Combustion performances of a novel micro-combustor with a backward facing step and an exhaust gas recirculation channel

Abstract

A novel micro-combustor with a backward facing step and an exhaust gas recirculation (EGR) channel was developed to operate at low mass flow rates of H2/air mixtures (1.33–5.34 × 10−6 kg/s). The function of EGR was realized by a high-speed jet issued from the combustor inlet with a diameter of 0.2 mm. Effects of inlet velocity (Vin) and equivalence ratio (ϕ) on combustion performances and EGR ability were investigated numerically. With the increase of inlet velocity, flame moved downstream and splitted at a critical velocity, and the flame temperature and outer wall temperature rise simultaneously. An increase in the equivalence ratio led to a larger reaction zone and higher flame temperature and wall temperature. The mass fractions of NOx emissions are very low with an order of magnitude of 10−10. Both the mass recirculation ratio (εm) and enthalpy recirculation ratio (εh) increased with the increase of incoming mass flow rate. However, for an identical mass flow rate, both εm and εh decreased with equivalence ratio. A dimensionless number, Fa, which was defined as the ratio of pressure difference between the inlet and outlet of EGR channel to the dynamic head of the jet flow, was introduced to evaluate the driving force of EGR. The Fa number decreased with the increase in the mass flow rate of jet flow, but exhibited a non-monotonic variation with equivalence ratio. The variation trends of εm, εh and Fa were interpreted by analyzing the variation regularities of parameters involved in their definitions.