MAP | CALMAIL |
Combustion Modeling Lab at UC Berkeley

Past Research

Development of Reduced Mechanisms for Numerical Modeling of Turbulent Combustion


J.Y. Chen (CML)

Workshop on "Numerical Aspects of Reduction in Chemical Kinetics"
CERMICS-ENPC Cite Descartes - Champus sur Marne, France, September 2nd, 1997


Abstract

Recent advances in automation of systematically reduced mechanisms are reported here with the aim to accelerate the development process. A computer algorithm has been developed enabling fast generation and testing of reduced chemistry. This algorithm has been used to develop various reduced mechanisms of methane-air combustion for modelling of turbulent combustion. A 10-step reduced chemistry has been extensively tested showing good performances in predicting a wide range of flame phenomena, including general flame characteristics, flame extinction limits, flame propagation speeds, and auto-ignition delay times. Strategies for using such extensive reduced chemistry for modelling turbulent combustion are briefly discussed.

Introduction

Modeling of coupling between turbulence and combustion is essential to applications which involve strong turbulence-chemistry interactions. For instance, in a lifted turbulent nonpremixed jet flame, strong turbulence-chemistry interactions are observed in regions near the jet nozzle exit where chemical reactions are not fast enough to compete with turbulent mixing. Partially mixed unburned mixtures of fuel and oxidizer are being formed at the base of jet and the unburned mixture is ignited later by burned products formed down stream of the jet. Several modes of combustion are possible, such as propagation of premixed flames with different mixtures and diffusion controlled flames at far downstream. Since turbulence-chemistry interactions play a decisive role in the stabilization of flame, modeling of lifted turbulent jet flames will need a reduced mechanism with sufficient description of chemistry important to ignition process. In addition, flame chemistry in diffusion controlled mode can be of different nature depending on the contents of the combustible mixture. The degree of turbulence-chemistry interaction in practical devices can vary from one mode to another depending on the operation regimes. Often a large detailed chemistry with several hundred steps is necessary to describe many different modes of chemistry regimes mentioned above. Consequently, development of reduced chemistry to cover a wide range turbulence-chemistry interactions presents a challenge task as only a small number of scalars is feasible under current turbulent combustion models.


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