Heat recirculation effects on flame propagation and flame structure in a mesoscale tube
Abstract
Heat recirculation effects on flame propagation and flame structure are theoretically and experimentally examined in a mesoscale tube as the simplest model of heatrecirculating burners. Solutions for steady propagation are obtained using a onedimensional twotemperature approximation. The results show that the low heat diffusivities of common solid materials permit significant heat recirculation through the wall only for a slowlypropagating condition, otherwise the flame behaves almost like a freelypropagating nonadiabatic flame. This limited heat recirculation sharply pinches and stretches two wellknown branches of the freelypropagating nonadiabatic flame, resulting in the appearance of two slowpropagation branches. On the upper slowpropagation branch flames can reach superadiabatic temperatures and on the lower one, which is stretched from the classical unstable lower branch, flames can be stable. As the tube inner diameter decreases, another burning regime appears where flames are barely sustained by the heat recirculation. Further reduction of the tube inner diameter makes no flame exist. It is also revealed that a flame in a mesoscale tube has two length scales, i.e. the conventional flame thickness and a convective preheat zone thickness, and that the latter should be much larger than the former for significant heat recirculation. It is theoretically predicted that a heatrecirculating, even superadiabatic, flame with positive propagation velocity against the gas flow can exist in a mesoscale tube. It is also found that a flame transition from one branch to another in a given tube is well described by only one dimensionless parameter. Finally, these theoretical results show good qualitative agreements with experiments, especially for the transition behaviours.
 Publication:

Combustion Theory and Modelling
 Pub Date:
 June 2012
 DOI:
 10.1080/13647830.2011.638400
 Bibcode:
 2012CTM....16..507L
 Keywords:

 heat recirculation;
 mesoscale combustion;
 propagation;
 structure;
 transition