Reaction Zone Weakening Effects under Hot and Diluted Oxidant Stream Conditions

Abstract

This paper reports computational results, to complement experimental observations, on the turbulence–chemistry interaction of nonpremixed jet flames issuing into a heated and highly diluted oxidant stream. It is found experimentally that large-scale vortices and flame stretch can lead to spatial thinning and a decrease in OH concentration. This reduction in OH is described as a weakening of the reaction zone. Accompanying reaction zone weakening is also an increase in H₂CO levels. The reduction in reaction rates is most noticeable at low oxidant stream O₂ levels. The heated and low oxygen oxidant conditions typify those of moderate or intense low oxygen dilution (MILD) combustion. The computational results indicate that the effects of the low oxygen levels of MILD combustion leads to both a reduction in reaction rates and an increase in transport of O₂ across the reaction zone. The relationship between the reaction rate and level of O₂ permeation suggests that a form of partial premixing can occur under MILD combustion conditions. This partial premixing leads to the formation of flame intermediates, which contribute to the stabilization of the flames. The permeation effects are most pronounced at high strain rates, which are commonly encountered in practical MILD combustors.

Keywords:

• JHC burner
• MILD combustion
• Reaction zone weakening

Acknowledgments

The authors would like to thank Dr. Zeyad Alwahabi for his assistance with this project. The financial support of The University of Adelaide and the Australian Research Council is gratefully acknowledged.