Optimization of a Mesoscale Combustor Using Heat Recirculation and Porous Inert Media

Continuous improvement of integrated circuitry has allowed for the development of small, sophisticated portable electronics and microelectromechanical systems (MEMS) for a wide range of applications. Compared to the electronics and other system components, the batteries powering small electronics and MEMS are large and heavy. Thus, smaller and lighter power systems are required to advance future products. Electricity for small systems may be supplied by miniature heat engines, which transform chemical energy of fuel into thermal energy, kinetic energy and electricity with the use of combustors, turbines and generators. Combustion at small scales is challenging because system heat losses to the surroundings are large and flow residence times are short. Heat recirculation can be used to improve combustion performance by reducing these heat losses and preheating reactants prior to ignition. Practical heat recirculation systems must be small to keep the overall system volume and mass small. The objectives of this study were: (a) to investigate heat transfer in miniature combustors, and (b) to identify effective means of reducing heat loss from small combustors. The analyses indicated that axial conduction through the combustor wall and radiation across the preheating annulus were the most significant pathways for heat loss from the system. Several design improvements, including extended surfaces and porous inert media (PIM) were analyzed. A design featuring PIM in the annulus with a gap between the PIM and outer wall was the most effective method of reducing system heat loss.