Preliminary Studies on a Pulse Detonation Rocket Engine without the Purging Process

The pulse detonation engine (PDE) has received considerable interest over the past few decades due to its potential for increased performance and hardware simplicity. There have been many studies on PDEs all over the world and several reviews have been published [1-6]. With oxidizer on board, a PDE works in rocket mode. Like traditional PDEs, PDRE operates by repeatedly producing detonation waves that propagate through fuel-oxidizer mixture and produce high chamber pressure intermittently which resulting in discrete impulses [7]. The basic structure of a PDE is a straight tube, also known as a detonation tube, with one end closed and the other open. A basic detonation cycle consists of the following processes: (a) filling process, i.e., the detonation tube is filled with fresh detonable fueloxidizer mixture; (b) the mixture is ignited near the closed end of the tube and detonation is initiated directly or indirectly through a deflagration to detonation transition (DDT) process; (c) a self-sustained detonation wave, which compresses the fuel-oxidizer mixture by shock waves and initiates combustion of reactants, propagates toward the tube open end; (d) the burned gas exhausts through a blow-down process; (e) purging process, i.e., the purge gas is injected into the tube to expel the burned gas and prevent pre-ignition of fresh detonable mixture in the next cycle. The purging process was needed to ensure stable operation of the multi-cycle PDRE in previous studies. For example, Lu et al. [8] used air as purge gas, Li et al. [9] utilized nitrogen as purge gas, helium was adopted as purge gas by Kasahara et al. [10] and Matsuoka et al. [11, 12]. However, it takes some time to accomplish injection of purge gas into the detonation tube during purging process, which occupies considerable proportion of a single detonation cycle and. Therefore, it limits the overall cycle time together with injection process, DDT process, propagation and blow-down process putting a ceiling on the upper limit of operating frequency for a certain PDRE. In addition, it requires independent supply system which increases the hardware complexity. Thus, if the purging process is eliminated, great benefit can be derived, e.g., the complex valves and injection systems in Refs. [11-14] can be simplified. This study presents a successful try to achieve stable operation of a PDRE without purging process. Oxygen-enriched air was utilized as oxidizer and liquid gasoline was used as fuel. Appropriate supply pressure for fuel and oxidizer was of great importance for such an operating mode. Different oxygen percentages were also tested to determine the upper and lower limits for stable operation without purging process. Additionally, different exhaust plumes were observed as operating frequency increasing in such an operating mode. Experimental results indicated that it was feasible for the PDRE to be operated without purging process.