Phenomenological Autoignition Model for Diesel Sprays Using Reduced Chemical Kinetics and a Characteristic Scalar Dissipation Rate

This study focuses on the development of an autoignition model for diesel sprays that is applicable to phenomenological multi-zone combustion models. These models typically use a single-step Arrhenius expression to represent the low-temperature chemistry leading up to autoignition. There has been a substantial amount of work done in the area of n-heptane autoignition in homogeneous mixtures. Reduced kinetic mechanisms with ten reactions or less have been proposed in the literature to represent the complex low-temperature oxidation of n-heptane. These kinetic models are attractive for multi-zone simulations because of the low number of reactions involved. However, these kinetic mechanisms and the multi-zone treatment of the fuel spray do not account for the effect of turbulence/chemistry interactions on the chemical reaction rate. In this work a correlation has been developed for the total ignition delay time that is a combination of the homogenous ignition delay and dissipation effects. The homogeneous ignition delay is predicted from a chemical reaction mechanism for n-heptane, and the dissipation effects are captured through a phenomenological expression for a characteristic scalar dissipation rate. The characteristic scalar dissipation rate includes effects of injection pressure, ambient density, and injector hole size. The characteristic scalar dissipation rate is compared to a critical scalar dissipation rate to assess the additional delay due to turbulence/chemistry interactions. The autoignition model was implemented into a multi-zone spray model and validated against constant volume ignition delay measurements of diesel sprays.