Thermal Performance of a Mesoscale Liquid Fueled Combustor

Abstract

Combustion in small scale devices poses significant challenges due to the quenching of reactions from wall heat losses as well as the significantly reduced time available for combustion. In the case of liquid fuels there are additional challenges related to atomization, vaporization and mixing with the oxidant in the very short time available. The present paper presents the experimental results from a novel meso-scale liquid fuel combustor. The liquid fuel employed here is methanol with air as the oxidizer. The combustor was designed based on the heat regenerating concept wherein the incoming reactants are preheated by the combustion products through heat exchange with the combustor walls. The combustor was fabricated from Zirconium phosphate, a ceramic with very low thermal conductivity (0.8 Wm -1 K -1 ). The combustor had rectangular double spiral geometry with combustion chamber in the center of the spiral formed by inlet and exhaust channels. Methanol and air were introduced immediately upstream at inlet of the combustor. The preheated walls in the regenerative section of the inlet channel also act as a pre-vaporizer for liquid fuel which vaporizes the liquid fuel and then mixes with air prior to the fuel-air mixture reaching the combustion chamber. Pre-vaporization by the hot narrow channel walls removed the necessity for a fuel atomizer. Self-sustained combustion of methanol-air was achieved in a chamber volume as small as 33 mm 3 . The results showed stable combustion under fuel-rich conditions. High reactant preheat temperatures (675K-825K) were obtained; however, the product temperatures measured at the exhaust were on the lower side (475K-615K). The estimated combustor heat load was in the range 50W-280W and maximum power density of about 8.5 GW/m3. This is very high as compared to macro scale combustors. Overall energy efficiency of the combustor was estimated to be in the range of 12 to 20%. This suggests further scope of improvements in fuel-air mixing and mixture preparation.