Large Eddy Simulation Of Partially Premixed Combustion Regime With Linear Eddy Model

The partially premixed combustion(PPC)is an intermediate regime lying between the premixed combustion and the non-premixed combustion.PPC has the potential advantages for avoiding excessive pressure rise rate,incomplete combustion,and severe pollutant emissions simultaneously.Thus,it has been widely adopted in advanced internal combustion engines with low-temperature combustion.The ignition process of PPC is affected by both turbulent mixing and chemical kinetics,whereas the mechanism of auto-ignition has not been well understood yet.Therefore,it is difficult to effectively control the combustion phase of PPC,which restricts its application.In the present study,the spray combustion of n-heptane in the partially premixed regime was investigated by large eddy simulation.Special attentions have been paid on the ignition process under different thermodynamic conditions,the sub-grid scale fluctuations of reactive scalars,and the flame structure in the partially premixed combustion.In the present study,the computational model was developed base on the KIVA-3V code,and the linear eddy model(LEM)was introduced to investigate the two-phase spray combustion.Hence,the PPC in the spray combustion can be well reproduced without any a priori assumption on the combustion regimes.By separating the sub-grid scale processes into the turbulent stirring,chemical reaction,and molecular diffusion,LEM can quantitatively describe the sub-grid scale fluctuations of reactive scalars and well represent the turbulence-chemistry interactions.In addition,parallel computation of the chemical reaction rate was implemented,which significantly improves the computational efficiency.The new code was validated against the experimental data in a constant-volume combustion chamber from Sandia National Laboratories.The ignition delay times and flame shape of n-heptane spray combustion were well reproduced.First,the effects of the initial ambient temperature,ambient oxygen concentration,pre-injection proportion,and main-injection timing on the ignition process of n-heptane spray combustion in a constant-volume combustion chamber were comprehensively investigated.In addition,a new parameter was proposed to quantitatively describe the impact of fuel-air mixing on the ignition process.The results showed that the mixing effect played a more dominant role under higher ambient temperatures and higher oxygen concentrations,which means that the ignition mainly occurred in the well-mixed region.On the contrary,the chemical kinetics affected more significantly on the ignition process,hence ignition mainly occurred in the hotter region that suffered less from the cooling effect of fuel evaporation.The effects of pre-injection on the ignition process mainly depended on the intensity of the chemical reaction in the premixed fuel-air mixture.The intensity of the chemical reaction was primarily determined by the residence time when the pre-injection proportion was low(20%),while it was primarily determined by the ambient temperature when the pre-injection proportion was high(40%).Because chemical reaction fundamentally takes place at the molecular level,the homogeneous mixing of the reactive scalars in the sub-grid scale is essential for the ignition.Therefore,the ignition always took place in the region with minor temperature fluctuation(less than 0.5%)despite of the variations of thermodynamic conditions.Second,the sub-grid fluctuations of temperature and fuel mass fraction in the two-phase spray combustion were investigated.The results showed that the mean temperature would be overpredicted if the temperature fluctuation was neglected,which demonstrates the importance of scalar fluctuations in the sub-grid scale.The temperature fluctuation was generally attributed to the cooling effect of fuel evaporation and the chemical reaction.On the other hand,the rapid high-temperature combustion resulted in intense turbulence which further led to high-frequency stirring in the sub-grid scale.At the same time,the sub-grid scale scalar field can be smoothed out,which made the temperature fluctuation reduce more than 99%.Therefore,the intense temperature fluctuation was mainly observed in the region where chemical reaction rate was low.The sub-grid scale fluctuation of temperature is the weighted mean of the fluctuations of all the other scalars,hence the peak value of temperature fluctuation(about 3.5%)was relatively lower than other scalar fluctuations(about 4.5%).Chemical reaction led to intermediate fluctuation(1.0% ~ 2.0%)of fuel mass fraction.The fluctuation can be observed due to the inherent unsteadiness of combustion,even if the fuel-air mixture is homogeneous.Finally,the flame structure of the two-phase spray combustion as well as its effects on the heat release rate and emissions were investigated.The premixed flame and the diffusion flame coexisted in the spray combustion,which is a typical partially premixed combustion process.The proportion relation of the premixed flame and the diffusion flame could be affected by the thermodynamic conditions.The heat release rate in the premixed flame was higher than that in the diffusion flame.The gross heat release rate was found to be dominated by the premixed flame,and the evolution of the heat release rate strongly depended on the flame structure.The initial ambient temperature had minor influence on the adiabatic combustion temperature.However,the equivalence ratio-temperature distribution varied with the initial ambient temperature.With higher initial ambient temperature,more proportion of the fuel-air mixture distributed in the region,which elevated the NO_x emission.