A Numerical Investigation On Premixed H₂/Air Combustion In Planar Micro-combustors

Currently,micro electromechanical systems are powered by chemical batteries.However,chemical batteries have a shortcoming of low energy density.Therefore,micro energy and power systems,which convert the high-energy-density chemical energy involved in fuels into electricity,have attracted extensive attentions and great progress has been made in this research field.Planar micro channel is frequently adopted structure for micro combustor.Meanwhile,hydrogen is most commonly used fuel.Numerical simulation is a method always adopted in the fundamental research and optimal design of micro-channel combustors.In the present thesis,we solved the following three issues which have not been fully addressed in the literature through numerical simulation:（1）The error exists in the numerical results when applying a two dimensional model and how to improve its prediction accuracy.（2）The effects of wall thickness and material on the critical inlet velocity at which flame inclination occurs.（3）Proposed a micro channel combustor with high and low conductive materials for its upstream and downstream walls to intensify flame stability in it.From the comparison between the results of 3D and 2D numerical simulation of a rectangular micro channel combustor,it is demonstrated that due to the underestimation of heat loss from external walls,large error will be brought about in 2D numerical simulation when the aspect ratio of micro combustor is relatively small.However,for the micro combustor with an aspect ratio above nine,the error of 2D numerical result can be reduced to below 5%.A modification is performed to ensure the heat loss ratio of 2D model be identical with that of 3D model,the prediction accuracy of 2D model is significantly improved.The effects of wall thickness and material on inclination limit（the critical inlet velocity at which flame inclination occurs）and blowout limit（the critical inlet velocity at which flame is blown out）of premixed H₂/air mixture are investigated through 2D numerical simulation for micro-channel combustors with large aspect ratio.The results show that the larger the wall thickness and thermal conductivity,the larger the flame inclination limit,or flame inclination does not appear at all.Similarly,the flame blowout limit rises with the increase of the wall thickness and thermal conductivity.Quantitative analysis reveals that the heat recirculation ratio via upstream wall to heat the unburned mixture will be increased while the heat loss ratio from external wall surface will be decreased when the wall thickness and thermal conductivity is larger.These will undoubtedly enhance the flame stability in the micro-channel combustor.To improve flame stability in micro-channel combustor made of quartz glass,we proposed a new type combustor that assemblied by Si C（high thermal conductivity）and quartz glass（low thermal conductivity）for its upstream and downstream walls respectively.Two dimensional numerical simulation results show that this new configuration can essentially increase flame inclination limit,blowout limit and combustion of premixed H₂/air mixture.Moreover,the longer the upstream Si C wall,the more evident of its positive effect.Analysis demonstrates that this is because the composite walls can increase the heat recirculation ratio and decrease the heat loss ratio of the micro-channel combustor simultaneously.In summary,the present study not only improves the simulation accuracy of combustion in micro-channels,but also provides a guidance for the optimal design of micro-channel combustors.