Experimental study on the temperature and structure of the exhaust plume in valveless pulse detonation engines

The experiments of multi-cycle detonation were carried out based on a direct-connected pulse detonation engine (PDE) and an air-breathing PDE. The ignition energy was less than 50 mJ generated by a spark plug. The gasoline and air were used as the fuel and oxidizer. The pressure variation along the length of the direct-connected PDE and air-breathing PDE at several different operating frequencies was measured by the piezoelectric pressure sensors. The temperature of the exhaust plume from the PDEs was measured by the temperature and water vapor concentration instrument based on infrared spectrum measurement principle. Meanwhile, the average thrust of the air-breathing PDE with three different nozzles, including convergent nozzle, divergent nozzle, and convergent-divergent nozzle was measured by the force sensor. The experimental results indicated that there was a reflected shock wave propagating upstream when the detonation wave reached the convergent nozzle, while there was no reflected shock wave for the divergent nozzle. The exhaust plume temperature increased slightly with the increasing operating frequency. The exhaust plume temperature of the PDE with the convergent nozzle was lower than that of the divergent nozzle and convergent-divergent nozzle, while the PDE with divergent nozzle obtained the highest exhaust plume temperature. Analogously, the average thrust of PDE with the convergent nozzle was the largest while the PDE with the divergent nozzle obtained the minimum average thrust. The spontaneous flow field of the exhaust plume under the condition of multi-cycle was observed by a high speed camera. Several Mach disks looked like diamonds were generated at the center of exhaust plume and the flow was supersonic for a while. The flow field was similar to that of the supersonic unexpanded free jet.