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 (θ = 90° 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 = 90° and 135° at Vin = 24 m/s, while the counterparts are 56.9% and 84.6% at Vin = 28 m/s. Numerical analysis demonstrates that at Vin = 24 m/s the intensity of elementary reactions as well as the heat release rate at flame tip is larger for θ = 90° 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 = 90° at low inlet velocities. At Vin = 28 m/s, the stretch effect becomes the key factor affecting flame stability. The maximum strain rate on the cavity ramp wall is 1.37×106s−1 for θ = 90°, which is much larger than the counterpart for θ = 135° (1.08×106s−1). The downstream flame is completely extinguished for θ = 90° at high inlet velocities. As a result, the combustion efficiency shows a sharp decrease for the combustor with θ = 90°.