Combustion And Heat Thansfer Characteristics In The Micro Combustors

With the rapid development of MEMS systems in recent years, the requirementof vast micro and small equipments for the energy supply system is getting higher andhigher. Whereas, the chemical batteries have been unable to meet their requirements.Due to hydrocarbon fuels having advantages of higher energy density (about100times that of lithium batteries), lower cost and more environmentally friendly,micro-energy system based on the combustion of hydrocarbon fuels as a power sourceof the micro device increasingly attract the people's attention. But when the burner isreduced to the microscale, the impact of the surface heat loss to the enviromentalincreases sharply. The fuel ignition and stable combustion becomes extremelydifficult within the limited space. It will restrict the application and development ofmicro and small energy systems using hydrocarbon combustion if being able toeffectively solve these problems.By surveying the status of micro or small combustion in the domestic andforeign research field, the mutual coupling mechanism of solid wall-chemicalreaction-convection heat transfer-mass transfer-thermal conduction in micro or smallspace is not understood deeply and thorough. The homogeneous combustion and heattransfer process in micro space were studied with the numerical simulation andnon-dimensional theoretical analysis in this article. And the catalytic combustion heattransfer characteristics were experimental studied. The work and the main results areas follows:By adding and optimizing the solid plug in the flow channel (e.g. fins),Numerical simulations of premixed combustion in a micro scale rectanglechannel with or without fins were studied to investigate the effects of solid wallsto small-scale combustion, flow and heat transfer process. The physical andmathematical model containing solid walls was built and the accuracy was fullyverified. The effects of physical and geometrical parameters on combustion, flow, heatand mass transfer characteristics in a confined space were studied. The results showedthat: The inlet velocity and the channel gap have very important impacts on thelocation of the flame front. And the location of the flame front shifts downstreamalong the fluid flow with the increase of inlet velocity and channel gap. The blow outwill happen when the inlet velocity is large. In order to improve the combustion in thetenuous situation, the combustion models of a tiny tube or a flat chennel with finswere built. Through studing the effect of the walls with lengthways and transverse finson the combustion, flow and heat transfer characteristics in a small space, it could beconcluded that: the increasing of transverse fins in a small tube could carry more heat energy that fuel combustion released to upstream region and preheat the unburned gaseffectively and prevent the blow out phenomenon occurring. The combustion in amicro-scale rectangle channel could not be improved by adding lengthways fins alongthe inner walls. But the transverse fins on the inner walls postphone the reaction timefor pre-mixed fluid in the small channel. It promotes the combustion stably.By changing the shape of the flow channel, Numerical simulations of smallscale combustion in a Swiss-roll burner were undertook to investigate the effectsof gas inlet velocity, the mixture equivalence ratio and wall thickness on theprocess of combustion, flow, and heat and mass transfer. The methane/aircombustion model inside the small-scale Swiss-roll burner with a reverse convectiveheat transfer channel was established. The research results show that the key factorsaffecting the flame position is inlet velocity. The flame stabilized at the center of theburner only in a very narrow range because of excessive warm-up effect. When theinlet velocity is large, the wall heat conduction effect has been weakened by thestrength convective heat transfer along the fluid flow direction. The flame maintains asustained response and always stay at the first corner of reactant channel even if themethane concentration is very thin. When the inlet velocity is small, the methane/airmixture could maintain the chain reaction within the Swiss-roll burner only in verynarrow range (e.g. when inlet velocity v0=1m/s, the equivalence ratio ф=0.8~1.2orwhen the inlet velocity v0=0.5m/s, the equivalence ratio ф=1.0).The theoretical analysis of combustion and heat transfer processes in theone-dimensional burner with a convective heat transfer channel usingnon-dimensional analysis method was undertook. The analysis model assume that:the combustion reaction zone is equivalent to a WSR reactor. The convection heattransfer coefficient of between both sides of the middle partition and gas and betweenthe burner outer wall and the environment is a constant. The middle partition wall'sthickness is infinitely thin. The two ends of the interval wall are adiabatic. Theentrance effect on the heat transfer coefficient is ignored. The results shows the curvesof the interval wall temperature, the reactant temperature (cold fluid) and the resultanttemperature (hot fluid) along the dimensionless length L and the trend line of WSRreactor outlet temperature with the dimensionless mass flow rate M for these fourcases. They are H=0and Bi→∞, H=0and Bi≠0, H≠0and Bi→∞, H≠0and Bi≠0.The Pt-Al2O3catalysts suitable for small spaces were prepared using theanodic oxidation and impregnation method. A microscale combustor wasdesigned and fabricated. And the catalytic combustion of hydrogen on thePt-Al2O3catalyst surface at low temperature was experimental studied. Theeffects of mixture flow rate and the equivalence ratio of H2/Air premixed mixture onthe flameless catalytic reaction and heat transfer in the micro-scale burner were got.The average wall temperature of the catalytic reaction burner was about490K when the flow rate of1.0equivalence ratio mixture was336ml/min. The wall's averagetemperature of the burner was gradually reduced with the flow rate reducing. Whenchanging the equivalence ratio of the premixed mixture, the outer wall temperature ofthe burner first increased until it reaches a certain temperature and remained thecertain value with the equivalence ratio increasing from0.8to1.2.