Numerical Studies of the Response of Flamelets to Unsteadiness in the Near-Field of Jets Under Diesel Conditions

Abstract

Turbulent time records of scalar dissipation rates are numerically generated in the near-field of a high Reynolds number turbulent jet. The response of one-dimensional unsteady diffusion flamelets to these records were studied under pressure and temperature conditions representative of those in diesel engines. n-Heptane was chosen as the diesel fuel surrogate, its oxidation chemistry modeled by a 1,540-step mechanism comprising 159 species. Unsteadiness in the scalar dissipation rates was observed to impact autoignition, extinction, and reignition in the near-field. Possible linkages between these observations and flame liftoff, as well as its prediction, in diesel engines are discussed. Steady flamelet models were shown to not capture unsteady extinction–reignition events and associated chemical phase-lag effects. Unsteady flamelet–progress variable (UFPV) models in which the progress variable uniquely identifies all the flame states corresponding to a given value of χ are more suitable for representing the physics.

Keywords:

• Diesel jets
• Extinction/reignition
• Laminar flamelets
• Unsteady effects

ACKNOWLEDGMENTS

This work was carried out while the second author was on sabbatical leave at the Combustion Research Facility (CRF) at Sandia National Laboratories in Livermore, CA. The authors thank Dr. Lyle M. Pickett at the CRF for many interesting and valuable discussions on diesel flame liftoff. They also thank him for reviewing a version of this paper, and offering valuable critical comments that helped to improve it. They thank Professor Vinicio Magi for useful discussions on the numerical aspects of the work. Computing resources from the National Center for Supercomputing Applications (NCSA) and the Rosen Center for Advanced Computing at Purdue University are gratefully acknowledged.