Thermal field structures in low Damkôhler reacting flows

Abstract

This paper is mainly concerned with the regime of distributed reaction zones of a well-stirred reactor, where the Damköhler number is below unity. It is an original study compared with the majority of work done in the field of the interaction between combustion and turbulence in premixed flames. The turbulent structures are subjected to opposing effects, i.e., reduction induced by chemical effects and expansion induced by the increase in mean temperature due to exothermic reactions. When Da >1, the first effect due to the chemistry can scarcely outweigh the expansion of the turbulent structures caused by the exothermic effect. This result is the major contribution of the present study. This phenomenon is not sufficiently known, both in terms of mechanism and quantitative effects. The first objective of this work is to try to provide a qualitative explanation, using an experimental and computational analysis based on certain hypotheses. These assumptions will then explain the difference observed between the behaviour of the temperature fluctuations in the case of reacting and non-reacting turbulent flow in low Damköhler number situations, where the exothermic effect is negligible. The present paper analyses and compares the fluctuating temperature structures inside this reactor for various mean temperature situations. The experimental study is conducted using fine-wire thermoanemometry. The results of numerical simulations obtained using the Navier-Stokes energy equations and the chemical species transport equations associated with a turbulence model are compared with available measurements. The predictions were within engineering accuracy of experimental data.