Transient Computational Fluid Dynamic Modeling of Baffled Tube Ram Accelerator

Abstract

Transient computational fluid dynamic modeling in laboratory frame of reference was used to investigate baffled tube ram accelerator operating characteristics in inert and reactive mixtures. The minimum entrance Mach number that allowed supersonic projectile transit through a baffled tube filled with inert gas and the thrust generated in reactive flow were examined. The primary parameters considered were chamber-to-projectile diameter ratio, projectile geometry, baffle thickness, baffle spacing and baffle geometry. Axisymmetric and three-dimensional simulations used a dynamic mesh for a projectile moving at constant Mach (ranging from 1.8 to 5.1) through stationary baffles. The maximum baffle thickness and minimum baffle spacing at velocities near the minimum allowable entrance Mach number were both found to be ~53% of the projectile diameter. Further reducing the baffle spacing resulted in the projectile pushing a normal shockwave ahead of it like in a jet engine unstart. It was also found that thicker baffles and closer baffle spacing increased the projectile drag coefficient. Reactive flow phenomena were investigated by first establishing the combustion on the projectile in a smooth bore tube before it entered the baffled-tube section. A key finding from premixed methane and oxygen propellant simulations was that aftward-slanted baffles generated higher thrust than when the baffles were normal to the projectile or forward-slanted. This increase in thrust was associated with more complete propellant combustion in the annular baffle chambers around the tail and base sections of the axisymmetric projectile, as well as immediately behind it. It was also determined that residence time of the propellant during combustion process in the vicinity of the projectile at any given baffle, influences the overall thrust production. As the projectile Mach increases, both the propellant residence time and overall thrust level decrease.