Development of Transient Pulse Detonation Engine Cycle Analysis and Performance Prediction (PDE-CAPP) Code

Abstract

New transient PDE cycle analysis and performance prediction (PDE-CAPP) code has been developed and partially validated. The current developed PDE-CAPP code accounts for an inlet, fuel manifold injection system, fuel-air mixer, single combustor (tube), and a single divergent nozzle. Each PDE component was modeled using an engineering level analysis approach, except for the combustor and nozzle models which were based upon a combined numerical and ZND/Chemical Equilibrium/Insentropic formulation. The developed PDE-CAPP code represents a new unsteady time-efficient design and predictive tool for PDEs. The PDE-CAPP was utilized to predict the performance of both subsonic and supersonic missions and was found to be very efficient and its predictions were found to be both qualitatively and quantitatively in agreement with available data and other predictions based on one and two-dimensional CFD models. However, the new developed PDE-CAPP code is Orders of magnitude faster than existing one-dimensional CFD codes. For example, the CPU time that was required to obtain 10 complete PDE cycles for a PDE configuration with an inlet, single combustor, and a nozzle was about 12 seconds on an Octane II SGI machine with 450 MHZ processor speed, thus making the developed PDE-CAPP code an enabling technology for rapid performance assessments of different PDE design configurations and optimization studies. INTRODUCTION Pulse detonation engines (PDEs) represent a new novel form of propulsion that offers the potential for high combustion efficiency and performance with reduced hardware complexity as compared to current rocket-based, turbines, ramjet/scramjet engines. The pulse detonation engine (PDE) concept utilizes a repetitive unsteady cycle of detonation wave initiation, propagation, and expulsion to produce thrust. The higher performance potential of the PDE concept relative to conventional steady flow propulsion systems, such as turbine and ramjet engines, can only be realized by achieving rapid fuel-air mixing, detonation transition and combustion product scavenging over the PDE cycle. The high pressure behind a detonation must be exploited to the *Senior Research Scientist 2 Copyright ©2008 by Author, Published by the American Institute of Aeronautics & Astronautics, Inc., with Permission 44 AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit AIAA-2008-4882 21-23 July 2008, Hartford, Connecticut 3 largest possible extent over a repetitive cycle in order to generate high thrust. In general, the combustion process in PDE takes place at supersonic speed and at a quasi-constant volume, for this reason, PDE can achieve better specific fuel consumption relative to conventional steady-flow engines. In addition, the absence of rotating machinery in the flow path and the consolidation of the compression (pressurization), combustion, and thrust generation processes within a single component without the need for pre-compression to peak operating pressures forms the basis for a propulsion system with potential for a high thrust-to-weight ratio. This propulsion system also provides potential for lower cost, simplicity and a high degree of reliability. The PDE performance potential has in recent years increased interest in exploiting the PDE for aircraft, rocket and missile applications [1-6]. The PDE cycle [7-14] begins as a mixture of fuel and air fills the combustor volume. Rapid transition to detonation is induced by rapid energy deposited through a spark or laser-based system. The detonation wave then propagates through the fuel-air mixture and is expelled from the combustion chamber. The expansion waves behind the detonation waves and reaction zone, which are created to match the closed end boundary condition, reduce the combustor pressure to the stagnation condition that exists between the front end of the chamber and the trail of the expansion waves. This process partially scavenges combustion products when the wave is expelled at the exhaust end of the combustor. Once the detonation wave leaves the combustor, expansion waves propagate upstream into the confinement to further scavenging combustion product. After the scavenging process is completed, the filling process begins to recharge the combustion chamber with a detonable mixture of fuel and air. The level of energy required to initiate and sustain a detonation wave depends on the detonative limits of the fuel mixture, on the ignition delay at the operating conditions within the engine, and on the interaction between the exhaust waves and the flow within the detonation tube. A strong coupling between the mixing process, kinetics of combustion and the geometry of engine, therefore, exists. At an operating frequency of several hundred cycles per second, the time available to complete fuel injection and mixing is no more than a few milliseconds. One way to achieve proper mixing is to exploit the unsteady turbulent flow within the engine. Ignition delay of the fuel air mixture determines the extent of the transition region of the detonation tube. Expansion from the aft end of the detonation tube also interacts with the detonation wave as it traverses the transition region. A pulse detonation engine relies on the periodic detonation of a fuel-air mixture to produce thrust. The PDE concept differs from typical air-breathing and rocket engines that rely on steady state, isobaric deflagration combustion. Detonation waves propagate at supersonic speeds of several thousands meters per second. This flame speed allows detonation combustion to occur nearly at constant volume, whereas traditional deflagration combustion is a constant pressure process. The constant volume detonation process can be modeled as part of a Humphrey cycle whereas the constant pressure deflagration process can be described as part of a Brayton cycle. Figures 1 and 2 compare the temperature, pressure, and volume characteristics of the Brayton and Humphrey cycles. The Brayton cycle (0-1-4-5-0) consists of two constant pressure processes (1-4, 5-0) and two isentropic processes (0-1 and 4-5). The Humphrey cycle is similar, except that the constant pressure process (1-4) is replaced by a constant volume heat addition process (1-2). The efficiencies of the constant pressure Brayton and constant volume Humphrey cycles can be computed from the pressure-volume and temperature-entropy diagrams in Figs. 1 and 2. The efficiency of a cycle is defined as the useful work output divided by the total heat energy input. 44 AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit AIAA-2008-4882 21-23 July 2008, Hartford, Connecticut 4 Fig. 1: Pressure-Volume Cycle Diagram Fig. 2: Temperature-Entropy Cycle Diagram The efficiency of the Brayton cycle depends only on the temperature change during either of the two isentropic compression and expansion processes (i.e., T0/T1 = T4/T5) and it is given as [7] 1 0 1 T T Brayton − = η The efficiency of the Humphrey cycle is given as [7];