Determining factor for the blowoff limit of a flame spreading in an opposed turbulent flow, in a narrow solid-fuel duct

This study clarified the blowoff mechanism for a flame spreading in an opposed turbulent flow in narrow solid fuel ducts. To clarify this mechanism, two experiments were conducted. The first experiment was to investigate the influence of ambient pressure and fuel duct size on the blowoff limit. The results indicated that the flow velocity at the point when blowoff occurred, Vg,tVg,t, increased with ambient pressure. This tendency could not be confirmed by a well-known expression for the Damköhler number, which is defined as the ratio of the characteristic flow time to the characteristic chemical time. Subsequently, to clarify the determining factor for the blowoff, the second experiment, which observed the flow field near the flame leading edge, was conducted. The results show that the flow separation in front of the flame leading edge, which provided sufficient residence time of oxidizer and gaseous fuel, is necessary for the flame to spread in an opposed oxidizer flow. From the results, it is found that the oxidizer friction velocity, u∗u∗, which is an indicator of the turbulent momentum transfer, is the determining factor for the flame blowoff limit. When the friction velocity is larger than a critical value, flame blowoff occurs in the fuel duct, due to the absence of flow separation.