Lewis-number-induced and buoyancy-driven self-excitations in nitrogen-diluted laminar non-premixed free-jet flames
Lewis-number-induced and buoyancy-driven self-excitations in nitrogen-diluted laminar non-premixed free-jet flames
复制标题
氮气稀释层流非预混自由喷射火焰中路易斯数诱导和浮力驱动的自激
DOI：
10.1016/j.fuel.2013.05.030
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发表时间：
2013
期刊：
影响因子：
7.4
通讯作者：
S. Keel
中科院分区：
文献类型：
--
作者：
W. Lee;S. Youn;Jeong Park;O. Kwon;Young Ju Kim;J. Yun;S. Keel
在水平和垂直注入层流自由射流火焰中进行了实验，并施加了−10 kV的直流电场，以探索刘易斯数诱导和浮力驱动的自激之间的明显差异，这两种自激都是ofO(1.0) Hz，因此无法区分。采用直流电场水平喷射的方法，通过消除边缘火焰前积聚的、预热的、部分预混的混合物，并将提升的火焰附着在喷嘴上，抑制热损失引起的自激和浮力效应，从而识别路易斯数引起的自激。在位移速度与距离的相图中也可以看出每一种自激。在此基础上，给出了水平和垂直喷射自由射流火焰中初始燃料摩尔分数和喷嘴出口速度随火焰稳定性的函数图。结果表明：随着Lewis数（归一化喷嘴出口速度）的增加（减小），Lewis数诱导自激的Strouhal数先增大后减小；讨论了自由射流火焰在喷嘴出口附近形成的路易斯数自激机理。由于部分预混的预热混合物在边缘火焰前积聚而产生的浮力驱动的自激也表现为Strouhal数与Richardson数、相应燃料与空气的分子量之比、火焰前宽与平均间隔距离之比的函数依赖关系；同时，用Strouhal数对Richardson数的函数很好地描述了火焰闪烁引起的浮力自激。结果还表明，浮力驱动的自激是由火焰闪烁引起的浮力诱导的自激引起的。一旦在没有热损失自激的情况下启动浮力驱动的自激，它就会压倒其他自激（刘易斯数自激和浮力自激）。
Experiments in horizontally and vertically injected laminar free-jet flames with applying a DC electric field of −10 kV were conducted to explore distinct differences between Lewis-number-induced and buoyancy-driven self-excitations, both of which have been known to be ofO(1.0) Hz and thereby indiscernible. A horizontal injection method with the DC electric field, with which one could eliminate accumulated, preheat, partially premixed mixture in front of edge flame and also attach lifted flame to nozzle, was adopted to suppress heat-loss-induced self-excitation and buoyancy effects, thereby identifying Lewis-number-induced self-excitation. Each self-excitation was also discernible in the phase diagram of displacement speed versus stand-off distance. Based on these, flame stability maps as functions of initial fuel mole fraction and nozzle exit velocity were presented in horizontally and vertically injected free-jet flame. The results showed that the Strouhal number for Lewis-number-induced self-excitation increased and then decreased in increase (decrease) of Lewis number (normalized nozzle exit velocity). The mechanism of Lewis-number-induced self-excitation in free-jet flame, formed near nozzle exit, was also discussed. The buoyancy-driven self-excitation due to the accumulation of partially premixed preheated mixture in front of edge flame was also characterized well by the function dependency of a Strouhal number upon a Richardson number, the ratio of molecular weight of corresponding fuel to air, and the ratio of flame front width to mean stand-off distance; meanwhile the buoyancy-induced self-excitation due to a flame flicker was well described by the function of Strouhal number on a Richardson number. The results also showed that the buoyancy-driven self-excitation was originated from buoyancy-induced self-excitation due to a flame flicker. Once a buoyancy-driven self-excitation was launched in a situation without having a heat-loss-induced self-excitation, it overwhelmed the other self-excitations (Lewis-number-induced and buoyancy-induced self-excitations).