Combustion characteristics of hydrogen-air mixture in a radial micro combustor for using in thermophotovoltaic devices

摘要

Requirement of a fast rechargeable and powerful energy source for micro scale devices leads to increase attentions to various micro combustors for usage as heat source in micro thermophotovoltaic devices. Meanwhile, extracting as much thermal energy as possible from chemical energy of fuel by testing a variety of shapes of micro combustion chambers is at the heart of micro combustion subject. Present research has been purposed to find out performance and feasibility of a radial micro combustion chamber by numerical study of combustion characteristics of H-2-air pre-mixture and calculation of emitter and total efficiency of the system. Governing equations are considered three-dimensional, incompressible, and steady state for turbulent combustion. In addition, detailed kinetics of H-2-air combustion is chosen for the combustion mechanism. Inlet velocity, equivalence ratio, and wall material properties consisting of emissivity and thermal conductivity with external thermal convection coefficient are parameters, which have been chosen for investigations. Results indicated drastic effect of inlet velocity on the location of combustion zone. In fact, increasement of the inlet velocity causes to transfer combustion zone to the wall, which leads to increase wall temperature and consequently, emitter and total efficiency of the system. Equivalence ratio has more impact on the combustion temperature. While wall thermal conductivity and wall radiation emissivity play a decisive role in the total efficiency of the system. It was founded as the wall radiation emissivity decreases, heat recirculation increases. It causes to expand high temperature zone through the fluid and wall so that the total efficiency increases. By considering the present radial micro combustion chamber as a heat source for a micro thermophotovoltaic device, it was founded that maximum total efficiency of 3.14% is accessible for the condition in which the outer wall surface has maximum average temperature.