Flame behavior near steps bounding layered flammable mixtures

A study was made of flame behavior near steps bounding layered flammable mixtures established over liquid or solid surfaces. Experiments were conducted by using a long and narrow tray filled with methanol at temperatures above the flash point and steps of various heights placed at one end of the tray. The flame movements near the steps were observed in detail by using high-speed schlieren photography. When the step height h is less than a few times the thickness δ u of the flammable mixture layer before ignition, a flame is shown to climb on the step and to propagate along its top surface. The distance l of flame propagation along the top surface is found to increase as h decreases or as δ u increases. The flame movements near steps are simulated by an inviscid theory in which convective motion ahead of the leading flame front is evaluated by replacing thermal expansion of gas due to combustion with an imaginary source behind the leading flame front. The results show that the theoretically simulated flame movements near steps agree fairly well with the experimentally observed ones. Both the experimental and theoretical results indicate that l/h depends on δ ue / h , the ratio of the effective initial thickness δ ue of the flammable mixture layer to h.