Proceedings of IMECE 2007 2007 ASME International Mechanical Engineering Congress and Exposition November 11–17, 2007, Seattle, Washington, USA IMECE2007–44068 PRACTICAL ISSUES IN GROUND TESTING OF PULSED DETONATION ENGINES Philip K. Panicker, Frank K. Lu, Donald R. Wilson Aerodynamics Research Center, Mechanical and Aerospace Engineering Department University of Texas at Arlington Arlington, Texas 76019 U.S.A. [email protected], [email protected], [email protected] ABSTRACT with or replaced by a low-pressure fan. Compact designs may be feasible, thereby achieving high thrust-to-weight ratios. In Pulsed detonation engines can potentially revolutionize aero- addition, various combinations and hybrids have been space propulsion and they are the subject of intense study. proposed, including ejector-augmented and combined cycle However, most of the studies involve single shot and very short engines [6–8], thereby extending the PDEs versatility. There duration test runs. Some of the practical issues in developing are also other potential non-aerospace applications of pulse PDEs are discussed from the viewpoint of developing ground- detonations, including electric power generation [9,10], slag based demonstrators. This represents only the beginning of a removal [11] and others [12]. roadmap toward the successful development of flightweight While a roadmap has been proposed for developing PDEs engines. Viable solutions are offered that may help overcome [13], there are no known operational PDEs presently. Instead, the difficulties posed by the high temperature and pressures on it appears that the lion’s share of experimental studies have the test rig and instrumentation. Commercial solenoid valves been performed using single-shot test beds or with short run and electronic fuel injectors are presented as means to achiev- times at frequencies below 50 Hz in the range of 10 to 20 s. ing higher operational frequencies. Issues concerning data ac- However, longer test times are required to move PDEs toward quisition, such as proper implementing procedures for pressure practice, either for propulsion or for power; see, for example transducers and choosing the appropriate sampling rates are [14] which reported test times exceeding 5 minutes. The longer discussed. Methods for mitigating electromagnetic interference test times are expected to introduce a host of issues. In this are discussed. paper, a number of these critical issues pertaining solely to ground testing will be reviewed. Many of these issues will INTRODUCTION remain as flightweight systems are developed. Before Pulsed detonation engines (PDEs) offer many advantages over reviewing these issues, a brief introduction of basic PDE conventional propulsion systems and are regarded as potential processes will be provided. replacements for airbreathing and rocket propulsion systems, A PDE is shown schematically in Fig. 1 [15]. The figure for platforms ranging from subsonic unmanned vehicles, long shows that the reactant, comprising of oxygen and propane, is range transports, high-speed vehicles, space launchers to space introduced on the left. The gases are metered and are at an vehicles [1]. Theoretical studies have indicated a higher equivalence ratio of unity. Downstream is an igniter, followed thermodynamic efficiency than achievable in conventional, by a deflagration-to-detonation (DDT) section, wherein a DDT deflagration-based systems [2–5]. Moreover, there are savings device is inserted to help transition the combustion process to a in weight and reduction in complexity and cost. For example, fully-developed detonation wave. the high compression achieved in the detonation process may allow the compressor in a conventional engine to be dispensed 1 Copyright © 2007 by ASME Figure 1. Schematic of a PDE operating with propane and oxygen [15]. Figure 2. Stages of a PDE cycle. From various studies, the Shchelkin spiral has proven to be blowdown or exhaust stage (6) after which the cycle repeats most effective in inducing DDT in relatively short run up itself. distances, although not much have been reported on its use in a The unsteady processes can be displayed in a displacement- PDE [15–18]. Previous studies [17] found that spirals with time diagram. Figure 3 shows such a diagram for a simple 55% blockage ratio are the most effective. ideal process which we call the unit cycle. The figure depicts Further downstream is a tube for measuring the transient relatively the time required for the various events. Therefore, pressures that are developed. Also shown in the figure to the ideally, the time for the unit cycle comprises of left is a port for introducing purge air. Finally, the rig is T = T +T +T +T +T (1) instrumented with a load cell. cyc ign fill prop exh purge The stages of a PDE cycle are shown schematically in Fig. The cycle frequency is thus given by 2. The detonation chamber is initially at quiescent, ambient (2) conditions (1). It is then filled with a fuel/oxidizer mixture (2), f = 1 Tcyc which in Fig. 1 is shown as end-wall injection. At some time The ignition time is not shown in Fig. 3 but ignition starts (3), the mixture is ignited, ideally such that the detonation wave just ahead of (3). Moreover, while the fill time is shown meets the mixture front at the exit of the detonation chamber explicitly in Eqn. (1), it is absorbed into the propagation time in (4,5). The detonation chamber is then scavenged by a Fig. 3. Obviously, any departure from the ideal unit process 2 Copyright © 2007 by ASME shown in Fig. 3, such as extra time required for filling or any other non-ideal effects such as viscous attenuation or mismatches in timing of various components, will lengthen the cycle time. Figure 3 shows that the PDE unit process is dominated by unsteady gasdynamics phenomena, namely, the filling/propaga- tion, exhaust and purging processes. However, the bulk of the fundamental studies have concentrated on ignition and detonation propagation, perhaps rightly so as these are crucial to the success of a PDE. As has been known for some time, direct initiation is practically impossible due to the exorbitant energy requirements [19]. Thus, less energetic ignition has been the approach but this requires a sufficient length for deflagration-to-detonation transition to occur, usually shortened by a detonation enhancement device such as a Shchelkin spiral. The aforementioned gasdynamics phenomena can be particularly vexing in bringing PDEs into practice. For example, the purging stage is particularly important as this cools the chamber as well as cleans it for a fresh charge [20]. Without this stage, the PDE may suffer serious damage as the heat release from the detonation process is much larger than ordinary deflagration processes; for example, see the review by Bazhenova and Golub [12], This specific issue of heating will be discussed later. This paper is organized into a number of topics that are pertinent in developing PDEs and their ground-based Figure 3. PDE unit process. demonstrators. affect the signals from transducers used for monitoring the MECHANICAL VALVES VS. SOLENOID VALVES performance of the engine. Finally, there also appears to be a Early PDE designs made use of mechanical valves, such as practical limit in the speed of mechanical valves for engine rotary valves [16,21–35]. Rotary valves possess a few applications. The fastest mechanical valves today are found in limitations. They are leaky and elaborate sealing techniques Formula 1 engines. For example, the Cosworth CA2006 V8 have been proposed [25]. Moreover, the difficulties in sealing engine achieved a Formula 1 landmark of 20,000 rpm in the also limit their operating pressure. Rotary valves are difficult 2006 Bahrain Grand Prix. This means that the twin intake to time precisely. Synchronization with the ignition is usually valves (and twin outlet valves) operate at 10,000 rpm (or 167 achieved by a sensor, such as a magnetic pickup or a Hz each). photodetector, to detect a reference position of the valve or The trend in the automobile industry is to move toward cam. However, transmission belts slip and the valves lose electronic fuel injection systems, where the injection is synchronicity as a result. Since the valves are ganged together accomplished by solenoid or piezo-driven valves. Such fuel to a common driveshaft, adjustments for minor departures in injectors are used for gasoline, diesel and alternative fuel timing cannot be done. Thus the loss of synchronicity leads to engines and their designs for gaseous and liquid fuel delivery improper filling and misfiring. are the subject of substantial recent research [37–45]. For Most of PDE designs fill from the closed end. This, diesel injectors, pressure boosters raise the injection pressure to together with the low operating pressures of the rotary valves, as high as 2500 bar [46]. Such a high pressure is required to means that the engine may not be filled rapidly for high spray the fuel directly into the engine, right after the frequency operation. (A simple analysis shows that thrust compression stroke. Modern gasoline injectors also inject scales with the frequency [36], a subject that will be addressed directly into the engine at high pressures. Moreover, valves for later.) This inability to fill rapidly causes many practical alternative automotive fuels, such as propane [47], methane, problems. For example, it is difficult to control the mixture LPG [48], biodiesel and ethanol-based blends [49] and, stoichiometry. The poor filling characteristics at high increasingly, hydrogen [50–52], are presently available off the frequencies or due to a long chamber mean that there is shelf. These gas injectors are extremely crucial in advanced uncertainty that a precise amount of reactant has been delivered engine concepts [53,54]. uniformly throughout the detonation chamber. Other issues to The favorable features of electronic valves such as their fast contend with include electromagnetic interference (EMI) and action (opening times of 5ms or less) and precision control by vibrations from the motor driving the valves, both of which can TTL signals from a computer make these valves attractive for 3 Copyright © 2007 by ASME a.