On the boundaries of the transitional regime of mass transfer during ethanol combustion on horizontal rear walls of a rib and a step

Mass transfer in the boundary layer on the surface behind a rib and a backward-facing step during combustion of evaporating ethanol on the horizontal top and bottom walls of the channel is studied. It is shown that there are two values of the air flow velocity at which flame blow-off occurs for each configuration. For a backward-facing step 3 mm high, two critical combustion modes are observed near the boundaries of the laminar-turbulent transition region. For a rib and for a backward-facing step, flame blow-off may occur at a low velocity of the air flow $\approx$ 6 m/s) if the flow regime approaches or deviates from the laminar mass transfer mode. At elevated velocities of the air flow ($\approx$ 19 m/s in experiments with a backward-facing step), flame blow-off is induced by the change from the transitional to turbulent mass transfer mechanism.