| In recent years,the rapid development of micro-electromechanical system(MEMS)has been widely used in the fields of optics,biology,automobiles,computers,aerospace,and military.As one of the most important power equipment,micro-burner is still insufficient in efficiency and stability of fuel combustion due to problems such as small size and large face to volume ratio.The development and application of high-efficiency and stable micro-burner has become a current hot issue.The use of hydrocarbon fuels and catalytic combustion are two effective ways to improve combustion efficiency and stability.Because hydrocarbon fuels have the advantages of high energy density,convenient replenishment of fuel,and ability to be used multiple times,catalytic combustion can reduce the activation energy of the reaction,avoiding flame-out due to free radicals generated by the flame,and can also use the larger surface area of the micro burner.The structure of the micro-channel combustion chamber will also have an important impact on the catalytic combustion of fuel.However,there are not many studies on the influence of the wall structure conditions of the micro-channel on the catalytic combustion characteristics of methane,especially the research on the effect of the rough structure on the wall on the catalytic combustion efficiency and stability of methane is even rarer.Therefore,it is necessary to carry out research on the effect of the rough structure of the micro-channel wall on the catalytic combustion of methane.In this paper,the detailed catalytic combustion reaction mechanism was compiled into a user subroutine and then imported into the Fluent platform for numerical calculation,and the effects of the rough structure of the micro-channel wall surface on the catalytic combustion of methane were carried out.Firstly,a comparison of methane combustion efficiency and combustion stability between rough-walled and smooth-walled micro-channels was carried out,and the advantages of rough-walled micro-channel were demonstrated,the effect of main reaction parameters on methane conversion ratio and the selection law of rough-walled micro-channel were obtained.Secondly,a detailed analysis of the catalytic combustion of methane in the rough-walled micro-channel was carried out,the catalytic combustion characteristics of methane in the rough-walled microchannels were clarified,and the reason why the rough-walled micro-channel could improve the efficiency and stability of methane combustion was revealed.Finally,the effects of the geometric parameters of the wall rough structure on the catalytic combustion of methane was studied,and the rough structure that could improve the methane conversion ratio and combustion stability under different rough structure forms was obtained,and the sensitivity law of the geometric parameters was analyzed.The main results of this article obtained are as follows:(1)The advantages of the rough-walled micro-channel were explained,and the law of the effect of the main reaction parameters on the methane conversion ratio and the selectivity of the rough-walled micro-channel were obtained.Under different reaction conditions,the conversion ratio of methane catalytic combustion in the rough-walled micro-channel is higher than that of the smooth micro-channel,and the extinction limit of methane in the rough-walled micro-channel is higher.Increasing the catalytic wall temperature,reducing the fuel inlet velocity,increasing the fuel inlet temperature and reducing the equivalence ratio can all increase the conversion ratio of methane.The catalytic wall temperature has the highest selectivity for rough micro-channel,followed by inlet temperature and equivalent ratio,the last is the inlet velocity,therefore,compared with other reaction parameters,appropriately increasing the temperature of the catalytic wall is the most effective way to increase the methane conversion ratio in the rough micro-channel.The selectivity of micro-channel with rough wall increases first and then decreases with the increase of catalytic wall temperature,gradually increases with the increase of inlet velocity and equivalence ratio,and remains basically unchanged with the increase of inlet temperature,therefore,the presence of the rough structures on the wall can inhibit the adverse effect of the increase in the inlet velocity or the equivalence ratio.(2)The catalytic combustion characteristics of methane in the rough-walled micro-channels were clarified,and the reason why the rough-walled micro-channel could increase the methane conversion ratio and stability was revealed.Near the rough structures on the wall of the micro-channel,free radicals,temperature,velocity,wall shearing force,methane reaction rate,and main components are all distributed in skipped manners.On both sides of the rough structure,the methane mass fraction reduces,the temperature rises,and the fuel flow rate increases.The mass fraction of methane at the outlet of the rough micro-channel is lower,the mass fraction of carbon dioxide and the temperature is higher.The presence of the rough structure provides more catalyst layout surfaces,which can generate more activated Rh(s)during catalytic combustion and enhance the elementary reactions near the wall.The wall shearing force on both sides of the rough structure is small,and the back of the rough structure can produce a low-speed recirculation zone,which strengthens the disturbance mixing of the fuel and prolongs its residence time in the micro-channel.The presence of the rough structure shortens the diffusion time of the reactant gas to the catalyst and increases the reaction rate of methane.Factors such as enhancing activation process,generating low-velocity reflux zone and increasing the reaction rate of methane affect each other and act synergistically,which are the main reasons for the improvement of methane combustion efficiency and stability of the micro-channel with rough wall.(3)The different rough structure forms that are helpful to improve the methane conversion ratio and combustion stability were obtained,and the sensitivity law of geometric parameters was analyzed.Appropriately increasing the ratio of height to diameter and width to length of the rough structure,increasing the number of rough structures,the shape of the rough structure being triangular,and arranging the rough structure downstream of the micro-channel are all beneficial to improving the catalytic combustion efficiency and extinction limit.In the sensitivity analysis of geometric parameters,after the height to diameter ratio of the rough elements are increased from 0.05 to 0.4,the average increase in methane conversion ratio is 15.2%,and the extinction limit is 18.9 m/s,which are higher than other rough structure forms,therefore,increasing the height to diameter ratio of the rough structure is the most effective way to improve the efficiency of methane catalytic combustion and the extinction limit.At the same time,it is necessary to avoid the adverse effects of excessively large height to diameter ratio of the rough structure or excessive number of rough elements,resulting in excessive return intensity near the outlet and temperature drop in the micro-channel.In this paper,without the combination of geometric parameters,the height to diameter ratio of the rough element should not exceed 0.3 and the number of rough elements should not exceed 4. |
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