Numerical study of Mild combustion with entrainment of burned gas into oxidizer and/or fuel streams

A numerical study has been performed to obtain basic knowledge of Mild combustion in which the fuel stream is preheated and diluted with various amounts of burned gas as well as the oxidizer stream. This situation occurs in application of Mild combustion in furnaces in which the burned gas might entrain into fuel and/or oxidizer streams via internal recirculation. For this purpose, a numerical model has been employed which consists of a network of plug flow reactors, counterflow diffusion flame solver and an equilibrium solver for generation of the burned gas. Detailed chemistry and multi-component transport model including Soret and Dufour effects have been used for the calculations. Stationary behavior of the model shows a considerable decrease in flame peak temperature with increasing dilution ratio for fuel dilution case as well as oxidizer dilution case. A decrease of the flame peak temperature is accompanied by a decrease of the NO formation. Further analysis of the NO reaction kinetics turned out that the prompt NO route plays a dominant role in the total NO formation. These observations were the motivation to investigate the autoignition behavior of each dilution case to study consequences of the application of each case. It appears that the most reactive mixture fraction is highly dependent on the dilution case and it can occur in a wide range of mixture fractions for different dilution cases. For a detailed comparison of autoignition behavior of dilution cases, the role of chemical and diffusion effects in this behavior has been clarified. Dilution ratio has been founded as a dominant parameter to control chemical effects and strain rate as a parameter to control diffusion effects. Dilution ratio appears to be an important parameter to determine the order of autoignition between dilution cases. Increasing strain rate delays the autoignition of dilution cases differently for each dilution case. (C) 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved.