Effect of fine water droplets on extinguishment of diffusion flame stabilized in the forward stagnation region of a porous cylinder
Effect of fine water droplets on extinguishment of diffusion flame stabilized in the forward stagnation region of a porous cylinder
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DOI：
10.1016/j.proci.2010.06.005
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发表时间：
2011
期刊：
影响因子：
--
通讯作者：
H. Naito;Toru Uendo;Y. Saso;Y. Kotani;A. Yoshida
中科院分区：
文献类型：
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作者：
H. Naito;Toru Uendo;Y. Saso;Y. Kotani;A. Yoshida
本文以甲烷逆流扩散火焰为实验对象，定量研究了细水滴的物理、热力和化学效应对高应变扩散火焰膨胀的影响。水滴由双流体雾化器和超声波雾化器产生。用PDPA测量了水滴直径的分布，发现水滴直径的分布接近正态分布，其数均直径的典型范围为15 ~ 25μm，Sauter平均直径的典型范围为25 ~ 30μm。速度梯度和无因次燃料喷射速度决定了燃料喷射的极限。在没有水滴的情况下，临界速度梯度为423 s-1。水滴的引入导致火焰区的速度梯度降低，从而导致火焰区的氧气冷却和耗尽。临界速度梯度随表面积参数φ的增大而减小，表面积参数φ用水滴质量分数与Sauter平均直径的比值表示。通过简单的分析，将液滴的寿命与停留时间进行了比较，发现存在一个阈值，低于该阈值，液滴可以在火焰区内蒸发。PDPA测量还表明，直径大于阈值的水滴通过火焰区向滞止面移动。阈值被发现依赖于速度梯度。由热电偶测量的最高温度随着水滴的质量负载以及速度梯度而减小。然而，对流扩散火焰的最高温度与水滴的质量负荷和速度梯度无关，对流扩散火焰的最高温度主要是热效应。
The roles of the physical, thermal and chemical effects of fine water droplets on extinguishment of diffusion flame subjected to high strain are quantified experimentally using a methane counterflow diffusion flame. Water droplets were generated by twin-fluid atomizers and ultrasonic mist generators. The distributions of water droplet diameter were measured by a PDPA and found to be nearly normal, with the number mean diameter ranging typically from 15 to 25μm and the Sauter mean diameter from 25 to 30μm. The limit of extinguishment was determined by the velocity gradient and the non-dimensional fuel-ejection velocity. The critical velocity gradient at extinguishment without water droplets was found to be 423s−1. Introduction of water droplets into air stream resulted in reduction of the velocity gradient at extinguishment due to evaporation which causes the cooling and depletion of oxygen in the flame zone. The critical velocity gradient at extinguishment decreases with the increase of the surface area parameter, Σ, which is expressed by the ratio of mass fraction of water droplets to Sauter mean diameter. By a simple analysis, the lifetime of a droplet was compared with the residence time and it was found that there exists threshold below which droplets can be evaporated within the flame zone. PDPA measurement also showed that the water droplets with diameters larger than the threshold pass through the flame zone towards the stagnation plane. The threshold was found to depend on the velocity gradient. The maximum temperature measured by a thermocouple decreases with the mass loading of water droplets as well as the velocity gradient. However, the maximum temperature at extinguishment is rather independent of mass loading of water droplets and also of velocity gradient and the thermal effect is predominant in the extinguishment of the counterflow diffusion flame.