This study investigates the continuous transition from flame-spreading to stabilized combustion near the blow-off limit in\nopposed forced flow by using expanding solid fuel duct that makes distribution of oxidizer velocity in the axial direction. The\nstabilized combustion is a diffusion flame that appears in the Axial-Injection End-Burning Hybrid Rocket. The boundary between\nflame-spreading and stabilized combustion has not been investigated in detail. Polymethyl methacrylate (PMMA) rectangular\nducts were used as a fuel, and gaseous oxygen was used as an oxidizer. All firing tests were conducted at atmospheric pressure. The\ndiffusion flame traveled in the opposed-flow field where the oxidizer velocity increases continuously in the upstream direction.\nThe combustion mode changed when oxidizer velocity at the flame tip exceeded a certain value. The oxidizer velocity used in this\nexperiment ranges from 0.6 to 32.8 m/s. Experimental results show that a threshold oxidizer velocity of the transition can be\ndetermined. In this study, the threshold velocity was 26.4 m/s.
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