Evaluation of the Flame Lift-off Length in Diesel Spray Combustion Based on Flame Extinction

Abstract

The interaction of spray and combustion processes forms a complex system of physical phenomena undergoing in IC engines. Studying this interaction is important to determine strategies for simultaneously reducing soot and NOx emissions from diesel engines. Spray combustion interactions are evaluated by the flame lift-off length - the distance from the injector orifice to the location of hydroxyl luminescence closest to the injector in the flame jet. Various works have been dedicated to successful simulations of lifted flames of a diesel jet by use of various combustion modeling approaches. In this work, flame surface density and flamelet concepts were used to model the diesel lift-off length. Numerical studies have been performed with the ECFM3Z model and n-Heptane fuel to determine the flame lift-off length under quiescent conditions. The numerical results showed good agreement with experimental data, which were obtained from an optically accessible constant volume chamber and presented at the Engine Combustion Network (ECN) of Sandia National Laboratories. It was shown that at a certain distance downstream from the injector orifice, stoichiometric scalar dissipation rate matched the extinction scalar dissipation rate. This computed extinction scalar dissipation rate correlated well with the flame lift-off length. For the range of conditions investigated, adequate quantitative agreement was obtained with the experimental measurements of lift-off length under various ambient gas O2 concentrations and ambient gas densities.