Numerical study on heat recirculation in a porous micro-combustor

Heat recirculation is crucial to sustaining and stabilizing flames in micro-combustion in which a strong thermal coupling between the combustor wall and gas mixture exists through thermal conduction. Filtration combustion, on the other hand, is able to recirculate heat through the solid matrix to the unburned gas mixture, representing a promising potential to further enhance heat recirculation, if applied in micro-combustion. A numerical study on heat recirculation in premixed H2/air filtration combustion in a planar micro-combustor with the channel height of H = 1 mm is carried out. Thermal non-equilibrium between the gas mixture and solid matrix is considered in the 2D numerical model. A parametric study is undertaken to examine the effects of key parameters on the two pathways of heat recirculation in the porous micro-combustor, they are, heat conduction in the combustor wall and conduction and radiation through the solid matrix. The porous micro-combustor has the low-velocity extinction limits as low as ∼0.2 m/s and the blowout limits in terms of critical equivalence ratios increase with increasing inlet flow velocity. Flame position and wall temperature are greatly influenced by the porosity (ɛ) and solid matrix thermal conductivity (ks) of the porous medium, but interestingly, the flame temperature seems unaffected within the velocity range studied. Upon quantifying the two pathways of heat recirculation, it is found that convective heat exchange between the gas mixture and solid matrix plays the dominant role, while thermal conduction in the combustor wall the secondary role in preheating the gas mixture. The gas-to-solid convection efficiency (ηg-s) increases with the decrease of ɛ or equivalence ratio (Φ), and the increase of ks. In contrast, the gas-to-wall convection efficiency (ηg-w) increases with the increase of ɛ or Φ. Careful selection of the wall material is important to ensure efficient heat recirculation through the combustor wall.