Combustion Characteristics Of Micro Scale Combustor In Micro Electro Mechanical System And Its Optimization

Micro-electro-mechanical system technology has received extensive attention in recent years.Combustion-based small power generation devices are considered as suitable alternatives to conventional batteries due to their advantages,such as high-power density,light weight,short recharging time,and long charging duration.In addition,these devices can be used in micro fuel cells,micro gas turbines,and micro thermoelectric and thermophotovoltaic(TPV)systems.However,the short residence time of the mixtures and the high heat loss due to the high surface area-to-volume ratio of small combustors result in flame instability and low blow off limit of the system.To address these problems,the flame stability must be enhanced and the operating range of the system must be extended.The combustion performance of a plat combustion chamber with mixture that has constant and kinetic theory-based thermal conductivity,dynamic viscosity coefficient and mass diffusivity at the millimeter scale was numerically investigated.Results show that when the height of the combustion chamber is equal or less than 14 mm,setting the above factors in accordance to kinetic theory yields has higher accuracy than setting them as constant.When the height of the combustion chamber is less than 16 mm and the equivalence ratio is less than 1,the combustion characteristics of the combustor with constant parameters differ from those of the combustor with molecular kinetic theory-based ones.With inlet velocity increases and equivalence ratio decreases,this difference increases.The 60 inlet flame stabilizer of the original micro-combustor were developed into120 inlet flame stabilizers,which were then staggered in different radial positions in the six-and seven-wafer micro-combustors.The performances of the improved and original micro-combustors were compared using numerical simulation,and the effects of the geometric parameters on the former were discussed.The findings reveal that the staggered structure in the inlet increases the blowout limit of the silicon-based six-wafer combustion chamber from 0.4g/s to 0.7g/s.When the equivalence ratio is 0.7,the blends react with the recirculation jacket.The staggered structure in the inlet of the silicon-based seven-wafer combustion chamber increases the blowout limit from 0.7g/s to 0.8g/s compared with the original one.The highest combustion efficiency is observed when the lengths of the exterior and inner slots are 1.1 and 0.1 mm,respectively;and the distance between the two is 0.9 mm.The designs of the rectangular rib,front cavity and triangular rib in the front cavity were improved,and the effects of the combustion performances of the original and improved designs were numerically compared.The results show that the rectangular rib enhances the heat transfer between the inner walls and the burned blends because its recirculation zone is larger than that of the original,which results in high wall temperature.In addition,the thermal resistance in the horizontal direction increases with the downstream and upstream step lengths.Consequently,the outer wall temperature of the improved combustor decreases with the step length.The combustion efficiency of the combustor with rib is slightly lower than that of the original one.The front cavity significantly improves the combustion efficiency,outer wall temperature uniformity,and average outer wall temperature of the micro-combustion chamber;increases the area of the high-temperature zone;and enhances the heat transfer between the wall and the mixture.Due to its relatively high uniform outer wall temperature and combustion efficiency and outer wall temperature,the micro-combustion chamber with a front cavity height of 0.5mm and length of 2mm is suitable for micro-TPV system applications.The triangular rib in the front cavity enhances the heat transfer between the mixture and the wall because the recirculation and low-velocity zone areas of the improved micro-combustion chamber are larger than those of the combustor without rib.These features also increase the residence time of the mixture in the cavities.The recirculation and low-velocity zone are as enlarge as the rib height of the improved micro-combustor increases.The micro-combustor with a rib height of 0.5 mm obtains the highest wall temperature and combustion efficiency and is therefore recommended for thermoelectric and TPV devices,which require sufficiently high combustor wall temperatures.