COMBUSTION AND HEAT TRANSFER IN MESO-SCALE HEAT RECIRCULATING COMBUSTORS

Abstract

Combustion in small-scale systems faces problems related to time available for chemical reaction to go to completion and the possible quenching of the reaction by the increased effects of interfacial phenomena (thermal quenching and radical quenching) that occur at the combustor walls due to higher surface to volume ratio. Heat recirculation, where in a portion of the energy from the products is fed back to the reactants through structural conduction is one of the strategies employed in meso-scale combustors to overcome the problems of thermal quenching of the flame. When liquid fuels are employed, structural conduction can help pre-vaporize the fuel and thereby removes the necessity for a fuel atomizer. This dissertation focuses on the design, development and operational characteristics of meso-scale combustors employing heat recirculation principle. Self-sustained combustion of propane-air and methanol-air flames were achieved in sub centimeter dimensions (32.6 mm3). The effects of design and operational parameters like wall thermal conductivity, heat exchanger size/channel length, combustion chamber geometry, equivalence ratio, Reynolds number, and external heat transfer (loss) coefficient on the combustor performance were investigated experimentally and numerically. The experimental procedure involved fabrication of combustors with different geometric features employing materials of different thermal conductivities and then obtaining their operating limits. Thermal performance with respect to various flow conditions was obtained by measuring the reactant preheating and exhaust gas temperatures using thermocouples. Numerical simulations were performed for both reacting and non-reacting flow cases to understand the heat transfer characteristics with respect to various design and operational conditions. Both experiments and numerical simulations revealed that wall thermal conductivity is one of the most important parameters for meso-scale combustor design. For typical meso-scale dimensions wall materials with minimal thermal conductivity (< 1W/m-K), especially ceramics would yield the best performance. Results showed that the most thermally efficient operating condition occurs for fuel lean cases at higher Reynolds numbers. Flame dynamics inside the combustor were investigated through high-speed imaging and flame acoustic spectrum mapping. Due to the small length scales involved, hydrodynamic instabilities have negligible effect on meso-scale combustion. Flame was observed to be extremely stable with negligible fluctuations. However, a significant amount of thermoacoustic phenomena is present within the combustion regime. Chemiluminescence imaging was employed to correctly map the flame zone inside the combustor.