Entropy generation analysis of unsteady premixed methane/air flames in a narrow channel

Abstract

The unsteady combustion process which is incurred by increased heat loss and thermal/radical quenching is easy to occur in micro channel. In order to control the unsteady flames in microscale, it is essential to figure out the formation mechanism of the instability process. In this work, transient numerical simulation of premixed methane/air combustion in a narrow channel is conducted using the DRM-19 reaction mechanism. Depending on the wall temperature profile, two types of flames were observed: the oscillating flame and flame repetitive extinction and ignition (FREI). High wall temperature profile results in oscillating flame and low wall temperature profile results in FREI. Entropy generation analysis method was used to identify the relative contributions of different factors to instability process. The major reactions during different phases in the both types of flames are confirmed. The main difference of major reactions between the two types of flames is related to the high hydrocarbons which are mainly formed at low temperature. The entropy generation by chemical reactions is dominant during the flame propagation phase, while in flame extinction phase, the thermal conductivity plays an important role in unsteady flame dynamics. As the flame gets weakened, the entropy generation by thermal conductivity is caused by heat loss through the wall and to the fresh mixture. The frequency of oscillating flame is much higher than that of FREI, but the maximum gas temperature of FREI is higher than that of oscillating flame. This work can provide the guideline for utilizing the unsteady flames or keeping the micro combustion stable in the micro power system.