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 heat-recirculating burners. Solutions for steady propagation are obtained using a one-dimensional two-temperature approximation. The results show that the low heat diffusivities of common solid materials permit significant heat recirculation through the wall only for a slowly-propagating condition, otherwise the flame behaves almost like a freely-propagating nonadiabatic flame. This limited heat recirculation sharply pinches and stretches two well-known branches of the freely-propagating nonadiabatic flame, resulting in the appearance of two slow-propagation branches. On the upper slow-propagation 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 heat-recirculating, 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.

Keywords:

• heat recirculation
• mesoscale combustion
• propagation
• structure
• transition

Notes

¹According to our experiments (unpublished), when a lean propane–air mixture at an equivalence ratio of, for example, 0.9 is supplied in a quartz tube (5 mm inner diameter, 1 mm thickness and 0.5 m length) with a mixture velocity less than 26 cm s⁻¹, flame propagation is on the fast-advancing branch of Figure 3. And if the mixture velocity is larger than 19 cm s⁻¹, the flame suffers from a thermo-acoustic instability over a certain range of axial position. In such a situation, if the mixture velocity is gradually increased, the flame smoothly transfers to the slow-moving branch of Figure 3, accompanied by the gradual suppression and then disappearance of the instability. It is believed that this stabilization is due to different flame structures (thicknesses) between the two branches, which may affect the synchronization between heat release rate fluctuation and acoustic pressure.