Dynamics of unsteady premixed flames in meso scale channels and the effects of varying the wall heating conditions

Abstract

Understanding the dynamics of flames at small scales opens up opportunities to enhance the performance of small-scale power generation devices, micro-reactors, fire safety devices, and numerous other systems that confine combustion to micro/meso scales. The current study delves into the dynamics of laminar premixed methane-air flames in mesoscale channels, subject to different wall heating conditions. Two external heaters, positioned at adjustable distances, are employed to create a bimodal wall heating profile on the combustor walls. The separation distance (d) between the heaters was varied, and the resulting flame dynamics were examined across a wide range of equivalence ratios and Reynolds numbers (Re). The observed dynamics were also compared against a baseline configuration that utilises a single heater to establish an axial unimodal wall temperature profile. Apart from the previously documented observations of unsteady flames with repetitive extinction and ignition (FREI) characteristics, this study identifies an additional unsteady propagating flame (PF) regime. While FREI appeared at stoichiometric and fuel-rich conditions, propagating flames were observed at the equivalence ratio of 0.8. Unlike the FREI regime where the flame extinguishes after a characteristic travel distance, propagating flames continue to travel till they reach the upstream end of the combustor tube, where they extinguish upon encountering a meshed constriction. These flames are associated with a characteristic heat-release-rate oscillation that couples with the pressure fluctuations at frequencies close to the natural harmonic of the combustor tube. These dynamics are discussed in detail with appropriate theoretical arguments to justify the observed trends.