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

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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