Soot Formation Modeling of n-Heptane Sprays Under Diesel Engine Conditions Using the Conditional Moment Closure Approach

Abstract

Numerical simulations of soot formation of n-heptane autoigniting spray in a constant-volume vessel under diesel engine conditions with different ambient densities (14.8 and 30 kg/m³) and ambient oxygen concentrations (8–21% O₂ mol fraction) were performed using two-dimensional, first-order conditional moment closure (CMC). Soot formation was modeled with a semiempirical two-equation model that considers simultaneous soot particle inception, surface growth, coagulation, and oxidation by O₂ and OH. Soot radiation was accounted for with an optical-thin formulation. Results are compared to experimental data by means of ignition delay time, lift-off length, and soot volume fraction distribution. Good predictions of ignition delay and lift-off for all nine cases are achieved. High volume fraction soot location and semi-quantitative distribution have been well described even with this comparatively simple soot model. The findings suggest that the conditional moment closure approach is a promising framework for soot modeling under diesel engine conditions.

Keywords:

• Autoigniting sprays
• Conditional moment closure
• Diesel engines
• Soot formation

ACKNOWLEDGMENTS

The present work has been conducted as part of the HERCULES-B project within EC‘s Seventh Framework Program, Contract SCP7-GA-2008-217878; financial support by the Swiss Federal Government (SER Contract Number 154,269, SFOE Grant Numbers 103,241 and 102,688) is gratefully acknowledged.