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

Abstract

Abstract Hydrogen is an idea fuel for micro energy conversion devices. This study scrutinizes the influence of cavity aft ramp angle (θ\xa0=\xa090° and 135°) 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 θ\xa0=\xa090° and 135° at Vin\xa0=\xa024\xa0m/s, while the counterparts are 56.9% and 84.6% at Vin\xa0=\xa028\xa0m/s. Numerical analysis demonstrates that at Vin\xa0=\xa024\xa0m/s the intensity of elementary reactions as well as the heat release rate at flame tip is larger for θ\xa0=\xa090° 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 θ\xa0=\xa090° at low inlet velocities. At Vin\xa0=\xa028\xa0m/s, the stretch effect becomes the key factor affecting flame stability. The maximum strain rate on the cavity ramp wall is 1.37 × 10 6 s − 1 for θ\xa0=\xa090°, which is much larger than the counterpart for θ\xa0=\xa0135° ( 1.08 × 10 6 s − 1 ). The downstream flame is completely extinguished for θ\xa0=\xa090° at high inlet velocities. As a result, the combustion efficiency shows a sharp decrease for the combustor with θ\xa0=\xa090°.