Dynamics and thermodynamics of co-flow non-premixed methane–air flame in a cylindrical micro combustor with heat recirculating wall

Abstract

Numerical studies have been made on co-flow non premixed methane air flame in a cylindrical micro combustor with heat recirculating wall. An optimum Peclet number Peopt based on air flow velocity has been observed at which the flame location from the combustor inlet is minimum and the rate of heat recirculation to fresh charge is maximum. The optimum Peclet number is almost independent of the outer wall Nusselt number NuE and the thermal conductivity ratio ks/kg. The rate of heat recirculation shows contrasting increasing and decreasing trends with NuE, depending upon whether Pe LT Peopt or Pe >Peopt respectively. A higher limit of Peclet number for flame blow-off and a lower limit of Peclet number for flame extinction have been identified for a given global equivalence ratio and given values of NuE, ks/kg and t/D (the ratio of wall thickness to tube diameter). The upper Pe limit decreases sharply with NuE, while the lower Pe limit is almost uninfluenced by both NuE and ks/kg. Heat diffusion and chemical reaction in the gas phase are found to be the dominant processes of exergy destruction. The second law efficiency shows a minimum value around 30% at an optimum value of Peclet number where flame stand-off becomes minimum.