One Method to Increase the Operating Frequency of Pulse Detonation Rocket Engines

Abstract

D ETONATION is an extremely rapid chemical energy release process in nature. However, detonations have been explored for propulsion applications only for the past 60 years or so, because of the difficulties involved in rapidly mixing the fuel and air at high speeds and initiating and sustaining a detonation in a controlled manner in a fuel–air mixture [1]. Although the design of practical combustionbased engineering devices is dominated by the easier-tamed deflagration, the dramatic energy release rates that can be achieved in the same mixtures in detonation conditions have for decades enticed engineers to seekways to harness suchwaves [2].Most of the interests in detonation lie in developing detonation-based propulsion devices, such as pulse detonation engines (PDEs) including airbreathing pulse detonation engines (APDEs) and pulse detonation rocket engines (PDREs) in the past few decades and rotating detonation engines in recent years. There have been numerous studies on PDEs all over the world, and several reviews have been published [1,3–7]. Although considerable attempts have been spent on detonationbased engines, a number of challenging fundamental and engineering problems have yet to be solved [2,5,7]. One of them is how to increase the operating frequency of a PDE without reducing the hardware simplicity, because, theoretically, near-constant and larger thrust can be obtained at higher operating frequency for a given single-cycle performance and tube size [8]. For a practical PDE, there is an optimum frequency at which the performance reaches the maximum because single-cycle performance will vary instead of keeping as a constant. Only in the region below such an optimum frequency, the performance increases with frequency; above the optimum frequency, the performance decreases with frequency. However, increasing the operating frequency is still a big challenge and is helpful to widen the scope of experimental study on PDEs and determine the optimum frequency at which the performance achieves themaximum.Many factors, such as efficient supply systems for fast injection of propellants, low-energy source to provide reliable and fast ignition, and geometry of the detonation tube to facilitate deflagration to detonation transition (DDT) at the lowest total pressure loss, restrict the operating frequency of the PDE. The current study focuses on a control methodology for fast and efficient supply of propellants. In previous studies, solenoid valves [9–12] and mechanical rotary valves [13–16] were often employed for periodic propellants supply of the PDE. High-speed solenoid valves can achieve very high operating frequencies (more than 100 Hz); however, their flow rates are too small to meet the requirement of practical PDEs. Obviously, parallel connection of several ones can break the flow rate limit, whereas too many solenoid valves will be needed when the detonation tube volume is large, leading to a complicated and costly system. Rotary valves are able to provide large flow rates at higher operating frequencies compared with solenoid valves, yet they require additional driving devices, such as electric motors, and have more complicated mechanical structures, both of which increase the hardware complexity. Also, there have been some attempts at the valveless PDE scheme, eliminating any mechanical valves [17–22]. In related work, the so-called gasdynamic valves were created by the pressure oscillations inside the detonation tube or combustor, which could interrupt the fill of fuel and oxidizer intermittently. However, the valveless scheme was usually used in airbreathing PDEs, where air was often used as an oxidizer. It was not stable in PDREs, where a more reactive oxidizer (e.g., oxygen) was employed. If the detonation tube volume is not very large, especially for a single-tube PDRE, an appropriate control methodology can result in higher operating frequency of the PDRE using solenoid valves without reducing the hardware simplicity. Thiswork provides a novel control method to increase the operating frequency of PDREs.