Abstract
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.
Highlights
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
In comparison to the Conditional Moment Closure (CMC) model, in the progress variable approach, transport equations are solved for the progress variable rather than for all the species from the detailed chemical mechanism conditioned on the mixture fraction
A model for autoignition and heat release in turbulent flow was formulated and validated against experimental data of a hydrogen jet issuing into turbulent coflow of hot air
Summary
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 experiment of Markides and Mastorakos [4] on hydrogen autoignition in a turbulent coflow was chosen as a test case for an LES combustion model, which combines a composite progress variable with a probability density function (PDF) (in the LES context, the term “filtered density function (FDF)” is used.) of presumed shape for the mixture fraction. This configuration was modelled with Conditional Moment Closure (CMC) by Mastorakos et al [5] and Stankovicet al. After outlining briefly the numerical setup for validation studies, model results are compared against measurements
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