Comparative study of combustion and thermal performance of a meso-scale combustor under co- and counter-rotating fuel and oxidizer swirling flows for micro power generators

Direct energy conversion systems, such as thermophotovoltaic and thermoelectric generators, have received increasing attention in micro power generation. Micro and meso-scale combustors are one of the most core components in these systems. So, developing combustion stabilization technologies for micro or meso-scale combustors is of great importance. In these systems, a hydrocarbon fuel with high energy density is burned in a micro or meso-scale combustor. Many studies have been conducted to explore various combustion stabilization techniques, but as a novelty, in this work, we study the combustion and thermal performance of a meso-scale micro power generator powered by a swirling fuel jet discharging into a co- or counter-rotating air coflow. To do so, we used 3D-printed axial swirlers with double annulus to form the swirling co- and counter-rotating fuel (methane) jet-air coflow configurations at various air and fuel flow rates. Blow-out limit, flame characteristics, combustor mean outer wall temperature, pollutant emissions, emitter efficiency, and normalized temperature standard deviation are investigated in this study. The results show that swirl addition enhances the blow-out limit significantly and co-rotating swirling flows generally enhances the flame blow-out limit when compared with the counter-rotating swirling flows mode at high fuel flow rates. Moreover, the combustor with co-rotating swirling flows has shorter lift-off height and longer flame length. The sensitivity of the flame lift-off height to an increment of the fuel mass flow rate is smaller in co-rotating than in counter-rotating swirling flows (more than $40\%$). Furthermore, it is observed that under the same operating conditions, co-rotating swirling flows exhibit lower values of $CO$ and $N{O}_x$ in the flue gas and higher values of mean outer wall temperature, combustion efficiency, emitter efficiency, and normalized temperature standard deviation. The enhancement of the emitter efficiency by implementing co-swirl configuration is about $35\%$, $26\%$, and $8\%$ for the methane flow rates of $0.050$, $0.100$, and $0.150slpm$, respectively when compared with the counter-swirl mode. The results of this work can provide useful information to choose between co- and counter-rotating flows for combustors of combustion-based micro power generators.