In this experimental work,an optically accessible rapid compression machine is used to study the ignition and combustion process under engine relevant operation conditions for five different air-natural gas equivalenc...In this experimental work,an optically accessible rapid compression machine is used to study the ignition and combustion process under engine relevant operation conditions for five different air-natural gas equivalence ratios(λ)ignited with either 12.2 mg or 6.7 mg of pilot diesel injected at 1,600 bar.Initial temperature of the ambient mixture,walls and injector was 333 K.Additionally,for the short(6.7 mg)diesel injection,the variation in the ID(ignition delay)for two higher ambient temperatures(343 K and 353 K)was measured.Pressure and piston displacement are recorded while two high-speed cameras simultaneously capture signals in the visible range spectrum and at 305 nm wavelength for OH^(*)chemiluminescence respectively.ID is measured both from OH^(*)and pressure rise.From the recorded data,the heat release ratio is estimated and compared with the visual signals.This gives an insight of the temporal and spatial evolution of the flame,as well as a qualitative perception of the transition from spray ignition into a premixed flame in the ambient fuel-air mixture.It was found that increasing the methane concertation delays the ignition,reduces the natural flame luminosity and enhances the OH^(*)chemiluminescence signal.展开更多
In this study,the multi-objective intelligent optimization of the crevice structure in a rapid compression machine(RCM)is carried out based on the RCM simulation model modified with the KIVA-3V program.A multi-objecti...In this study,the multi-objective intelligent optimization of the crevice structure in a rapid compression machine(RCM)is carried out based on the RCM simulation model modified with the KIVA-3V program.A multi-objective optimization simulation model of the crevice structure based on the large eddy simulation model coupled with the genetic algorithm NSGA-Ⅲis established.Six optimization parameters and seven optimization objectives are selected in the optimization process.The results show that the genetic algorithm can quickly find the values of the optimized parameters.The crevice volume ratio shows a trade-off relationship with the dimensionless temperature ratio T_(max)/T_(aver)and the tumble ratio.A larger crevice volume can reduce the flow of boundary layer cryogenic gas into the combustion chamber,thus improving the temperature uniformity.In addition,the crevice entrance width and the connecting channel length should be smaller,while the volume of the crevice main chamber should be larger,so as to sufficiently introduce the low-temperature gas of the boundary layer into the crevice and reduce their influence on the temperature field of the combustion chamber.When the crevice volume accounts for10%of the total volume,the temperature uniformity of the combustor is significantly enhanced,and when the crevice volume accounts for 30.4%,there is almost no low-temperature vortex in the combustion chamber.展开更多
The diffusion of fuel spray in swirl flow is vital for the combustion of diesel engine,however,the researches on this is still mysterious due to the obstacles on direct investigations on a real engine.The research of...The diffusion of fuel spray in swirl flow is vital for the combustion of diesel engine,however,the researches on this is still mysterious due to the obstacles on direct investigations on a real engine.The research of intake swirl in engine at present normally use CFD simulation or based on data analysis of combustion and exhaust emission,the specific mixing process of fuel in swirl flow still not very clear.In this paper,a rapid compression machine(RCM)with an optical combustion chamber was established with the mean compression velocity of 7.55 m/s.Three kinds of flow fields,including intense swirl field,weak swirl field and turbulent field,were conducted to investigate the diffusion characteristics of injected fuel,the mixing processes of fuel spray in the swirl flow fields were recorded by the high-speed camera.Experimental results show that the mixing of fuel spray in air consists of four stages:fuel injection,wall approaching,rapid diffusion and final diffusion.The fuel spray is firstly bended by the lateral flow of the swirl field,then liquid fuel is blown onto,and coats on,the sidewall.The fuel on the sidewall is finally enrolled into the chamber plenum afterwards in gaseous phase via the spiral arms(in swirl fields)or random routes(in turbulent field)At last the fuel disperses all over the chamber.The results give an intuitive observation and help better understand the air-fuel mixing process in diesel and direct inject gasoline engines.展开更多
Cemented paste backill(CPB)is a susta inable mining technology that is widely used in mines and helps to improve the mine environment.To investigate the relationship between aggregate grading and different affecting f...Cemented paste backill(CPB)is a susta inable mining technology that is widely used in mines and helps to improve the mine environment.To investigate the relationship between aggregate grading and different affecting factors and the uniaxial compressive strength(UCS)of the cemented paste backill(CPB),Talbol gradation theory and neural networks is used to evaluate aggregate gradation to determine the optimum aggregate ratio.The mixed aggregate ratio with the least amount of cement(waste stone content river sand content=7:3)is obtained by using Talbol grading theory and pile compactness function and combined with experiments.In addition,the response surface method is used to design strength speaific ratio experiments.The UCS prediction model which ues the ISTM and considers the aggregates gradation have high accuracy.The root mean square error(RMSE)of the prediction results is 0.0914,the coefficient of determination(R^(2))is 0.9973 and the variance account for(VAF)is 99.73.Compared with back propagation neural network(BP-ANN),extreme lea ming machine(ELM)and madal basis function neural network(RBF ANN),LSTM can efectively characterize the nonlinear relationship between UCS and individual affecting factors and predict UCS with high accuracy.The sensitivity analysis of different affecting factors on UCS shows that all 4 factors have significant effect on UCS and sensitivity is in the following ranking:cement content(0.9264)>slurry concentration(0.9179)>aggregate gradation(waste rodk content)(0.9031)>curing time(09031).展开更多
Granular material mechanics,finite element analysis and crushing theory are applied to study the compressive comminution mechanism of particle beds in this paper.This is a new method by which we have established an eq...Granular material mechanics,finite element analysis and crushing theory are applied to study the compressive comminution mechanism of particle beds in this paper.This is a new method by which we have established an equivalent model of granular material,determined the values and distributions of contact forces and discovered a crushing law.The model has been tested on the newly designed equipment and proved to be correct.Some new characteristics and laws of compressive comminution of particle beds have been found.展开更多
Recent combustion research has focused on low temperature combustion to meet engine emission regulations and to advance the development of low temperature homogenous compression ignition engines.Autoignition studies i...Recent combustion research has focused on low temperature combustion to meet engine emission regulations and to advance the development of low temperature homogenous compression ignition engines.Autoignition studies in this temperature regime are primarily performed by Rapid Compression Machines(RCMs)which are sensitive to the heat transfer characteristics of the experimental device.RCMs are widely used to measure autoignition data such as ignition delay and species concentration.Measured ignition delays from RCMs are typically reported at an adiabatic condition;however,this assumption may produce a systematic error in ignition delay measurement as heat transfer is observed to reduce the pressure and temperature during the autoignition process,e.g.,a longer ignition delay has a greater pressure and temperature drops.RCMs are custom built and have unique design characteristics that affect the heat transfer during the autoignition process.In addition,depending on the diluent composition(e.g.,helium versus nitrogen or argon),different heat transfer characteristics are expected.As a result,autoignition results at similar conditions may vary from facility to facility or depending on the used diluent.The dependency of the measured data on the used facility or diluent may produce uncertainty in the data which impact the development of high-fidelity combustion mechanisms.In this work,a new method is developed and utilized to eliminate heat transfer from the ignition delay data.To evaluate the new method,the autoignition of n-pentane mixtures in the low temperature regime were investigated using an RCM.To vary the heat transfer,the compression ratio of the RCM was changed and the ignition delays were measured at similar pressure and temperature conditions.The tests were performed at an equivalence ratio of approximately one and nitrogen and argon as diluents.By applying the new method,the effect of heat transfer on the ignition delays were eliminated successfully and ignition delays at adiabatic condition were determined.A detailed kinetic model of n-pentane was used to simulate the measured adiabatic ignition delay,which agreed well with the experimental data.展开更多
Ignition delay times of butanol isomers/n-heptane mixture were measured using a rapid compression machine at compressed pressures of 15,20 and 30 bar,in the compressed temperature range of 650–830 K and equivalence r...Ignition delay times of butanol isomers/n-heptane mixture were measured using a rapid compression machine at compressed pressures of 15,20 and 30 bar,in the compressed temperature range of 650–830 K and equivalence ratio of 1.0.Sensitivity analysis and reaction fluxes analysis were performed using a detailed mechanism of blend fuels so as to evaluate the impact of n-heptane addition and temperature variation on the ignition and combustion process.Over the experimental conditions in this study,the blend fuels displays apparent low and high temperature reactions and a negative-temperature-coefficient(NTC)behavior.With increasing butanol isomers mole fraction in the mixtures,the ignition delay times increase.It is worth noting that the suppression to n-heptane ignition from tert-butanol is very limited.The ignition delay time of 40/60 tert-butanol/n-heptane mixture is smaller than other three kinds of blends.With the increasing of tert-butanol mole fraction,the increasing range of its ignition delay time is very large.Moreover,compressed pressure has a limited effect on the ignition of blend mixture at low temperature but certain influence at medium temperature arrange.Tert-butanol/n-heptane mixture is not sensitive to the pressure.The chemical analysis indicates that butanol isomers also present the NTC behavior because of the low temperature reactivity radicals pool produced by n-heptane.Reaction fluxes analysis shows that the n-heptane addition has little impact on the reaction path.Sensitivity analysis shows that for the pure n-butanol,2-butanol and iso-butanol fuel,H-abstraction from the?-carbon plays the dominant role in the reactions having the inhibiting effect on the low-temperature branching,while the H-abstraction from the?-carbon can promote the ignition;for tert-butanol/n-heptane mixtures,reaction R16.H2O2(+M)<=>OH+OH(+M)plays the leading role.For n-butanol/n-heptane,iso-butanol/n-heptane mixtures,the major promoting reactions include some H-abstraction from n-heptane and OH branching reactions,the influence of H-abstraction from?-carbon is weaken;For 2-butanol/n-heptane,tert-butanol/n-heptane mixtures,R16 plays an absolutely dominant role,while the major inhibiting reactions add some elementary reactions of small radicals.展开更多
N-hexadecane is a potential candidate of diesel surrogate fuels and is also the largest linear alkane(n-alkanes)with known chemical kinetic models.The objective of this study is to investigate the autoignition charact...N-hexadecane is a potential candidate of diesel surrogate fuels and is also the largest linear alkane(n-alkanes)with known chemical kinetic models.The objective of this study is to investigate the autoignition characteristics of n-hexadecane in the lowto-intermediate temperature region and to validate the existing kinetic models.In this study,the ignition delay times(IDTs)of nhexadecane were measured using a heated rapid compression machine(RCM)at two pressures of 7 and 10 bar,and over equivalence ratios ranging from 0.5 to 1.3.Two-stage ignition characteristic and the negative temperature coefficient(NTC)behavior of total ignition delay time were experimentally captured.This study paid special attention to the influence of pressure,equivalence ratio,and oxygen content on the IDTs of n-hexadecane.It is observed that both the total IDTs and the first-stage IDTs decrease with the rise of those parameters.It is worth noting that the first-stage IDT is found to show a greater dependence on temperature but a weaker dependence on other parameters compared to the total IDT.The observed IDT dependence in the lowtemperature region(LTR)were quantitatively described by ignition delay time correlations.The newly measured IDTs were then validated against two kinetic models(LLNL and CRECK).Simulation results show that both models underpredict the first-stage IDT but generally capture the temperature dependence.The CRECK model well predicts the total IDTs of n-hexadecane while the LLNL model significantly underpredicts the total IDTs at most investigated conditions.To the best of our knowledge,this study is the first investigation on n-hexadecane autoignition under low-to-intermediate temperatures,which deepens the understanding of large n-alkane oxidation and contributes to the improvement of the existing kinetic models.展开更多
基金the European Research Council with a“Horizon Europe:Marie Skłodowska-Curie Actions”grant and it can be disseminated freely.
文摘In this experimental work,an optically accessible rapid compression machine is used to study the ignition and combustion process under engine relevant operation conditions for five different air-natural gas equivalence ratios(λ)ignited with either 12.2 mg or 6.7 mg of pilot diesel injected at 1,600 bar.Initial temperature of the ambient mixture,walls and injector was 333 K.Additionally,for the short(6.7 mg)diesel injection,the variation in the ID(ignition delay)for two higher ambient temperatures(343 K and 353 K)was measured.Pressure and piston displacement are recorded while two high-speed cameras simultaneously capture signals in the visible range spectrum and at 305 nm wavelength for OH^(*)chemiluminescence respectively.ID is measured both from OH^(*)and pressure rise.From the recorded data,the heat release ratio is estimated and compared with the visual signals.This gives an insight of the temporal and spatial evolution of the flame,as well as a qualitative perception of the transition from spray ignition into a premixed flame in the ambient fuel-air mixture.It was found that increasing the methane concertation delays the ignition,reduces the natural flame luminosity and enhances the OH^(*)chemiluminescence signal.
基金supported by the National Natural Science Foundation of China(No.52076011)the Fundamental Research Funds for the Central Universities(No.2021JBM020)。
文摘In this study,the multi-objective intelligent optimization of the crevice structure in a rapid compression machine(RCM)is carried out based on the RCM simulation model modified with the KIVA-3V program.A multi-objective optimization simulation model of the crevice structure based on the large eddy simulation model coupled with the genetic algorithm NSGA-Ⅲis established.Six optimization parameters and seven optimization objectives are selected in the optimization process.The results show that the genetic algorithm can quickly find the values of the optimized parameters.The crevice volume ratio shows a trade-off relationship with the dimensionless temperature ratio T_(max)/T_(aver)and the tumble ratio.A larger crevice volume can reduce the flow of boundary layer cryogenic gas into the combustion chamber,thus improving the temperature uniformity.In addition,the crevice entrance width and the connecting channel length should be smaller,while the volume of the crevice main chamber should be larger,so as to sufficiently introduce the low-temperature gas of the boundary layer into the crevice and reduce their influence on the temperature field of the combustion chamber.When the crevice volume accounts for10%of the total volume,the temperature uniformity of the combustor is significantly enhanced,and when the crevice volume accounts for 30.4%,there is almost no low-temperature vortex in the combustion chamber.
基金The work at Xi’an Jiaotong University was supported by the National Natural Science Foundation China(Nos.51576159 and 91741110)the Shaanxi Provincial Key R&D Plan(Grant Nos.2019ZDLGY15-10 and 2019ZDLGY15-07).
文摘The diffusion of fuel spray in swirl flow is vital for the combustion of diesel engine,however,the researches on this is still mysterious due to the obstacles on direct investigations on a real engine.The research of intake swirl in engine at present normally use CFD simulation or based on data analysis of combustion and exhaust emission,the specific mixing process of fuel in swirl flow still not very clear.In this paper,a rapid compression machine(RCM)with an optical combustion chamber was established with the mean compression velocity of 7.55 m/s.Three kinds of flow fields,including intense swirl field,weak swirl field and turbulent field,were conducted to investigate the diffusion characteristics of injected fuel,the mixing processes of fuel spray in the swirl flow fields were recorded by the high-speed camera.Experimental results show that the mixing of fuel spray in air consists of four stages:fuel injection,wall approaching,rapid diffusion and final diffusion.The fuel spray is firstly bended by the lateral flow of the swirl field,then liquid fuel is blown onto,and coats on,the sidewall.The fuel on the sidewall is finally enrolled into the chamber plenum afterwards in gaseous phase via the spiral arms(in swirl fields)or random routes(in turbulent field)At last the fuel disperses all over the chamber.The results give an intuitive observation and help better understand the air-fuel mixing process in diesel and direct inject gasoline engines.
基金This study was supported by the National Key Research and Development Program of China(2018YFC 1900603,2018YFC0604604).
文摘Cemented paste backill(CPB)is a susta inable mining technology that is widely used in mines and helps to improve the mine environment.To investigate the relationship between aggregate grading and different affecting factors and the uniaxial compressive strength(UCS)of the cemented paste backill(CPB),Talbol gradation theory and neural networks is used to evaluate aggregate gradation to determine the optimum aggregate ratio.The mixed aggregate ratio with the least amount of cement(waste stone content river sand content=7:3)is obtained by using Talbol grading theory and pile compactness function and combined with experiments.In addition,the response surface method is used to design strength speaific ratio experiments.The UCS prediction model which ues the ISTM and considers the aggregates gradation have high accuracy.The root mean square error(RMSE)of the prediction results is 0.0914,the coefficient of determination(R^(2))is 0.9973 and the variance account for(VAF)is 99.73.Compared with back propagation neural network(BP-ANN),extreme lea ming machine(ELM)and madal basis function neural network(RBF ANN),LSTM can efectively characterize the nonlinear relationship between UCS and individual affecting factors and predict UCS with high accuracy.The sensitivity analysis of different affecting factors on UCS shows that all 4 factors have significant effect on UCS and sensitivity is in the following ranking:cement content(0.9264)>slurry concentration(0.9179)>aggregate gradation(waste rodk content)(0.9031)>curing time(09031).
文摘Granular material mechanics,finite element analysis and crushing theory are applied to study the compressive comminution mechanism of particle beds in this paper.This is a new method by which we have established an equivalent model of granular material,determined the values and distributions of contact forces and discovered a crushing law.The model has been tested on the newly designed equipment and proved to be correct.Some new characteristics and laws of compressive comminution of particle beds have been found.
基金supported by a Science,Mathematics,and Research for Transformation(SMART)fellowship hosted by U.S.Army’s Combat Capabilities Development Command(CCDC)Ground Vehicle Systems Center。
文摘Recent combustion research has focused on low temperature combustion to meet engine emission regulations and to advance the development of low temperature homogenous compression ignition engines.Autoignition studies in this temperature regime are primarily performed by Rapid Compression Machines(RCMs)which are sensitive to the heat transfer characteristics of the experimental device.RCMs are widely used to measure autoignition data such as ignition delay and species concentration.Measured ignition delays from RCMs are typically reported at an adiabatic condition;however,this assumption may produce a systematic error in ignition delay measurement as heat transfer is observed to reduce the pressure and temperature during the autoignition process,e.g.,a longer ignition delay has a greater pressure and temperature drops.RCMs are custom built and have unique design characteristics that affect the heat transfer during the autoignition process.In addition,depending on the diluent composition(e.g.,helium versus nitrogen or argon),different heat transfer characteristics are expected.As a result,autoignition results at similar conditions may vary from facility to facility or depending on the used diluent.The dependency of the measured data on the used facility or diluent may produce uncertainty in the data which impact the development of high-fidelity combustion mechanisms.In this work,a new method is developed and utilized to eliminate heat transfer from the ignition delay data.To evaluate the new method,the autoignition of n-pentane mixtures in the low temperature regime were investigated using an RCM.To vary the heat transfer,the compression ratio of the RCM was changed and the ignition delays were measured at similar pressure and temperature conditions.The tests were performed at an equivalence ratio of approximately one and nitrogen and argon as diluents.By applying the new method,the effect of heat transfer on the ignition delays were eliminated successfully and ignition delays at adiabatic condition were determined.A detailed kinetic model of n-pentane was used to simulate the measured adiabatic ignition delay,which agreed well with the experimental data.
基金supported by the National Natural Science Foundation of China(Grant No.2013CB228405)
文摘Ignition delay times of butanol isomers/n-heptane mixture were measured using a rapid compression machine at compressed pressures of 15,20 and 30 bar,in the compressed temperature range of 650–830 K and equivalence ratio of 1.0.Sensitivity analysis and reaction fluxes analysis were performed using a detailed mechanism of blend fuels so as to evaluate the impact of n-heptane addition and temperature variation on the ignition and combustion process.Over the experimental conditions in this study,the blend fuels displays apparent low and high temperature reactions and a negative-temperature-coefficient(NTC)behavior.With increasing butanol isomers mole fraction in the mixtures,the ignition delay times increase.It is worth noting that the suppression to n-heptane ignition from tert-butanol is very limited.The ignition delay time of 40/60 tert-butanol/n-heptane mixture is smaller than other three kinds of blends.With the increasing of tert-butanol mole fraction,the increasing range of its ignition delay time is very large.Moreover,compressed pressure has a limited effect on the ignition of blend mixture at low temperature but certain influence at medium temperature arrange.Tert-butanol/n-heptane mixture is not sensitive to the pressure.The chemical analysis indicates that butanol isomers also present the NTC behavior because of the low temperature reactivity radicals pool produced by n-heptane.Reaction fluxes analysis shows that the n-heptane addition has little impact on the reaction path.Sensitivity analysis shows that for the pure n-butanol,2-butanol and iso-butanol fuel,H-abstraction from the?-carbon plays the dominant role in the reactions having the inhibiting effect on the low-temperature branching,while the H-abstraction from the?-carbon can promote the ignition;for tert-butanol/n-heptane mixtures,reaction R16.H2O2(+M)<=>OH+OH(+M)plays the leading role.For n-butanol/n-heptane,iso-butanol/n-heptane mixtures,the major promoting reactions include some H-abstraction from n-heptane and OH branching reactions,the influence of H-abstraction from?-carbon is weaken;For 2-butanol/n-heptane,tert-butanol/n-heptane mixtures,R16 plays an absolutely dominant role,while the major inhibiting reactions add some elementary reactions of small radicals.
基金supported by the National Natural Science Foundation of China(Grant No.51425602).
文摘N-hexadecane is a potential candidate of diesel surrogate fuels and is also the largest linear alkane(n-alkanes)with known chemical kinetic models.The objective of this study is to investigate the autoignition characteristics of n-hexadecane in the lowto-intermediate temperature region and to validate the existing kinetic models.In this study,the ignition delay times(IDTs)of nhexadecane were measured using a heated rapid compression machine(RCM)at two pressures of 7 and 10 bar,and over equivalence ratios ranging from 0.5 to 1.3.Two-stage ignition characteristic and the negative temperature coefficient(NTC)behavior of total ignition delay time were experimentally captured.This study paid special attention to the influence of pressure,equivalence ratio,and oxygen content on the IDTs of n-hexadecane.It is observed that both the total IDTs and the first-stage IDTs decrease with the rise of those parameters.It is worth noting that the first-stage IDT is found to show a greater dependence on temperature but a weaker dependence on other parameters compared to the total IDT.The observed IDT dependence in the lowtemperature region(LTR)were quantitatively described by ignition delay time correlations.The newly measured IDTs were then validated against two kinetic models(LLNL and CRECK).Simulation results show that both models underpredict the first-stage IDT but generally capture the temperature dependence.The CRECK model well predicts the total IDTs of n-hexadecane while the LLNL model significantly underpredicts the total IDTs at most investigated conditions.To the best of our knowledge,this study is the first investigation on n-hexadecane autoignition under low-to-intermediate temperatures,which deepens the understanding of large n-alkane oxidation and contributes to the improvement of the existing kinetic models.