Lunar ridges are a kind of familiar linear structures developed on the lunar surface. The distribution pattern, formation mechanism and research significance of lunar ridges are discussed in this paper. Single lunar r...Lunar ridges are a kind of familiar linear structures developed on the lunar surface. The distribution pattern, formation mechanism and research significance of lunar ridges are discussed in this paper. Single lunar ridges are usually distributed in the form of broken lineation, and, as whole, lunar ridges are trapezoidal or annular in shape around the maria. As to the formation mechanism, only volcanism or tectonism was emphasized in the past, but the two processes are seldom taken into combined consideration. On the basis of detailed analyses, the authors thought that tectonism is a prerequisite for the formation of lunar ridges, while volcanism is the key factor controlling their particular shapes. Finally, the authors pointed out that it is very significant in the study of lunar ridges to link the course of lunar structure evolution with the stress state in the lunar crust.展开更多
Olivine exposures at the central peak of Copernicus crater of the Earth＇s Moon have been confirmed by telescope observations and Clementine spectra data. Using these exposures as training sites, this study used a met...Olivine exposures at the central peak of Copernicus crater of the Earth＇s Moon have been confirmed by telescope observations and Clementine spectra data. Using these exposures as training sites, this study used a method of combining two spectral indices （950 nm/750 nm and 2000 nm/1500 nm）, one maturity index （Is/FeO）, and four chemical content indices （FeO, Mg, Al, Ca）, through a decision tree classifier, to map olivine-rich units on the west lunar surface based on mosaicked Clementine image （500 m pixel size）. Most classified olivine exposures are found inside craters or on their rays, suggesting that olivine exposures are directly associated with the impact processes. The results have been validated in several selected sites, though further validations with data from the newest missions are strongly needed.展开更多
Future high-power-density engines require high level of intake boost.However,the effects of boosting on mixing,combustion and emissions in existing studies are inconsistent.In this paper,the mixing,combustion and emis...Future high-power-density engines require high level of intake boost.However,the effects of boosting on mixing,combustion and emissions in existing studies are inconsistent.In this paper,the mixing,combustion and emission characteristics with intake pressures of 100-400 kPa at low,medium and high loads are studied.The results show that the increase of intake pressures is conducive to the enhancement of air entrainment,while the air utilization ratios are reduced,thus requiring injection pressure to be optimized to effectively improve the mixing.For the intake pressures of 100 kPa,the average chemical reaction path is low-temperature reaction route,while the path of higher intake pressures is dominated by high-temperature pyrolysis.For soot emissions,when the equivalence ratio is lower than 0.175,the oxygen in the cylinder is suficient,so the effect of temperature decrease is more significant,which leads to the increase of soot emissions with the increase of intake pressures.Otherwise,the effect of increasing oxygen concentration is more significant,so soot decreases accordingly.When the peak of global temperature is lower than 1800 K,the effect of the increase of oxygen concentration is more dominant,so the NO_(x)emission increases with the increase of intake pressures.Otherwise,the rule of NOxemissions is consistent with temperature changes.展开更多
Natural gas/diesel dual-fuel combustion strategy has a great potential to reduce emissions for marine engines while the high fuel consumption is the major problem.Pre-chamber system is commonly employed as the ignitio...Natural gas/diesel dual-fuel combustion strategy has a great potential to reduce emissions for marine engines while the high fuel consumption is the major problem.Pre-chamber system is commonly employed as the ignition system on large-bore dual-fuel marine engines especially under lean-burn condition,due to its advanced ignition stability and engine efficiency.However,the ignition and combustion mechanism in such dual-fuel pre-chamber engine is still unclear and the effects of in-cylinder swirl flow and mixture stratification on combustion require further investigation specifically.This paper numerically studied the detailed ignition mechanism and combustion process in a marine engine equipped with a pre-chamber ignition system,and revealed the flame development process in main chamber.Moreover,the effects of mixture stratification and swirl ratio on the combustion rate and further engine thermal efficiency are investigated under decoupled condition.The results mainly show that the jet flame develops along the pre-chamber orifice centerline at the initial stage and premixed combustion play an important role,while after that,heat release zone only exist at flame surface,and premixed flame propagation controls the combustion process.In addition,with higher swirl ratio the combustion rate increases significantly due to the wider ignition area.Mixture stratification degree plays a role in accelerating the combustion,either too high or too low stratification degree reduce the combustion rate,while a moderate stratification increases the combustion rate.And appropriate stratification degree by verifying the gas injection parameters can reduce fuel consumption in 0.3%.展开更多
A good understanding of the thermophysical properties of hydrocarbon fuels at supercritical pressure is important to research on experiment and numerical simulation of fuel supercritical spray.Experimental measurement...A good understanding of the thermophysical properties of hydrocarbon fuels at supercritical pressure is important to research on experiment and numerical simulation of fuel supercritical spray.Experimental measurements are difficult to conduct directly because of the extremely high pressure and high temperature.In this study,back propagation(BP)neural network,BP optimized by mind evolution algorithm(MEA-BP)and BP neural network optimized by genetic algorithm(GA-BP)are established to determine the nonlinear temperature-dependent thermophysical properties of density,viscosity,and isobaric specific heat(C_(2))of hydrocarbon fuels at supercritical pressure.Meanwhile,approximate formulas for these properties prediction are primarily proposed using polynomial fitting.In this paper,models that can predict three types of physical properties of three kinds of hydrocarbon fuels and their mixtures in a wide temperature range under supercritical pressure are established.In the prediction of density and C_(2),BP neural network has a good prediction effect.The results show that the MAPE is lower than 2%in the prediction of density and C_(2),but the MAPE of viscosity prediction is slightly higher than 5%using BP.Furthermore,MEA and GA are used to optimize the prediction of viscosity.The optimization effect and computation of the MEA is better than that of GA because MEA does not have the local optimization and prematurity problems.The present work offers an efficient tool to predict the thermophysical properties of hydrocarbon fuels over a wide range of temperatures under supercritical pressure which can be easily extended to other fuels of interest.It will be beneficial to the experiment and numerical simulation studies of supercritical sprays.展开更多
文摘Lunar ridges are a kind of familiar linear structures developed on the lunar surface. The distribution pattern, formation mechanism and research significance of lunar ridges are discussed in this paper. Single lunar ridges are usually distributed in the form of broken lineation, and, as whole, lunar ridges are trapezoidal or annular in shape around the maria. As to the formation mechanism, only volcanism or tectonism was emphasized in the past, but the two processes are seldom taken into combined consideration. On the basis of detailed analyses, the authors thought that tectonism is a prerequisite for the formation of lunar ridges, while volcanism is the key factor controlling their particular shapes. Finally, the authors pointed out that it is very significant in the study of lunar ridges to link the course of lunar structure evolution with the stress state in the lunar crust.
基金supported by the National High Technology Research and Development Program(No.2010AA12220101 and 2009AA12Z310)National Natural Science Foundation of China(No.40871202 and 41002120)
文摘Olivine exposures at the central peak of Copernicus crater of the Earth＇s Moon have been confirmed by telescope observations and Clementine spectra data. Using these exposures as training sites, this study used a method of combining two spectral indices （950 nm/750 nm and 2000 nm/1500 nm）, one maturity index （Is/FeO）, and four chemical content indices （FeO, Mg, Al, Ca）, through a decision tree classifier, to map olivine-rich units on the west lunar surface based on mosaicked Clementine image （500 m pixel size）. Most classified olivine exposures are found inside craters or on their rays, suggesting that olivine exposures are directly associated with the impact processes. The results have been validated in several selected sites, though further validations with data from the newest missions are strongly needed.
基金supported by the Natural Science Foundation of China(No.51921004 and U2241262)。
文摘Future high-power-density engines require high level of intake boost.However,the effects of boosting on mixing,combustion and emissions in existing studies are inconsistent.In this paper,the mixing,combustion and emission characteristics with intake pressures of 100-400 kPa at low,medium and high loads are studied.The results show that the increase of intake pressures is conducive to the enhancement of air entrainment,while the air utilization ratios are reduced,thus requiring injection pressure to be optimized to effectively improve the mixing.For the intake pressures of 100 kPa,the average chemical reaction path is low-temperature reaction route,while the path of higher intake pressures is dominated by high-temperature pyrolysis.For soot emissions,when the equivalence ratio is lower than 0.175,the oxygen in the cylinder is suficient,so the effect of temperature decrease is more significant,which leads to the increase of soot emissions with the increase of intake pressures.Otherwise,the effect of increasing oxygen concentration is more significant,so soot decreases accordingly.When the peak of global temperature is lower than 1800 K,the effect of the increase of oxygen concentration is more dominant,so the NO_(x)emission increases with the increase of intake pressures.Otherwise,the rule of NOxemissions is consistent with temperature changes.
基金the financial supports provided by the Low-speed Marine Diesel Project(Project No.CDGC01-KT0308)National Natural Science Foundation of China(Grant No.91941102)。
文摘Natural gas/diesel dual-fuel combustion strategy has a great potential to reduce emissions for marine engines while the high fuel consumption is the major problem.Pre-chamber system is commonly employed as the ignition system on large-bore dual-fuel marine engines especially under lean-burn condition,due to its advanced ignition stability and engine efficiency.However,the ignition and combustion mechanism in such dual-fuel pre-chamber engine is still unclear and the effects of in-cylinder swirl flow and mixture stratification on combustion require further investigation specifically.This paper numerically studied the detailed ignition mechanism and combustion process in a marine engine equipped with a pre-chamber ignition system,and revealed the flame development process in main chamber.Moreover,the effects of mixture stratification and swirl ratio on the combustion rate and further engine thermal efficiency are investigated under decoupled condition.The results mainly show that the jet flame develops along the pre-chamber orifice centerline at the initial stage and premixed combustion play an important role,while after that,heat release zone only exist at flame surface,and premixed flame propagation controls the combustion process.In addition,with higher swirl ratio the combustion rate increases significantly due to the wider ignition area.Mixture stratification degree plays a role in accelerating the combustion,either too high or too low stratification degree reduce the combustion rate,while a moderate stratification increases the combustion rate.And appropriate stratification degree by verifying the gas injection parameters can reduce fuel consumption in 0.3%.
基金supported by the National Natural Science Foundation of China (Grant Nos. 51922076 and 91941102)
文摘A good understanding of the thermophysical properties of hydrocarbon fuels at supercritical pressure is important to research on experiment and numerical simulation of fuel supercritical spray.Experimental measurements are difficult to conduct directly because of the extremely high pressure and high temperature.In this study,back propagation(BP)neural network,BP optimized by mind evolution algorithm(MEA-BP)and BP neural network optimized by genetic algorithm(GA-BP)are established to determine the nonlinear temperature-dependent thermophysical properties of density,viscosity,and isobaric specific heat(C_(2))of hydrocarbon fuels at supercritical pressure.Meanwhile,approximate formulas for these properties prediction are primarily proposed using polynomial fitting.In this paper,models that can predict three types of physical properties of three kinds of hydrocarbon fuels and their mixtures in a wide temperature range under supercritical pressure are established.In the prediction of density and C_(2),BP neural network has a good prediction effect.The results show that the MAPE is lower than 2%in the prediction of density and C_(2),but the MAPE of viscosity prediction is slightly higher than 5%using BP.Furthermore,MEA and GA are used to optimize the prediction of viscosity.The optimization effect and computation of the MEA is better than that of GA because MEA does not have the local optimization and prematurity problems.The present work offers an efficient tool to predict the thermophysical properties of hydrocarbon fuels over a wide range of temperatures under supercritical pressure which can be easily extended to other fuels of interest.It will be beneficial to the experiment and numerical simulation studies of supercritical sprays.
