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

对后台阶内形成的扩散火焰的火焰稳定特性进行了实验研究。实验工作在台阶高度h=10- 50 mm，台阶至喷油孔距离lf=20- 200 mm，燃油喷射速度Vf=5、10、15 m/s，主气流速度=30、40、50 m/s，喷油角θ =30 °、45 °、60 °、90 °的条件下进行。利用激光多普勒测速技术对回流区的速度分布进行了测量。结果表明：（1）再附着点与台阶高度的比值lr/h（5.5）几乎为常数，与台阶高度无关; (2)在火焰稳定性极限条件下燃烧的存在不会显著改变速度分布以及再附着长度。认为用等温回流流场特性来检验火焰稳定性是合理的。(3)当燃油沿与回流方向相反的倾斜方向喷射时， ...更多信息 火焰稳定极限的喷射角取决于燃料射流速度和从台阶到喷射出口的距离。当燃料在相对近的台阶区域喷射时，随着喷射角的减小，燃料射流对回流的逆流作用足够强，使得火焰稳定性不受燃料射流速度的影响。因此，可以抑制火焰稳定性对燃料喷射速度的依赖性。当燃油在再附着区附近喷射时，湍流度大大增加，因此燃油喷射速度和喷射角的影响不明显。(4)基于所提出的有效当量比Φ I ^*（在该有效当量比Φ I ^* 下，火焰燃烧速度将平衡局部流动速度），火焰稳定条件可以由于燃料被喷射的位置而被区分。在回流流速和湍流强度高的区域，火焰被预测为稳定的燃料之间的喷射燃料和夹带的空气到回流区的富混合条件。另一方面，在回流流速和湍流强度较低的区域，火焰被预测为稳定与贫混合条件。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