Simulation of the Effect of Spatial Fuel Distribution Using a Linear-Eddy Model 2007-01-4131
Prior HCCI optical engine experiments utilizing laser-induced fluorescence (LIF) measurements of stratified fuel-air mixtures have demonstrated the utility of probability density function (PDF) statistics for correlating mixture preparation with combustion. However, PDF statistics neglect all spatial details of in-cylinder fuel distribution. The current computational paper examines the effects of spatial fuel distribution on combustion using a novel combination of a 3-D CFD model with a 1-D linear-eddy model of turbulent mixing.
In the simulations, the spatial coarseness of initial fuel distribution prior to the start of heat release is varied while keeping PDF statistics constant. Several cases are run, and as the initial mixture is made coarser, combustion phasing monotonically advances due to high local equivalence ratios that persist longer. The effect of turbulent mixing is more complex. For the case where the length scale of the initial distribution matches the integral length scale of turbulence, turbulent mixing leads to moderation of peak heat-release rate. The randomness of turbulence is captured in the simulation, and for the above case, cycle-to-cycle variation of the combustion is evident. In contrast, when the initial fuel distribution is significantly finer or coarser than the turbulence length scale, turbulent mixing does not affect combustion for two different reasons. For fine distributions, molecular diffusion alone homogenizes the fuel-air mixture prior to ignition, so turbulence adds nothing. For initial distributions that are coarse compared to the turbulence length scale, diffusion and turbulence are both ineffective at mixing, so again turbulence has a minimal effect on combustion.