been hypothesized a priori based on the relative values for four fuel combustion property targets used in the formulation of surrogate jet fuel mixtures in recent studies by the authors. In order to validate a priori speculation, the pure chemical kinetic reactivities of these fuels have been compared by performing oxidation reactivity experiments in a high pressure flow reactor and fundamental flame measurements. The oxidation reactivity profile for Sasol IPK demonstrates no low temperature oxidative characteristic of the occurrence of active alkyl peroxy radical oxidation mechanisms, whereas the remaining jet fuels demonstrate extensive reactivity between 550 and 750 K. Despite sharing similar derived cetane numbers, Shell SPK shows the more pronounced low temperature reactivity compared to the other alternative jet fuels. Two fundamental flame measurements at near-limit condition, both extinction limits of diffusion flames and critical flame initiation radii of outwardly propagating premixed flames, were used to investigate differences chemical kinetic/transport coupled high temperature combustion behaviors. Strained extinction measurements identify the pronounced high reactivity of Shell SPK, whereas critical radius measurements exhibit the distinctive low reactivity of Sasol IPK. Additional analyses along with available literature measurements indicate that the proposed four fuel combustion property targets can be used to predict the relative pre-vaporized global combustion properties of the tested petroleum-derived and alternative jet fuels. Finally, further detailed understanding and improvements on DCN and TSI determination methodologies, particularly for alternative jet fuels, are addressed as a prerequisite condition to better parameterizing the combustion property targets, thus improving the quantitative predictability of a provisional combustion property target (CPT) Index. The CPT index appears useful in screening the relative global combustion properties of real petroleum derived fuels, non-petroleum alternative fuels, and their mixtures.
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