Numerical Investigation On Combustion And Emission Mechanism In The Near Wall Region After Spray Impingement

Spray impingement is an unavoidable phenomenon in modern internal combustion engines.And it is a promising method of controlling the mixture formation and near wall combustion.Thus,understanding the spray atomization,mixture formation and combustion in the near wall region becomes extremely important.In this work,a variety of numerical simulation methods coupled with experiments in both constant volume system(CVS)and IC Engines were conducted to explore the spray/wall impact and near wall combustion.A boundary control method was then presented to improve the near wall combustion.Firstly,the thresholds of droplet splashing under different boundary conditions were proposed according to numerical and experimental results.It was found that the outcomes of droplet impact were determined by incident energy and boundary conditions.Clearly splash criterion was observed when low energy drop(Re<4000)impacted dry wall with room temperature.And the splash criterion could be defined by a combination of Oh number and Re number(Oh Re=17).However,uncertainty splash situation was detected when the droplet either containing higher Re number(Re>4000)or hitting hotter wall with temperature over fuel boiling point.In the same way,corona splash criterion existed(Oh^-0.4We=1400)when the wall film was less than 1/10 droplet diameter(H*<0.1)for wet wall impingement.But it became unclear when the film got thicker(H*>0.1).Based on the study of single droplet impingement,a sub model was proposed to estimate the droplet splashed mass and the distribution of secondary drops.Secondly,a new spray/wall impingement model and a modified wall film model were proposed.Spary/wall impact experiments in CVS with three different fuels,including n-tridecane,diesel and gasoline,were used for model validation.The result showed that the new model was able to predict the spray outcome and film development in a wider range of boundary conditions.Then the near wall combustion after spray impingement was investigated.With lower ambient temperature,typical pool fire was observed as the flame spread from the spray outer region to the inner region.With the increase of wall temperature,the unstable near wall flame could be improved owing to faster fuel/air mixing.When the ambient temperature is highly enough,stable near wall flame was formed.The mixture formation of downstream spray could be further improved by decreasing the impact angle.During the entire near wall combustion,soot was mainly produced in the rich fuel region,which was controlled by spray mixing and film distribution.While NOx was generated in high temperature region.Based on the study of near wall combustion,Thermal Barrier Coating(TBC)was designed for IC engine to control the near wall combustion.A mathematical model was built on Kiva3v to study the influence of TBC on engine combustion.The wall heat losses was reduced with TBC,resulting in higher in-cylinder temperature and peak pressure.With the expantion of high temperature region in the near wall zone,the soot oxidation was accelerated.Meanwhile,more NOx was generated.Then,the effect of thermal properties of TBC was studied.The result showed that reducing thermal conductivity combined with increased porosity would ensure a more adiabatic engine cycle.Finally,the near wall combustion process and pollutant formation of RCCI and GCI combustion were investigated based on the spray impingement model and TBC model.Spray impingement was observed in RCCI combustion with early injection.While in GCI combustion,no spray impingement was observed.With the application of TBC,the deposited fuel could be greatly inhibited,and the temperature of near wall region was increased with advanced combustion phase.Both HC and CO were reduced owing to increased combustion efficiency.Besides,the high soot concentration region shrunk accompanied by enlarged NOx region.