Christopher A. Stevens

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.

Detonation Propagation Through Ducts in a Pulsed Detonation Engine

A study of configurations to allow a consistent and predictable transition of a detonation from one detonation tube to another is presented. Development of a continuously operating pulsed detonation engine (PDE) without a high energy ignition system or a deflagration-todetonation transition (DDT) device will increase engine efficiency, reduce cost, improve performance, and reduce weight. The intent of this study was to minimize energy losses of a detonation wave in order to directly initiate another detonation wave. Detonation tube fill fraction, purge fraction, equivalence ratio, cross-over length and cross-over geometry were varied to determine their effect on direct initiation via a cross-over tube. Velocities at or above the upper Chapman-Jouguet (C-J) velocity point are desired and considered successful detonations. It was found that a cross-over tube with a “U” shaped geometry and a width at least 75% of the initiated tube’s width provided the best conditions for direct initiation.

Overview of Performance, Application, and Analysis of Rotating Detonation Engine Technologies

Recent accomplishments related to the performance, application, and analysis of rotating detonation engine technologies are discussed. The pioneering development of optically accessible rotating detonation engines coupled with the application of established diagnostic techniques is enabling a new research direction. In particular, OH* chemiluminescence images of detonations propagating through the annular channel of a rotating detonation engine are reported and appear remarkably similar to computational fluid dynamic results of rotating detonation engines published in the literature. Specific impulse measurements of rotating detonation engines and pulsed detonation engines are shown to be quantitatively similar for engines operating on hydrogen/air and ethylene/air mixtures. The encouraging results indicate that rotating detonation engines are capable of producing thrust with fuel efficiencies that are similar to those associated with pulsed detonation engines while operating on gaseous hydrocarbon fuels....

Comparison of Transient Response of Pressure Measurement Techniques with Application to Detonation Waves

During the testing of a Rotating Detonation Engine coupled with an ejector, pressure measurements indicated a deficiency in the understanding of the pressure probes. The pressure histories came from an array of Infinite Tube Pressure (ITP) static probes, Capillary Tube Average Pressure (CTAP) static probes, and direct Kiel stagnation pressure probes. Upon examination of the pressure histories, the average of the Kiel probe pressure over several laps of detonation, it was revealed that the ITP static pressures measured higher than the Kiel stagnation pressures both within the mixing chamber and at the exit of the ejector. This research conducts an unsteady calibration of the different probe configurations in order to quantify the unsteady response of the probe configurations. The unsteady calibration allows the pressure at the entrance of the probes to be reconstructed from the recorded pressure histories. The unsteady calibration of the pressure probes was performed by subjecting each of the probes to a single detonation in a detonation tube. The pressure history of a planar detonation such as the one in a detonation tube is well understood, and accurately modeled by the Zeldovich-vonNeumann-Doring (ZND) model. A transfer function for each probe type was constructed from the model pressure history and the measured pressure history. The transfer function for each configuration accounts for damping and phase lag introduced by the probe configuration. Each of the probe configurations has distinct phase lag and damping and the transfer function for each will be dominated by different phenomena. The CTAP consists of a 0.0625 in (1.6 mm) diameter, 36 in (0.91 m) long tube connected to the RDE at one end and a pressure transducer at the other. In the tube, viscous dissipation results in a temporal average of the pressure. The dissipation results in strong damping of any perturbations. The length of the tube results in longer phase lag than any other configuration. In the ITP configuration, the pressure transducer is connected to the RDE by 1.5 in (3.8 cm) of 0.0625 in (1.6 mm) diameter tubing and to ambient air by 72 in (1.83 m) of 0.0625 in (1.6 mm) diameter tubing. The proximity of the transducer to the probe entrance reduces impact loading on the transducer and the open ended tube does not reflect shockwaves. The distance from the tube entrance to the transducer results in little phase lag, but the small diameter tubing significantly damps pressure perturbations. The chamber connecting the transducer

The Effects of Repetitively Pulsed Nanosecond Discharges on Ignition Time in a Pulsed Detonation Engine

An experimental investigation of the effectiveness of a nanosecond duration repetitivelypulsed plasma discharge device for ignition of a pulsed detonation engine was carried out. Ignition of C2H4/air mixtures and aviation gasoline/air mixtures at atmospheric pressure produced a maximum reduction in ignition time of 17% and 25%, respectively, as compared to an automotive aftermarket multiple capacitive-discharge spark ignition system. It was found that the ignition time is reduced as total energy input and pulse repetition frequency is increased. Further investigation of ignition events by Schlieren imaging revealed that at low pulse-repetition frequency (0-5 kHz), individual ignition kernels formed by the discharge do not immediately interact, while at higher pulse-repetition frequencies ( ≥ 10 kHz) ignition kernels combine and result in a faster transition to a self-propagating flame front.

Ball Valve Pulsed Detonation Engine

An experimental study assessed the feasibility of utilizing a fully ported ball valve to inject fuel/air mixture into the thrust tubes of a Pulsed Detonation Engine (PDE). A fully ported, two inch ball valve was installed upstream of a detonation tube and rotated at constant speed. A solenoid controlled purge flow was replaced with a water injection event when the airflow proved insufficient to cool the detonation tube and burned gases. The water injection at the end of each cycle prevented premature ignition of fresh incoming mixture of ethylene/air, but failed to prevent premature ignition of hydrogen/air mixture. Measurements of pressure drop across the valve, detonation wave speeds, and detonation tube temperature were collected to verify successful operation of the engine. Leakage of combustible mixture was noted when the valve was closed at the end of testing. Disassembly of the valve after experiments revealed wear and charring of the valve seal. Performance characteristics are compared to a similar engine configuration utilizing a poppet valve equipped automotive cylinder head to control mixture and purge air timing. The measurements of total pressure loss show that the ball valve presents significantly less restriction to flow, but the valve also displayed a propensity to backfire due to leakage between the PTFE seals and the ball.

Optical Measurement of Detonation with a Focusing Schlieren Technique

In the past cell size measurements were limited to time-integrated images along the side and end walls of detonation tubes. Soot or soft metal lining these walls recorded the paths of triple points in the detonation front, and the detonation inscribed a pattern on the material. These kinds of measurements are fundamentally limited to measurement influenced by the boundary conditions of the tube wall. There was no capability to capture the behavior of triple points between the walls of detonation tubes. In this research, a modified schlieren imaging system was able to bring a small, planar slice of a three-dimensional cellular structure into focus, while de-focusing the remainder of the detonation front. Focal planes as thin as 0.4 mm were achieved and as a result, the free cell size was measured without influence from the boundary. The cell size of hydrogen-air and ethylene-air detonations was measured in a 38 mm by 38 mm rectangular cross-section. A minimum of 50 samples of the cell size were acquired at each of two equivalence ratios for each fuel. The uncertainty using the new method was significantly smaller than the scatter in previously published data (where error bars are published at all). A scaling factor was explored to match the measured cell sizes collected with the new technique to previously published data, but large uncertainty prevented identification of a universal scaling factor for all mixtures. The focusing schlieren technique will continue to be a useful tool for developing empirical data on cell size in future efforts.

Cell Width of Methane Air Mixtures at Elevated Initial Pressure and Temperature

Detonation combustors have performance and efficiency advantages over constant pressure combustors due to the natural pressure rise of a detonation. The design of detonation combustors requires knowledge of detonation properties such as cell width and detonability limits. The cell width and detonability of common fuel/oxidizer mixtures remains unknown. Natural gas is commonly used in power generating gas turbines, and methane is its primary component. Despite its widespread use, the cell width of methane has been measured only at atmospheric conditions. This research measures cell width and length at conditions similar to the burner inlet of a modern gas turbine (P = 4 atm, T = 600 K). Inside a detonation tube, a mixture of methane and air was ignited and transitioned to detonation before passing over a soot foil attached to the wall of the tube. Triple points in the detonation front impressed a cellular pattern on the soot foil which was measured to determine the cell width and height.

Interactions of Detonations with Ramps

In the development of pre-detonators for pulse detonation engines (PDE’s), it is necessary to transition a detonation from the small diameter of the pre detonator tube to the larger diameter of the PDE thrust tube. Experiments have shown that confined detonations encountering abrupt expansion in cross sectional area will diffract and are prone to failure. The failure of the detonation occurs when the initial detonation wave is narrower than 13 cell widths. Some limited success was achieved by limiting the expansion ratio. This work combines numerical results with experimental observations to investigate two geometries that may be able to propagate a detonation beyond the step expansion. Results from the numerical study predict that expanding the detonation gradually through a linear ramp does not prevent diffraction of the detonation wave, but a properly placed incline is capable of restarting a diffracted detonation downstream of a step expansion. The experimental data confirmed these predictions, but higher temporal and spatial resolution was needed to observe the cellular structure in experiments.