The Effects of Nozzle Shapes and Pressures on Boundary Layer Flashback of Hydrogen-Air Combustor

A new apparatus for the study of bunsen burner flames at pressures up to 100 atmospheres is described. With hydrogenoxygen mixtures the flames at elevated pressures are always turbulent unless burner tubes with an inner diameter much smaller than 0.03 cm are used. The flash-back conditions of turbulent hydrogen-oxygen flames at atmospheric pressure and at 14.6 atmospheres are given by the critical velocity gradient at the wall of the burner tube. The temperature of the burner tip has a large effect on the flash-back tendency of the flame. For turbulent flames the critical flash-back gradient is larger than for laminar flames. The limit between stable flame and flash-back was found to be very sharp. From measurements of the flame pressures and from photographs of the flame cones it is concluded that the burning velocity of turbulent flames is not larger than that of laminar flames. Depending on the mixture ratio the burning velocities of hydrogen-oxygen flames at 14.6 atmospheres are 2 to 3 times as large as those of atmospheric flames. From a theoretical consideration it is concluded that the burning velocities of hydrogen-oxygen flames increase with pressure because the flame temperatures increase with pressure, and thus also the heat conduction from the burning zone into the unburned gas.