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微尺度火焰及微燃烧器的稳燃强化技术研究

Study on Micro Flames and Combustion Stability Enhanced Technology in Micro Combustor

【作者】 蒋利桥

【导师】 赵黛青;

【作者基本信息】 中国科学技术大学 , 热能工程, 2008, 博士

【摘要】 基于碳氢燃料燃烧的微动力/发电系统具有高的能量密度,能为便携式微型装置直接提供动力或作为电源供应电能,是传统化学电池的潜在替代产品,其用应前景广阔。微尺度燃烧特性及微燃烧器技术的研究对发展基于碳氢燃料燃烧的微动力/发电系统至关重要。本论文首先从微尺度火焰结构及其熄火特性详细探讨了微火焰稳定性变差的机理,然后针对微尺度下S╱V(表面积/体积)增大导致微燃烧器热损失急剧增大的问题,本论文提出一种基于热、质逆流组织壁面渗透燃烧场的方式来降低微燃烧器热损失,并从理论、实验及用应详细探讨了该技术的特点,研究结果对推动微尺度燃烧研究和微燃烧器技术发展有着积极的意义。论文第二章从基于燃烧的微动力/发电系统、微燃烧器、微通道内燃烧及微火焰几方面评述了国内外研究机构的主要研究工作成果及最新进展。通过分析认为尽管微动力/发电系统具有高能量密度的优点,但是由于微尺度燃烧方向的研究是门新兴学科,制约微燃烧器和微动力/发电系统性能的许多关键问题还没有解决,特别需要对微尺度燃烧过程、微燃烧器稳定燃烧技术等核心问题进行细致、系统的研究。论文第三章从燃烧过程中的流动、传热和传质以及燃烧化学反应无量纲参数的变化出发,在理论上简要分析了微尺度效应对燃烧过程的影响,得出了影响微尺度下燃烧器热损失的主要因素。针对本研究论文所提出的热、质逆流组织燃烧场机理降低热损失方法,以圆柱多孔介质壁面渗透燃烧过程为物理模型,建立和分析了该燃烧方式下壁面内、以及火焰与壁面之间的换热模型,并给出了壁面附近未燃气膜内温度场分析解。论文第四章对静止空气中的自由射流甲烷扩散和甲烷/空气预混合微火焰进行了详细的实验研究,从火焰结构和熄火特性考察了燃烧过程的微尺度效应。实验结果表明微尺度下射流火焰均为层流火焰,火焰均呈蓝色;无因次化参数H╱d(火焰高度/喷管内径)与喷管出口Re数成正比线性关系,并分别拟合了扩散和预混合微火焰的H╱d~Re经验关系式,对于预混合微火焰,H╱d~Re关系式斜率还随混合气燃料当量比减小而减小。喷管直径的变小对微火焰的熄火吹熄极限速度影响很小,但随着喷管直径的变小,微火焰容易发生淬熄。特别是当微喷管当量直径小于0.5mm后,火焰的淬熄极限速度随喷管直径变小急剧上升,同时获得稳定预混合微火焰的临界当量比也随喷管直径减小急剧上升,实验拟合了微扩散火焰淬熄火极限速度与喷管直径变化经验关系式。微火焰燃烧过程数值分析结果表明,随着喷管直径的减小,热量扩散作用增强所导致的散热增大是微火焰熄火的主要原因,而对于大的当量比下的预混微火焰,其内层预混和外层扩散的双层火焰结构强化了微火焰的稳定性。研究结果表明微火焰的熄火机制是热量扩散和组分扩散共同作用的结果。论文第五章设计了一种管状多孔壁面渗透燃烧原理实验的微燃烧装置原型,其燃烧室尺寸为高度×直径=19.5mm×10mm。实验对比观察了不同进气方式下微燃烧装置内火焰形态及其稳定位置,结果证实了采用多孔壁面进预混气的壁面渗透燃烧方式可以在微燃烧室内获得稳定火焰。在壁面进预混气的基础上,通过在前端面中心进部分空气,可以拓展稳定燃烧总混合气当量比极限范围,但不能增加燃烧装置内的燃烧热负荷。在组织壁面渗透燃烧时,燃烧室内形成蓝色管状火焰沿整个燃烧室壁面分布,火焰厚度为1mm左右,火焰面与壁面之间的距离为1mm左右。燃烧工况(燃料当量比和燃料流量)变化时,火焰面上温度高于1200℃,而燃烧装置外壁面温度均低于350℃,多孔壁面与微火焰之间存在一层未燃气薄层,该气膜隔断了高温火焰与壁面直接接触,在火焰与壁面之间形成大的温差,使得微燃烧装置的热损失显著降低。热损失降低的原因为:(1)未然气膜薄层隔断了火焰与壁面的对流换热,使得微燃烧装置外壁面温度大大降低;(2)接近于燃烧室内表面积的管状火焰面使得燃烧室内火焰温度分布均匀,不存在局部高温,有效减少与温度四次方成正比的火焰辐射热量损失;(3)多孔壁面与冷预混气的高效换热将大部分壁面热损失回收。采用壁面渗透燃烧的微燃烧装置内火焰稳定性得到了很好的强化,其机理为:(1)壁面渗透燃烧场组织方式有效延长了反应区燃料的物理停留时间;(2)热损失的降低和预混气在多孔壁面内的高效预热阻止了化学反应时间的延长;(3)壁面渗透燃烧场组织方式有效地防止了壁面化学熄火。论文第六章实验研究了基于壁面渗透燃烧的自隔热微型燃烧器性能。微燃烧器内燃料转换率均大于97%,当量比在0.7~1.0范围时,燃烧效率稳定在90%以上,混合气流量(燃烧热负荷)的变化对燃烧效率影响不大,高的热负荷下微燃烧器热损失比例降低;在热负荷范围为60~140W/cm~3稳定燃烧时,整个燃烧器外侧壁面热损失率低于15%。

【Abstract】 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.

  • 【分类号】TK16
  • 【被引频次】6
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