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.