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微尺度环境下预混火焰稳燃方法的研究
Methods for Stabilizing Premixing Flame in Micro Scale
【作者】 汪洋；
【作者基本信息】 浙江大学 ， 工程热物理， 2010， 博士
【摘要】 现代便携式电子设备要求其配套的能源系统具有更长的供电时间。使用微型能源系统供电,由于能量密度大大提高,可大大延长续航时间。将庞大的火电厂系统,通过MEMS技术微型化,作为便携式电源是可行的。但微型能量系统的核心部件,微尺度燃烧器的尺寸为毫米级甚至微米级。在如此小的尺度内实现稳定燃烧而不发生熄火或吹脱,是微动力能源系统研究领域的难点问题。宏观尺度的基本稳燃方法主要通过提升燃料混合气体的温度、促进对流等方法提高反应强度,改善燃烧过程。因此在微尺度稳燃方案中可以依据相同的原则,使用内部加热,外部加热,催化燃烧,壁面热回流等方法提升反应温度或者降低熄火温度,从而改善燃烧稳定性。实验对象主要为内径为2mm的石英玻璃圆管,使用氢气/空气预混气体作为燃料。本研究通过试验研究各种稳燃方案在不同运行工况下的效果。测量燃料气体不同流量、当量比工况下的可燃极限的改变。并测量特定工况下的燃烧器表面温度分布,跟踪燃烧器运行过程中的能量散失情况。并结合数值模拟方法分析燃烧器内部的详细燃烧过程。抑制熄火由满足燃烧点火能入手。经过各种稳燃方法的试验,外部热风加热,或者直接预热燃料混合气体均可以稳燃。以预热稳燃为例：燃料混合气体总流量0.12L/min时,不预热时可燃极限当量比为0.339-3.639。在250℃预热温度时扩展到0.317-4.304。使用催化剂降低熄火温度,也可以抑制热熄火。对微型刚玉陶瓷燃烧器使用Pt催化剂的稳燃效果实验。结果显示总流量在0.12L/min时,催化剂使用前后浓相的可燃极限当量比由11.90上升到18.03。催化剂稳燃实验中观察到使用催化剂后,由于非均相反应对气相反应的抑制作用,催化燃烧器中反应强度变弱,壁面具有更为均匀和较低的温度分布。为观察均相反应和气相反应并存的燃烧过程,在不同材质的催化燃烧器中进行了实验对比,并结合数值模拟对燃烧器内部火焰进行研究。结果显示壁面材料影响催化燃烧器中的反应模式。具有低导热率的燃烧器有利于反应模式向气相反应转变,有助于提高反应强度。比如在燃料混合气体总流量0.12L/min,石英玻璃燃烧器中OH浓度数量级为10-3气相反应占主导。而在紫铜燃烧中OH浓度降低为10-10,非均相反应占主导。相应的反应中心温度由1474K降低到约1000K。由于反应强度变弱,催化燃烧器在抑制熄火同时,较易发生吹脱。
【Abstract】 The portable electronical device in the modern times requires the compatible power source with longer operation period. The micro power system has higher power density, thus prolongs the operation period effectively. Therefore, minimizing the large power plant system into portable power source, through MEMS technology, is feasible. However, the micro combustor, which is the core of micro power system, has scale of micro or micron meter. It is difficult to overcome quenching or blowout and stabilize flame in such small scale.The methods for stabilizing flame in macro scale is mainly enhancing the reaction intensity, through increasing the temperature of fuel mixture, or improving the circumfluence section. The similar principle is applied in the micro scale combustor. The micro flame stability is improved, through increasing the reaction temperature or decreasing the critical quenching temperature with methods of heating, catalyst, heat recirculation, etc. Experiment is conducted in the quartz-glass combustor with straight-tube shape, having inner diameter of 2 mm. The combustor is operated with H2/air premixture.The effects of different stabilizing methods are tested under various operation conditions. The stability limits, surface temperature distribution, and heat loss of the micro combustor during operation are measured under different flow rates and equivalence ratio of fuel gases. Numerical simulation is also applied to analyze the details of internal combustion processes.Inhibiting quenching requires sufficient heat for ignition energy. According to the experimental results,both heating the combustor and preheating premixture stabilize the flame. Take preheating as an example:at the total flow rate of 0.12 L/min, the equivalence ratio extends from 0.339-3.639 to 0.317-4.304 after the premixture temperature increases from environment temperature to 250℃. Catalyst stabilizes the combustion through decreases the quenching temperature. According to the experimental results of catalytic combustor, at 0.12 L/min, the equivalence ratio in the rich case increases from 11.90 to 18.03 after applying catalyst.In the catalytic combustor, the reaction intensity is weakened, because the heterogeneous reaction inhibits the homogeneous one. As a result, the combustor wall has relatively even and low temperature distribution. For the purpose of investigating the interaction of two reaction modes, performances of catalytic combustors with different wall materials are compared. Moreover, numerical simulation is applied to analyze the details of combustion processes. According to the simulation results, wall material affects the reaction mode. The combustor wall with lower thermal conductive coefficient converts the reaction mode to homogeneous reaction, thus enhances the reaction intensity. For example, at the total flow rate of 0.12 L/min, the order of magnitude of OH concentration is -3 in the quartz-glass combustor, indicating homogeneous reaction domains. But in the copper combustor, the order of magnitude decreases to -10, thus heterogeneous reaction domains. Accordingly, the reaction temperature decreases from 1474 K to 1000 K. Although the catalytic combustor inhibits quenching effectively, the weak reaction intensity may induces blowout.