Effects Of Heterogeneous Reaction On Homogeneous Reaction In Microscale Catalytic Combustors

With the rapid development of micro-machining technology,a variety of small or miniature machines and electromechanical equipment are extensively applied in various fields.The demand for power supply for these machines and equipment have increased significantly,and has led to many scholars paying more attention to micro power devices.Combustion based micro-power systems are expected to meet the requirements of MEMS,owing to their advantages of high energy density,small volume and light weight.However,there are some obstacles in the miniaturisation process of the systems,especially in the combustion process which is the core of energy conversion,such as low combustion efficiency and instability of flame.Catalytic combustion is well adopted in the micro combustors by taking advantage of its large surface area,leading to the enhancement of the catalytic reaction intensity.In this regard,catalytic combustion can extend the flammability limit,improve the combustion stability and increase the fuel conversion rate.However,the addition of catalyst significantly increases the complexity of the micro combustion.It is critical to clarify the coupling relationship between the heterogeneous reaction(catalytic reaction)and homogeneous reaction(gas-phase reaction)in the micro catalytic combustors.In this work,based on the review of current research progresses of micro combustion,the effect of heterogeneous reaction on the homogeneous reaction has been systematically investigated in the micro catalytic combustors by means of experimental measurement and numerical simulation.The research results with significance and practical value have been acquired:(1)The combustion characteristics under the non-/catalytic conditions were compared via changing the inlet velocities,equivalent ratios and oxidizers in the micro combustor which was fueled with hydrogen.The results showed that the heterogeneous reaction could increase the wall temperature and improve the uniformity of the wall temperature distribution.With the increase of the inlet velocity,the wall temperature increased gradually.When the equivalent ratio was 1.0,the highest wall temperature of the combustor was recorded.The wall temperature under hydrogen-oxygen premixed combustion was higher than that under hydrogen-air premixed combustion at the same fuel flow rate.(2)The effects of the inlet shapes,the channel heights,the thermal conductivity of the wall materials,the catalytic surface shapes and the catalytic surface areas on the hetero-/homogeneous combustion were analyzed via numerical simulation and experiment.The results showed that the fuel conversion rate in the rectangular cross-section combustor was higher than that in the triangular,square and circular cross-section combustors.When the inlet velocity was the same,the wall temperature of the catalytic combustor increased with the increase of the channel height.The wall temperature of the catalytic combustor decreased with the increase of the channel height,while the same hydrogen flow rate was adopted.For the wall material,the temperature difference of the outer wall decreased,the uniformity of the wall temperature distribution increased and the heat loss of the wall also increased with the higher thermal conductivity.When the triangular catalytic surface was employed in the combustor,the fuel conversion rate and the wall temperature were both higher than those in application of the circular and rectangular catalytic surfaces.With the increase of the catalytic surface area,the wall temperature and hydrogen conversion rate increased,but the inhibition effect of heterogeneous reaction on the homogeneous reaction was gradually enhanced.(3)Effects of heterogeneous reaction on the homogeneous reaction were obtained by analyzing the heterogeneous reaction path,net reaction rate of the critical species,the heat release rate in the catalytic combustor,heat transfer at the gas-solid interface and heat loss ratio of the combustor.The results showed that in the catalytic combustor,the heterogeneous reaction process on the catalytic surface was divided into three stages;the rapid depletion of reactants,fast consumption of free radicals and reaction balance state.The heterogeneous reaction intensity raised near the inlet due to the abundant reactants.The absorbed free radicals interdicted the homogeneous reaction at the vicinity of catalytic surface,resulting in the occurrence of the suppression effect of heterogeneous reaction.The part of the fuel was consumed by the heterogeneous reaction,so that the heat release rate of the homogeneous reaction in the catalytic combustor was lower than that in the combustor without catalyst.The intensity of heat transfer from the catalytic wall to gas mixture and the temperature of mixture both increased due to the exothermicity induced by heterogeneous reaction,thereby increasing the homogeneous reaction intensity.The heat loss ratio of the catalytic combustor was larger than that of the non-catalytic combustor,and the heat loss ratio of the outlet increased with the increase of the inlet velocity.(4)Effects of the products and heat release from heterogeneous reaction on the homogeneous reaction of hydrogen-oxygen mixture were investigated in the catalytic combustor.The results showed that the intermediate from the heterogeneous reaction could promote the homogeneous reaction on a small scale.A large amount of the final product was created in the heterogeneous reaction,and it enhanced the third-body effect in the homogeneous reaction,resulting in an inhibition effect of heterogeneous reaction on the homogeneous reaction.The maximum mixture temperature,OH concentration and the homogeneous reaction intensity near the wall were increased,due to the heat release from the heterogeneous reaction.The inhibition effect of the final product from heterogeneous reaction on the homogeneous reaction was higher than the promotion effect of intermediate and released heat from the heterogeneous reaction.Thus the homogeneous combustion was suppressed by the heterogeneous reaction in the catalytic combustor.(5)The combustion characteristics of methane-air premixed mixture under catalytic conditions were discussed.The effects of the heterogeneous reaction on the homogeneous reaction were analyzed by comparing with methane and hydrogen as the fuel at the same calorific value.The influence of heterogeneous reaction on the homogeneous reaction of methane-air mixture was studied.The results showed that the high temperature zone and the high species concentration region were located near the inlet in the catalytic combustor,and the product concentration near the inner wall of the inlet was higher than that on the centerline of the channel.When the methane was used,the combustion efficiency and wall temperature in the combustor were both lower than that of burning hydrogen.It indicated that the heterogeneous reaction intensity in the catalytic combustion of methane was higher than that in the catalytic combustion of hydrogen.In the catalytic combustion process of methane,the heterogenous reaction inhibited the homogeneous ignition,and the homogeneous ignition distance increased with the increase of heterogeneous reaction intensity.The heterogeneous reaction had a promotion effect on the homogeneous reaction at a low velocity.At a large velocity,the heterogeneous reaction suppressed the homogeneous reaction,attributed to the decrease of the heat release rate of homogeneous reaction,the decline of reaction rate,the reactants competition between heterogeneous and homogeneous reaction and a strong adsorption effect of catalytic surface on the intermediate from the homogeneous reaction.The results of this paper provide a basic theory for the analysis of the intrinsic chemical reaction mechanism of catalytic combustion,and also provide the technical prototype and basis for designing of the microscale catalytic combustor.