Study on Micro Flames and Combustion Stability Enhanced Technology in Micro Combustor
|School||University of Science and Technology of China|
|Course||Thermal Power Engineering|
|Keywords||Micro combustion micro flames quenching penetration combustion lower heat loss combustion stability enhancing self-thermal insulation micro combustor|
Micro Power/Generation Systems based on burning hydrocarbon fuels have higher energy density than those of existing batteries, so they are expected to substitute for conventional batteries and be the next-generation power source for portable and micro devices which have been developed quickly in recent years. It is urgent to elucidate micro-scale effects which influence micro combustion characteristics and micro combustor technology when R&D the high-performance micro power/generation systems. In this paper, the combustion stability and extinction mechanism of micro free-jet flames has been studied by investigating the flame structure and its quenching characterisitcs firstly. Then focusing on the problem that the heat loss of micro combustor increases quickly due to its larger surface/volume (S/V) ratio, a way based on the principle of opposite direction of heat transfer and mass flow to form wall penetration combustion for reducing heat loss has been conceived, and the mechanism of reducing heat loss and enhancing combustion stability has been investigated carefully in theory and experiment. The research results are significant to improve the researching in micro combustion field and to develop novel micro combustor technologies.In charter 2, the advancement of different micro power/generation systems, micro combustor and reactor technologies, and the micro combustion and flames have been reviewed in recent ten years. Because of the researching actives in this field just beginning lately, many problems, which limited the development of these micro systems and micro combustor technologies, such as how to keep combustion stable, to avoid extinction, and to improve the combustion efficiency, heat efficiency and energy conversion efficiency of overall system, need to be clarified.In chapter 3, it has analyzed the micro-scale effects in theory by comparing the changed characteristics of dimensionless parameters during flow, heat and mass transfer, and combustion process. Moreover, the factors which influenced the heat loss of micro combustor have been elucidated, and a penetration combustion model of micro combustor with cylindrical porous wall has been established.In chapter 4, both the combustion characteristics and the extinction mechanism of micro free-jet methane diffusion flames and methane/air premixed flames were investigated experimentally in quiescent air. The micro tube character size (inner diameter d) is varied from 0.2 to 2.0 mm for investigating micro-scale effects. Experimental results showed that the micro flames were blue laminar flames because of very small Re number of tube port. Flame height (H) was proportional to mean ejection velocity and Re number. The relationship between dimensionless parameter H/d and Re was proportional to linear both in diffusion and premixed micro flames, but to premixed micro flames, the slope ratio of H/d-Re was decreased as equivalence ratio decreased. When d decreased, the limit velocity of quenching increased quickly, but limit velocity of blow off was not varied much. Especially when d<0.5mm, the quenching limit velocity of diffusion flames increased sharply as d reduced, and the minimum fuel equivalence ratio of micro premixed flame increased quickly with smaller d. The results of computation simulation showed that heat loss increasing caused by heat diffusion enhanced had led to flame quenching while d decreased; At larger equivalence ratio of micro premixed flames, the double-flames structure, inside premixed flame and outside diffusion flame, made premixed flames more stable. It has demonstrated that the heat and mass diffusion control the micro combustion process. The mechanism of quenching of micro flames was caused by the effect of heat and mass diffusion enhanced together.In chapter 5, a concept and principle of the micro combustor prototype with wall penetration combustion based on mixture entering from wall has been designed and fabricated. The combustion room size was 10mm×19.5mm (Diameter d×Height H). Experiment results showed the blue flame was tubular with flame-front area as large as inside porous wall surface area, and the thickness of flame was about 1mm. The temperature of flame was above 1200℃, and an unburned gas film was formed between the flames and porous wall. The outside wall temperature of the combustor was no more than 250℃, and the temperature of porous wall was below 400℃. The lower temperature and small gap size of porous wall can stop flashback. It was demonstrated that the heat loss of the micro combustor was reduced remarkably because of forming about 1000℃temperature difference between flame and combustor’s outside wall. The mechanism of heat loss reducing is by the following three aspects: Firstly, the unburned thin film cuts off convection heat transfer between flame and porous wall, so heat loss from flames to wall is mainly by flame radiation. Secondly, larger flame-front area decreases the overall temperature of flames, so flame heat radiation which is proportional to four times power of temperature reduces significantly. Thirdly, lots of heat loss is recovery by preheating cold premixed gas because of very good heat exchanging effect in the porous wall. It also can enhance flame stability by the following aspects: Firstly, residence time is extended because of larger flame-front area declining flowed velocity of mixture in reaction zone. Secondly, preheating shortens the time of methane/air mixture preheated→ignition→combustion process, and heat loss reducing stops chemical reaction time prolonging. Thirdly, it can avoid chemical quenching with the thin unburned film to stop the active radicals moving from flames to lower-temperature porous wall.In chapter 6, the performance test of a self-thermal insulation combustor prototype showed that the CH4 conversion ratio was above 97% and the combustion efficiency more than 90% when equivalence ratio varied from 0.7 to 1.0. The combustion load was 50-140W/cm~3 in the 1.5cm~3 chamber. The combustion efficiency didn’t vary much when combustion load increased, and the total side wall heat loss ratio was less than 15%. The maximum NO emission was below 12ppm. This kind of micro combustor can be used to supply heat scourcs for micro power/generation systems directly.