Fred Schauer

Detonation Initiation Studies and Performance Results for Pulsed Detonation Engine Applications

Abstract : An in-house computational and experimental program to investigate and develop an air breathing pulse detonation engine (PDE) that uses a practical fuel (kerosene based, fleet-wide use, JP type) is currently underway at the Combustion Sciences Branch of the Turbine Engine Division of the Air Force Research Laboratory (AFRL/PRTS). PDEs have the potential of high thrust, low weight, low cost, high scalability, and wide operating range, but several technological hurdles must be overcome before a practical engine can be designed. This research effort involves investigating such critical issues as: detonation initiation and propagation; valving, timing and control; instrumentation and diagnostics; purging, heat transfer, and repetition rate; noise and multi-tube effects; detonation and deflagration to detonation transition modeling; and performance prediction and analysis. An innovative, four-detonation-tube engine design is currently in test and evaluation. Preliminary data are obtained with premixed hydrogen/air as the fuel/oxidizer to demonstrate proof of concept and verify models. Techniques for initiating detonations in hydrogen/air mixtures are developed without the use of oxygen enriched air. An overview of the AFRL/PRTS PDE development research program and hydrogen/air results are presented.

Parametric Testing of a Unique Rotating Detonation Engine Design

The Rotating Detonation Engine, RDE, has been studied over the last several decades as a possible alternative means of propulsion 1-5 . The RDE offers a potential performance improvement resulting from the thermodynamic advantages of the Humphrey cycle 6-9 . However, the design and operation of RDEs is at a very low Technology Readiness Level, TRL. Significant development investment is required to verify that the RDE is viable and can provide the promised thermodynamic advantages. To date, the majority of RDE development has followed the design path first explored by prominent Russian scientists 10-12 . The subject of this paper is a device following a different path explored by Nicholls and Edwards 14 . Such a device has been built and successfully demonstrated for short durations on hydrogen and ethylene fuels. This paper presents the basic design of the device, validation of its operation and some discussion on its operating characteristics.

Experimental Study of the Performance of a Rotating Detonation Engine with Nozzle

A rotating detonation engine is experimentally tested with various nozzle configurations for the purpose of measuring the propulsive performance of these devices in terms of thrust and specific impulse. Particular attention is given to comparing different internal nozzle configurations, which include bluff body, aerospike, and choked aerospike arrangements. The nozzle throat exit choke present in the rotating detonation engine exhaust is analyzed to provide insight into the stagnation pressure gain nature of the device.

Flowfield Characterization of a Rotating Detonation Engine

A rotating detonation engine (RDE) at the Air Force Research Lab (AFRL) has been modified to allow optical access to the annulus while in operation. High speed video of chemiluminescence was taken for three operating conditions to characterize the RDE flowfield. Two-dimensional representations of the entire RDE are presented to show basic flow structure. Detonation height, detonation angle, oblique shock wave angle, shear layer angle, and contact surface angle were measured. Average value for each of these measurements did not change drastically over the range tested, but large deviations of the values were observed. These considerable deviations of the flowfield point toward device variation as a major factor to be understood.

INTEGRATION OF A PULSED DETONATION ENGINE WITH AN EJECTOR PUMP AND WITH A TURBO-CHARGER AS METHODS TO SELF-ASPIRATE

Abstract : Two methods, an ejector pump and a turbo-charger, are evaluated as a means to self-aspirate a Pulsed Detonation Engine (PDE). For the experiments pertaining to the ejector pump, a pulsed detonation engine is run on hydrogen and air at frequencies up to 40 Hz. equivalence ratios from 0.5 to 1.0, and fill fractions from 0.25 to 1.0. Flow visualization is used to determine the combination of fill fraction and equivalence ratio that successfully induced a secondary flow in the ejector pump. Pressure traces at the inlet and along the ejector pump are used to understand the performance of the ejector pump. The induced secondary flow is found to be approximately triple the primary detonation flow. Fill fraction and equivalence ratio are found to affect the performance of thee ejector. High fill fractions and high equivalence ratios results in an oscillatory flow at the ejector inlet. Hydrogen and air are used as the frtel and oxidizer during the experiment with the turbo-charger also. Air flow and pressure at the exit of the compressor are used to evaluate the potential for self-aspirating the PDE. By fltnning two detonation fltbes simultaneously though the turbo-charger self-aspiration is achieved. The centrifugal style turbine and compressor of the turbo-charger showed no signs of discoloration or pitting after a 25 minute self-aspiration run where the detonation tube and turbo-charger attained thermal equilibrium. Throughout the course of the testing the turbine experienced 35 K plus detonation events and reached a rotational operating speed of 80 K rpm.

Propagation of Detonation Waves in Tubes Split from a PDE Thrust Tube

Abstract : A Pulse Detonation Engine (PDE) combusts a fuel air mixture through detonation. Existing designs require spark plugs in each separate thrust tube to ignite premixed reactants. A single thrust tube could require the spark plug to fire hundreds of times per second for long durations. This paper reports on the use of a continuously propagating detonation wave as both a thrust producer and a single ignition source for a multi-tube system. The goal was to minimize ignition complexity and increase reliability by limiting the number of ignition sources. The work includes a systematic investigation of single tube geometric effects on detonations. These results were subsequently used to further examine conditions for splitting detonations i.e. the division of a detonation wave into two separate detonation waves. Einally a dual thrust tube system was built and tested that successfully employed a single spark to initiate detonation in separate thrust tubes.

Liquid Hydrocarbon Detonation Branching in a Pulse Detonation Engine

Abstract : Pulse detonation engines operate on a fill-detonate-exhaust cycle with thrust directly proportional to the cycle frequency. That is, a decrease in cycle time results in increased thrust. This paper shows that the detonate portion of the cycle can he shortened by using a branched detonation as the ignition source as opposed to a spark plug type of ignition. The combustion energy from a branched detonation allows ignition and deflagration-to-detonation transition to occur more quickly, shortening overall cycle time. Further, while detonation branching has been previously accomplished using gaseous hydrogen fuel, this paper reports the first application of detonation branching using liquid hydrocarbon fuel. For this application, a pressurized heating system was designed to vaporize the fuel and mix it with an airstream to stoichiometric conditions.

A Comparison of Fluidic and Physical Obstacles for Deflagration-to-Detonation Transition

Abstract : A fluidic obstacle has been proposed as an alternative to conventional deflagration-to-detonation transition (DDT) enhancement devices for use in a Pulsed Detonation Engine (PDE). Experimental results have been obtained utilizing unsteady reacting and steady non-reacting flow to gain insight on the relative performance of a fluidic obstacle. Using stoichiometric premixed hydrogen-air, transition to detonation has been achieved using solely a fluidic obstacle with comparable DDT distances to that of a physical orifice plate. Flame acceleration is achieved due to the intense turbulent mixing characteristics inherent of a high-velocity jet and the blockage created by the virtual obstacle. Turbulence intensity (T.I.) measurements, taken downstream of both obstacles with hot-film anemometry during non-reacting steady flow, show a conservative trend that a fluidic obstacle produces approximately a 240% increase in turbulence intensity compared to that of a physical obstacle. Ignition times were reduced approximately 45%, attributable to the increase in upstream T.I. levels relative to the fluidic obstacle during the fill portion of the PDEs cycle. Transition to detonation was obtained for injection compositions of both premixed stoichiometric hydrogen-air and pure air.

Performance Studies of Pulse Detonation Engine Ejectors

An experimental study on the performance of pulse detonation engine ejectors was performed. Time-averaged thrust augmentation produced by straight and diverging pulse detonation engine ejectors was measured using a damped thrust stand. The ejector length-to-diameter ratio was varied from 1.25 to 5.62 by changing the length of the ejector and maintaining a nominal ejector diameter ratio of 2.75. In general, the level of thrust augmentation was found to increase with ejector length. Also, the ejector performance was observed to be strongly dependent on the operating fill fraction. A new nondimensional parameter incorporating the fill fraction was proposed. When the pulse detonation engine ejector data were represented as a function of this new parameter, the ejector data were reduced to one representative thrust augmentation curve for ejectors of similar internal geometry. Straight pulse detonation engine ejectors compared well with the available data on straight steady-flow ejectors. Diverging pulse detonation engine ejectors produced nearly twice the thrust augmentation as their straight-ejector counterparts due to the additional thrust surface area the divergence provided. All pulse detonation engine ejectors tested were seen to be sensitive to the axial position of the ejector as well. The optimum ejector axial placement was found to be a function of fill fraction due to a tradeoff between the detonation wave induced drag and increased mass entrainment. Downstream ejector placements performed the best at the low fill-fraction operating conditions.

Experimentation of a Premixed Rotating Detonation Engine Utilizing a Variable Slot Feed Plenum

A premixed supply of fuel and oxidizer was successfully fed through five concentric feed slots and detonated in a rotating detonation engine (RDE). This study reports the first known successful air-breathing premixed RDE. The premixed flow maintained a boundary layer velocity gradient that successfully arrested flashback. An operating map was constructed and compared to a non-premixed RDE. Mixing effects were shown to be important through measured wave speeds that were slower than theoretical Chapman-Jouguet detonation velocities, CFD results, and similar non-premixed RDE. High speed imagery indicated that the wave speed measurements should account for the circumferential velocity of expanding gases and the Chapman-Jouguet calculation should account for partial mixing with combustion products.