Performance of a Solid-Fuel Ramjet Combustor with Bypass Air Addition

The influence of bypass air addition on the performance of a solid-fuel ramjet combustor was investigated. Experiments were performed with cylindrical center-perforated hydroxyl-terminated polybutadiene fuel grains and unvitiated heated air at flight-relevant pre-ignition combustor inlet total temperatures. The core air mass flow rate was kept constant, whereas the bypass ratio was varied from 0 to 30% with consistent pre-combustion chamber pressures. The average regression rate and mass flow rate of the fuel were greatest for the 0% bypass ratio case. The regression rate decreased for the 30% bypass ratio and for cases in which carbon black was not added to the fuel grain. Increasing the mass flow rate of air bypassed to the secondary combustion zone reduced the overall chamber pressure. The thrust increased with the increasing bypass ratio. The effect of bypass air on the chamber pressure yielded lower characteristic velocities at the 30% bypass ratio than at the 0 and 15% bypass ratios. The combustion efficiency of the device was maximized for the 15% bypass ratio. The key outcomes of the bypass air addition are to lower the global equivalence ratio and to reduce the combustion chamber pressure such that there is a nonzero bypass ratio for which the combustion efficiency is maximized.