Study On Flame Stabilization Mechanism In Micro-combustor Partially Filled Porous Medium

The characteristic size of micro-combustors is in the millimeter scale,resulting in flame quenching and poor flame stability.Filling porous medium enhances heat recirculation in the micro-combustor and effectively improves the flame burning velocity and consequently extends flame stability.It is found that different from conventional-scaled porous combustors,stationary flame can be achieved over a wide range of flow conditions in porous micro-combustors.However,the research on the flame stabilization in micro-combustors filled porous medium is still insufficient.In this thesis,a physical model of hydrogen/air premixed combustion in a micro-channel partially filled with porous medium is established.The effects of solid wall,porous medium,heat losses to the ambient on the flame characteristics are compared and analyzed,and the flame stabilization mechanism is presented.By analyzing the single-temperature model,it is found that the flame burning velocity in a porous medium is several times that of the free flame.The flame burning velocity increases monotonically with the increase of the equivalent ratio when the wall thickness is infinitesimal.The heat loss effect of solid wall(thickness of 0.5 mm)on low-velocity flame is prominent,which leads to the flame easy to be blown off.With the increase of the flow velocity,the preheating effect of the wall is gradually significant,and the flame burning velocity is increased.Therefore,the equivalent ratio corresponding to the stationary flame exhibits a"U"shape with the increase of the flow velocity.The two-temperature model takes into account the heat transfer between gas and solid phases in the porous medium,successfully capturing two distinct flame patterns(submerged flame and surface flame),and gives a wider range of stationary flame.The stabilization mechanism of both submerged flames and surface flames is closely related to the heat recirculation in the combustor.Although the main reaction zone of the surface flame is apart from the porous medium,the porous medium still play a vital role in heat recirculation.At the same time,the one-dimensional flame thickness of stationary flames is larger than that of free flames.The ratio(T_r)of the flame peak temperature to the adiabatic flame temperature and the ratio(R)of the total preheating to the total heat losses could reflect the heat recirculation in the micro-combustor.The results show that the higher the flow velocity is,the higher the flame burning velocity is,and the higher the required heat recirculation efficiency.In the vicinity of the flashback limit,the heat recirculation required by the flame changes slowly with the equivalent ratio,but the heat recirculation provided by the micro-combustor varies drastically along the axial direction.Hence,the flame is easier to be destabilize at the middle upper region of the porous medium.The heat losses from the outer wall increases the variation of the heat recirculation along the axial direction,which is beneficial to widening the range of stationary combustion.