Visualization of burning flow around a ram accelerator projectile by coaxial simultaneous shadow/direct photography

Abstract.

Experimental investigations with flow visualization of a ram accelerator are reported. The ram accelerator at Hiroshima University uses a tube with a rectangular cross section and a two-dimensional projectile. Observation windows installed on the flat wall of the rectangular tube enable visualization of the flow field around the projectile. In addition to the instantaneous shadow photography method conventionally used for visualization, a direct photography method has been introduced. This direct photography can detect the precise position and brightness of combustion that cannot be observed by shadowgraph. The experiments were conducted at various projectile velocities in the thermally choked mode. The propellant mixture is stoichiometric methane-oxygen diluted by carbon dioxide. It is observed that a tiny flamelet, assumed to be the remains of the starting process, adheres to the projectile forebody, however, visible light emission could not be detected there by direct photography. The brightest emission from combustion is observed around the rear of the projectile, and strong emission is found in the boundary layer on the surface of the projectile afterbody and at the center of the tube behind the projectile. It is also found that the gas mixture is burned over a shorter distance when the projectile moves more slowly.