Session Z15: Reacting Flows: Emissions and Soot (12:15pm - 1:00pm CST)

Fuel wall film effects on flame quenching and emissions in Gasoline Direct Injection (GDI) engines

The thermal efficiency and power output of Gasoline Direct Injection (GDI) engines have improved dramatically in recent years. Yet, these engines still suffer from relatively higher soot and unburned hydrocarbon (UHC) emissions. The injection of liquid fuel in GDI engines causes formation of a liquid film on the combustor wall, especially during cold start. This can cause fuel rich zones to form in the near-wall region which could fundamentally affect flame structure and lead to quenching. In this work, the interaction of a laminar premixed flame and a fuel wall film is studied using 1D direct numerical simulation with complex chemistry and transport under constant volume conditions. Fuel vaporization off the wall film is implemented as a boundary condition by using an analytical function which takes flame position into account. Parametric studies are conducted with various initial temperature of 500 to 650 K and pressures of 7 to 15 bar at a constant wall temperature of 400 K. Since gasoline usually consists of multiple hydrocarbon components, the fuel wall film composition is varied systematically to understand the effect on flame quenching characteristics. Emissions of UHC and soot precursors before and after flame quenching are also investigated in detail.   

Global pathway analysis for soot formation and evolution in pressurized sooting turbulent jet flames

Understanding on soot formation and evolution at pressurized flames are of significant interest due to the increasing operating pressures in different combustors and the accompanying increased soot emissions. In this study, a series of pressurized turbulent sooting flames at 1atm, 3atm and 5atm, are simulated to study the pressure effect on the soot formation and evolution. The inflow conditions are chosen such that the Reynolds number at different pressures keep constant. Via a radiation flamelet progress variable approach with a conditional soot sub-filter probability density function to consider the turbulence-chemistry-soot interaction, quantitatively good agreements are achieved for soot volume fraction predictions compared with the experimental data at different pressures. More importantly, a soot based global pathway analysis is employed to reveal the dominant chemical global pathway from fuel to soot in the mixture fraction, progress variable and physical space. The key global pathways that control the soot nucleation, condensation, surface growth and oxidation processes are also identified to show the significance of these processes under different pressures.   

Formation of soot spike in n-dodecane spray combustion

Numerical simulations are conducted to identify the underlying mechanism that governs the early soot evolution process in \emph{n}-dodecane spray flames at 21\,\%~O\textsubscript{2} level. The early evolution of soot mass, in particularly the soot spike phenomenon, is captured in the present large eddy simulation~(LES) case, but not in the Unsteady Reynolds Averaged Navier-Stokes~(URANS) case. Hence, a comparison of simulation results from LES and URANS is conducted to provide a better insight of this phenomenon. LES is shown to predict a rapid increasing in soot mass during the early stage of soot formation due to having a large favorable region for soot formation~(equivalence ratio~$> 1.5$ and local temperature~$> 1800\, \mathrm{K}$). This favorable region increases and then decreases to reach a quasi-steady state in LES, while it continues to increase in URANS during the early time. In addition, the formation rate does not increase continuously as soot precursor reaches a plateau, whereas oxidation rate continues to increase significantly in LES due to the ever increasing oxidizing species. This leads to a relatively dominant soot oxidation process over the soot formation process, which consequently results in the formation of soot spike in the LES case.