Numerical investigations on the liftoff velocity of H 2 -air premixed combustion in a micro-cylindrical combustor with gradually changed section area

Sustaining and stabilizing flames are crucial issues in micro-combustion. In some micro-electro-mechanical systems such as the micro-thermophotovoltaic system, the flame should be formed in the combustion chamber, not outside it (combustion without liftoff). So, study of the liftoff phenomenon is important and vital in these systems. The aim of this study is to evaluate effect of changing combustor section area on the critical liftoff velocity in a micro-cylindrical combustor. For this purpose, the critical liftoff velocities are numerically identified for four combustor configurations (convergent, divergent, convergent-divergent and divergent-convergent combustion chamber). Premixed mixture of hydrogen-air has been used as reactants for the current investigation. Turbulence model implemented in this paper is RNG k-epsilon and combustion reaction was modeled with 10 species and 21 steps scheme using Eddy Dissipation Concept model. Two non-dimensional numbers d₁/d₂ (inlet to outlet diameter ratio) and d₁/d₃ (inlet to throat diameter ratio) are defined. For d₁/d₂ > 1.0, the combustion chamber is convergent, otherwise it is divergent. When d₁/d₃ > 1.0, the micro combustor is convergent-divergent and for d₁/d₃ < 1.0, the micro combustor is divergent-convergent. The results indicate that with increasing d₁/d₂, the liftoff occurs in a lower inlet flow velocity. With varying d₁/d₃, from 0.71 (2.0/2.8) to 1.0 (2.0/2.0), the liftoff velocity is reduced. Based on the numerical results, it can be said that the use of convergent and convergent-divergent combustion chamber decreases liftoff velocity. Meanwhile, the combustor with diverging and diverging-converging structure can enhance liftoff velocity. In the same condition, critical liftoff velocity of divergent-convergent micro combustor is the highest among all cases and this configuration is appropriate for Micro Electro-Mechanical Systems that work with high inlet velocity.