Large Eddy Simulation of Autoignition in a Turbulent Hydrogen Jet Flame Using a Progress Variable Approach

The potential of a progress variable formulation for predicting autoignition and subsequent kernel development in a nonpremixed jet flame is explored in the LES (Large Eddy Simulation) context. The chemistry is tabulated as a function of mixture fraction and a composite progress variable, which is defined as a combination of an intermediate and a product species. Transport equations are solved for mixture fraction and progress variable. The filtered mean source term for the progress variable is closed using a probability density function of presumed shape for the mixture fraction. Subgrid fluctuations of the progress variable conditioned on the mixture fraction are neglected. A diluted hydrogen jet issuing into a turbulent coflow of preheated air is chosen as a test case. The model predicts ignition lengths and subsequent kernel growth in good agreement with experiment without any adjustment of model parameters. The autoignition length predicted by the model depends noticeably on the chemical mechanism which the tabulated chemistry is based on. Compared to models using detailed chemistry, significant reduction in computational costs can be realized with the progress variable formulation. 1. Introduction Autoignition in nonpremixed and partially premixed turbulent flow is of interest for industrial applications such as sequential gas turbines or HCCI (Homogeneously Charged Compression Ignition) piston engines, see [1] for a recent review. The present work is motivated by the need for a more accurate model of reheat combustion in sequential gas turbine combustors [2, 3]. In a sequential gas turbine system, hot gas produced in a first combustion chamber expands through a high pressure turbine before additional fuel is injected in a second “reheat combustion chamber.” Since highly preheated exhaust gases enter the reheat combustor, autoignition plays an important role in determining flame position and thus emissions. A characteristic of autoignition is an induction period, during which a pool of radicals is built up without significant heat release. During that phase, temperatures are typically rather low, such that steady-state or partial equilibrium relations may not be invoked to reduce the chemical mechanism. The induction period is followed by a fast heat release phase, during which the intermediate radicals from the radical pool are consumed rapidly and combustion products are formed. If autoignition occurs in turbulent flows, the chemical and the fluid-mechanical time scales are often of the same order. Simplifying assumptions like fast (or slow)