First steps for the Development and testing of a Pulse Detonation Engine for UAV application

Abstract

Due to its thermodynamic cycle, the pulsed detonation engine (PDE) has theoretically a higher performance than an other classical propulsion concept using the combustion process (+ 20 to 25% in term of thermal efficiency). Nevertheless, it is necessary to verify that this advantage is not fully compensated by the difficulties, which could be encountered for practical use of the PDE concept or by the complex technology, which could be needed to implement it in an operational flying system. Moreover, a PDE a priori generates a severe vibration environment, which can imply higher more severe requirement for all on-board vehicle equipments or subsystems. On the basis of knowledge acquired during the last ten years, MBDA and DSO started a collaboration devoted to the pre-development of a small-scale PDE demonstrator that could be flight tested within the next years. This demonstrator should use storable fuel, be throttle-able, provides a good specific impulse in a small-size engine with a good thrust-to-weight ratio and minimum maintenance cost. This demonstrator should also be able to operate in a complete airbreathing mode, without onboard oxidizer, so the design of the ignition device is one of the key points of this engine, the other being the inlet integration and the capability to provide continuous inlet flow even with a pulsed combustion chamber operation. Since the repetitive and direct initiation of a detonation inside a PDE requires large amount of energy and hence high power consumption, two other low-energy mechanisms are generally used, they are deflagration-to-detonation transition (DDT) and Shock-to-detonation transition (SDT). Pre-detonation tubes with Shchelkin spirals are commonly found on most PDE concept. The mechanism involved is both DDT and SDT. Inside the pre-detonation tube, the flame accelerates from low-velocity laminar (or turbulent in some cases) to medium velocity turbulent regime, up to the thermal blockage velocity, and ultimately up to real CJ detonation velocity. For fuel – air mixture, the pre-detonation tube diameter is generally marginally larger than one detonation cellsize, so when the detonation diffracts into the main chamber, the sudden expansion from confinement results in decay of the reaction zone and decoupling from the shock wave in the subcritical regime (complete propagation failure as shock decouples from the reaction zone). In this case, re-ignition of the detonation could only be achieved by shock – shock interactions (main shock and reflected shock waves from the walls), so after the DDT inside the pre-detonation tube we could observe a SDT inside the main chamber. During the detonation phase, the pressure inside the chamber is higher than the total inlet pressure so without careful design some parts of the hot gases could be exhausted thru the inlet, decreasing both impulse from the detonation and engine operating frequency. The paper describes the numerical and experimental investigations on shock waves generated by fast flames as well as some work related to engine integration, performed by DSO and MBDA, in order to develop a first version of an operational ignition device for the envisioned PDE.