Numerical Study On Fundamental Flame Characteristics Of Premixed H₂/Air Combustion In A Porous Micro-combustor

Micro-/meso-scale combustion which is researched with the development of MEMS has a great potential in generating portable power.Combustion technology with porous media become an effective technology to overcome the stability flame in small scale by enhancing thermal conduction,and thus allowing more effective heat recirculation.However,at the beginning of its study from the middle of the 1990’s,there is still a number of scientific and technical problem needs to be further researched.In this paper,A CFD study on fundamental flame characteristics of premixed hydrogen-air combustion in a planar porous micro-combustor is carried out using thermal equilibrium model and thermal non-equilibrium model.Compared with the thermal equilibrium model,the thermal non-equilibrium model considers the convective heat transfer coefficient between the gas mixture and solid matrix and thermal dispersion effects.The effects of flow conditions(U0 and Φ)and properties of the porous medium(ε and ks)and thermal conductivity of wall on the wall temperature,flame temperature,flame location,flame speed and so on are examined individually using thermal equilibrium model.The numerical results indicate that a porous micro-combustor gives a higher wall temperature and a lower flame temperature than a free flame counterpart.In the presence of porous medium,temperature distribution as well as species(OH)concentration is more uniform.Flame locations in a porous micro-combustor exhibit a U-shaped pattern against the change of inlet flow velocity.Choosing the porous material properly to have ks and kw in the same order of magnitude is helpful to localize flames.And the flame speed is found to be in a linear relation to the inlet flow velocity.A numerical simulation study on fundamental flame characteristics of porous micro-combustor is carried out by thermal non-equilibrium model as a useful supplement of the calculation of thermal equilibrium model.Effects of flow conditions and properties of the porous medium on the heat recirculation are mainly studied by thermal non-equilibrium model.The numerical results indicate that the thermal equilibrium model tends to underestimate the flame temperature and to predict the flame position more upstream than the thermal non-equilibrium model does.However,the difference between two models reduces when thermal conductivity of porous media is small.Flame position and wall temperature are greatly influenced by the porosity and solid matrix thermal conductivity of the porous medium,but interestingly,the flame temperature seems unaffected.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 heat recirculation through thermal conduction in the combustor wall plays a secondary role in preheating the unburned gas mixture.The gas-to-solid convection efficiency(η_g-s)increases with the decrease of ε or Φ,and the increase of ks.The capability of transferring heat through wall conduction is limited by the wall thermal conductivity(kw),and therefore the gas-to-wall convection efficiency(η_g-w)is smaller than the gas-to-solid convection efficiency(η_g-s)in a general way.Different from the gas-to-solid convection efficiency(η_g-s),the gas-to-wall convection efficiency(η_g-w)goes up with the increase of ε or Φ,and the decrease of kw.The above finding could provide some guidance to improve the efficiency of micro combustion.