FLAME HOLDING BEHAVIOR OF DIFFUSION FLAME FORMED IN A BACKWARD FACING STEP

对后向台阶扩散火焰的持火行为进行了实验研究。实验采用台阶高度h=10~50 mm，台阶距喷油孔距离lf=20~200 mm，喷油速度Vf=5，10，15m/S，主风速度=30，40，50m/S，喷射角分别为30ﾟ，45ﾟ，60ﾟ，90ﾟ。利用激光测速技术测量了回流的速度分布。得到的结论如下：(1)再附着点与台阶高度的比值接近于恒定值Lr/h(5.5)，不依赖于台阶高度。(2)火焰稳定极限条件下燃烧的存在对速度分布和再附着长度没有明显影响。在等温条件下，用回流流场特性来考察火焰稳定性是合理的。(3)当燃油喷射方向与回流方向相反时，t…对火焰稳定性的影响。喷射角与火焰稳定极限的夹角主要取决于喷油速度和台阶到喷油出口的距离。当燃油喷射在相对较近的阶跃区时，随着喷射角的减小，燃油射流的逆流对回流的影响很大，火焰稳定性不受燃油射流速度的影响。因此，可以抑制火焰稳定性对燃油喷射速度的依赖。当燃油喷射到再附着区附近时，湍流大大增加，喷油速度和喷射角的影响都不明显。(4)根据提出的有效当量比PHi^*，在该条件下，火焰燃烧速度将平衡局部流动速度，根据燃料喷射的位置，可以区分火焰稳定条件。在回流速度和湍流强度较高的区域，喷油与夹带空气进入回流区的富燃料混合条件下，火焰将趋于稳定。另一方面，在回流速度和湍流强度较低的区域，预测了稀薄混合条件下火焰的稳定。较少

Flame holding behavior of diffusion flame formed in a backward facing step has been investigated experimentally. The experimental work is done with step height, h=10-50mm, distance from step to fuel injection hole, lf=20-200mm, fuel jet velocity Vf=5,10,15m/s, main air flow velocity=30,40,50m/s and injection anglerheta=30ﾟ, 45ﾟ, 60ﾟ, 90ﾟ. The measurement of velocity distribution of recirculating flow are performed by using LDV technique. Conclusions obtained are as fonows :(1) The ratio of the reattachment point to the step height shows nearly constant value lr/h (5.5) and does not depend on the step height.(2) The presence of combustion at the flame stability limit condition does not significantly change the velocity profile as well as the reattachment length. It is considered reasonable enough to examine flame stabilization by recirculation flow field characteristic in its isothermal condition.(3) When fuel is injected in inclined direction against the recirculation flow, effect of t … More he injection angle to the flame stability limit depends on the fuel jet velocity and the distance from step to the injection exit. When fuel is injected at relatively near step region, as injection angle decreases, effect of counter flow of fuel jet against the recirculation flow is strong enough such that flame stability is not affected by fuel jet velocity. Thus, the dependency of flame stability to the fuel injection velocity can be suppressed. When fuel is injected near the reattachment region, turbulence much increases so that effect of either fuel jet velocity or injection angle is not apparent.(4) Based on a proposed effective equivalence ratioPHI^* at which condition the flame burning velocity will balance the local flow velocity, the flame stabilization condition can be distinguished due to the location where the fuel is injected. At region where recirculation flow velocity and turbulence intensity is high, flame is predicted to be stabilized with fuel rich mixing condition between the injected fuel and the entrained air to recirculation region. On the other hand, at region where recirculation flow velocity and turbulent intensity is low, flame is predicted to be stabilized with lean mixing condition. Less