| Recently,with the development and improvement of micro-mechanical manufacturing technique,a variety of microelectromechanical systems with wide applications are develpoed,such as micro-satellite propellers,micro-sensors,micro-aircrafts,micro-medical equipments,and micro-motors.The applications of these microelectromechanical systems in military and civilian fields have improved the force of national defense and the quality of people's life.Currently,these microelectromechanical systems are driven by traditional power systems.However,these traditional power systems have the shortcomings of low energy density,large volume and short life time.Therefore,micro power generation systems with small size,low weight,high energy density and long duration time are desired to provide continuous and reliable power for these microelectromechanical systems.Micro power generation systems are a viable solution with hydrogen or hydrocarbon fuel combustion as the driving force.The micro-thermophotovoltaic system has the good applicable prospect due to its simple structure and no moving parts.The micro combustor as one of the most important components in the micro-thermophotovoltaic system is the key to converse the chemical energy of fuel into the electricity power.The improvement of combustion and heat transfer performance of micro combustors is a method to achieve high energy density and high energy conversion efficiency for the micro-thermophotovoltaic system.This work is supported by the National Natural Science Foundation of China[51176045],[51676066]and the High-level graduate program of China Scholarship Committee“Research on synergetic mechanism of heat flow in combustion process of a novel micro combustor”[201606130020].The theoretical and numerical methods are employed to investigate the combustion and heat transfer characteristics of hydrogen/air premixed flame in the micro tube combustors.The research results offer not only important guide for improving the thermophotovoltaic conversion efficiency of the micro-thermophotovoltaic system,but also the theoretical basis and reference for optimizing micro combustors.The main work and its novelty are summarized as follows:?1?In order to investigate the effects of different factors on the performance of micro tube combustor in the micro-thermophotovoltaic system,orthogonal experiment design and fuzzy grey relation theory were combined to investigate different factors?hydrogen/air equivalence ratio,gas mass flow rate,wall thermal conductivity and wall emissivity?on the emitter efficiency and combustion efficiency of micro circular tube combustor.The results suggested that the hydrogen/air equivalence ratio had the greatest influence on the emitter efficiency of the micro circular tube combustor in these four factors,and the wall emissivity had the most significant effect on the combustion efficiency of the micro circular tube combustor.Therefore,in order to improve the radiation performance and combustion performance of the micro circular tube combustor,it is necessary to firstly consider the hydrogen/air equivalence ratio and the emissivity of the wall material in the design of the micro circular tube combustor.?2?In order to improve the heat transfer performance of the micro circular tube combustor,a micro circular tube burner with rectangular rib was developed.And the mean wall temperature and wall temperature difference of the micro circular tube combustors with and without rectangular rib were compared under different hydrogen/air equivalent ratios and hydrogen mass flow rates.It was found that the micro circular tube combustors with rectangular rib owned higher and more uniform wall temperature.This was due to that the insertion of rectangular rib inside the micro combustor formed a recirculation zone behind the rib,and enhanced the heat transfer between the inner wall of the micro combustor and the flame.Then,the effects of the position and height of the rectangular rib on the outer wall temperature of the micro combustor were investigated.Results showed that the mean outer wall temperature was increased with the increase of dimensionless rib height when the dimensionless rib position was fixed at a constant and it exceeds 4/9,while the outer wall temperature difference was decreased with the increase of dimensionless rib height.The improved combustor with the dimensionless rib position of 5/9 and dimensionless rib height of 0.4 had the highest and most uniform wall temperature????w=1284.63K and?35?Tw=61.01K?where the H2 mass flow rate and H2/air equivalence ratio were kept at 5.25×10-7 kg/s and 1.0,respectively.The optimum dimensionless rib position was related with the inlet velocity of H2/air mixture and it was increased with the inlet velocity of H2/air mixture.?3?In order to improve the heat transfer performance of the micro circular tube combustor,a micro circular tube combustor with gradually reduced wall thickness is developed.And the mean wall temperature and wall temperature difference of the micro circular tube combustors with a step and with gradually reduced wall thickness were compared under different hydrogen/air equivalent ratios,hydrogen mass flow rates,wall materials and inlet/outlet diameter ratios.It was found that the micro circular tube combustors with gradually reduced wall thickness owned higher and more uniform wall temperature,which was beneficial to improve the energy conversion efficiency of micro-thermophotovoltaic system.?4?In order to understand the effects of cross sections of micro tube combustors on combustion characteristics of H2/air premixed combustion,combustion characteristics of H2/air premixed flame in the micro circular and elliptical tube combustors were compared under various H2 mass flow rates and H2/air equivalence ratios.Moreover,the effects of major/minor length ratio of the cross section of combustion chamber on the combustion characteristics were analyzed.It was found that the micro elliptical tube combustor had larger pressure loss and heat loss efficiency,while it owned higher emitter efficiency and combustion efficiency.If the advantage of the micro elliptical tube combustor was well utilized,the energy conversion of the micro-thermophotovoltaic system system was able to be greatly improved.?5?In order to improve the total output power of the micro-thermophotovoltaic system,coflow and counterflow double-channel micro combustor were developed.The mean wall temperature and its uniformity of the two double-channel micro combustors were compared under different hydrogen mass flow rates,hydrogen/air equivalent ratios and wall materials.Results suggested that the difference of the mean wall temperature between coflow double-channel micro combustor and counterflow double-channel micro combustor was small.However,the wall temperature uniformity of the counterflow double-channel micro combustor was higher than that of the coflow double-channel micro combustor.This is due to that the distance between the two individual flame centers in the counterflow double-channel micro combustor is longer than that in the coflow double-channel micro combustor.If the counterflow double-channel micro combustor wants to keep the merit,the H2 mass flow rate of each channel should be lower than 7×10-7kg/s and the H2/air equivalence ratio of each channel should be higher than 0.7,and the wall material should select steel and silicon carbide with higher thermal conductivity.?6?In order to extend the application value and range of the counterflow combustion in the optimization design of the multi-channel micro-combustor,the counterflow combustion was employed in the single-layer four-channel micro combustor and the double-layer four-channel micro combustor.The mean wall temperature and its uniformity of single-layer four-channel micro combustors and the double-layer four-channel micro combustors with various structure configurations were compared under different hydrogen mass flow rates,hydrogen/air equivalent ratios and wall materials.The results showed that whatever it was a single-layer four-channel micro combustor or double-layer four-channel micro combustor,the wall temperature distribution is mainly determined by placed directions of the adjacent combustion channels at each side of the micro combustors.The more counterflow channels existed in the adjacent channels at each side of the micro combustors,the more uniform the wall temperature of the micro combustors.Thus,the purely counterflow single-layer four-channel micro combustor and the purely counterflow double-layer four-channel micro combustor were the best project of single-layer four-channel micro combustor and double-layer four-channel micro combustor,respectively. |
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