Numerical study of wavy-wall effects on premixed H2/air flammability limits, propagation modes, and thermal performance of micro combustion chambers

A numerical study was conducted on planar micro combustion chambers with width of 2mm to explore the impact of wavy-walls on premixed H2/air flames and thermal performance. The numerical model incorporates radiation heat transfer, conjugate heat transfer, detailed chemical kinetics, and transport phenomena. Results indicate that wavy-walls facilitate the keeping and propagation of self-sustaining flames within the micro combustion chamber. In the equivalence ratio range of 0.4 to 0.6, the upper flammability limit of wavy-walls is three times higher than the flat walls. Various flame propagation modes, including closed symmetric flame, open symmetric flame, closed asymmetric flame, and open asymmetric flame, were observed depending on inlet velocity, equivalence ratio, and wall thermal conductivity. Quantitative analysis of these flame modes considered parameters related to eccentricity and asymmetry, revealing the significant role of wall thermal conductivity in flame asymmetry. Inlet velocity enhancement limits the equivalence ratio band for flames with eccentricity. In terms of efficiency, the wavy-wall micro combustion chamber succeeded in recording thermophotovoltaic efficiency of 8.89%, making it as a satisfying option for micro thermophotovoltaic applications. From a chemical perspective, the formation of open and closed flames was explained. Decreasing flame maximum temperature directs major reaction pathways towards increased production of OH and H2O near the wall, leading to absorb extra heat near the wall and reducing heat transfer to the centerline. This results in decreased centerline temperature, burning speed, and the formation of an open flame. Conversely, an increase in flame maximum temperature decreases the net production rate of H2O, promoting increased heat transfer to the centerline, elevated burning speed, and the formation of a closed flame.