Effect of the cavity aft ramp angle on combustion efficiency of lean hydrogen/air flames in a micro cavity-combustor

Hydrogen is an idea fuel for micro energy conversion devices. This study scrutinizes the influence of cavity aft ramp angle (theta = 90 degrees and 135 degrees) on the conversion ratio of lean hydrogen/air combustion in a micro-cavity combustor. It is shown that the combustion efficiencies are 95.8% and 93.5% for theta = 90 degrees and 135 degrees at V-in = 24 m/s, while the counterparts are 56.9% and 84.6% at V-in = 28 m/s. Numerical analysis demonstrates that at V-in = 24 m/s the intensity of elementary reactions as well as the heat release rate at flame tip is larger for theta = 90 degrees owing to a shorter flame height and smaller stretch rate. Therefore, flame tip opening is alleviated and higher hydrogen conversion efficiency is achieved in the combustor with theta = 90 degrees at low inlet velocities. At V-in = 28 m/s, the stretch effect becomes the key factor affecting flame stability. The maximum strain rate on the cavity ramp wall is 1.37 x 10(6) s(-1) for theta = 90 degrees, which is much larger than the counterpart for theta = 135 degrees (1.08 x 10(6) s(-1)). The downstream flame is completely extinguished for theta = 90 degrees at high inlet velocities. As a result, the combustion efficiency shows a sharp decrease for the combustor with theta = 90 degrees. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.