Adam Rasheed

Experimental Investigations of the Performance of a Multitube Pulse Detonation Turbine System

A multitube pulse detonation combustor consisting of eight unvalved tubes arranged in a can-annular configuration was integrated with a single-stage axial turbine nominally rated for 10 lbm=s, 25,000 rpm and 1000 hp. The multitube pulse-detonation-combustor-turbine hybrid was operated using ethylene-air mixtures for runs of 5 min to achieve thermal steady state in order to quantify performance at two operating conditions using simultaneous and sequential firing patterns. Analysis of these data reveal that turbine efficiency under pulsedetonation-combustor-fired operation was indistinguishable from steady performance within the 8 points measurement uncertainty. Furthermore, the pulse-detonation-combustor-turbine hybrid system demonstrated a potential 25% improvement in efficiency once corrections were made for the suboptimal fueling design, which resulted in lower-than-anticipated combustion efficiency. The present work is promising as it suggests that a pulsedetonation-combustor-turbine hybrid engine performance benefit can be realized with further development.

Experimental Investigation of a Pulse Detonation Engine with a Two-Dimensional Ejector

A parametric study into the integration of a pulse detonation engine (PDE) with an ejector was performed. High-speed shadowgraph visualizations of the flow inside the two-dimensional ejector provided a qualitative method of determining the performance of the integrated system. The performance was observed to be sensitive to the inlet geometry of the ejector as well as its axial position relative to the exhaust plane of the PDE. Significant levels of entrainment were obtained when the ejectors inlet was contoured, whereas flow separation reduced entrainment efficiency in the ejector with a straight thin inlet lip

Wave Attenuation and Interactions in a Pulsed Detonation Combustor-Turbine Hybrid System

A large-scale, multi-tube pulsed detonation combustor with eight tubes arranged in a can-annular configuration was integrated with a single-stage axial turbine nominally rated for 8 lbm/s, 25000 RPM and 1000 hp. This PDC-turbine system was operated using ethyleneair mixtures with each tube firing at 20 Hz. High frequency pressure transducers were installed throughout the flow path to investigate the wave interactions and attenuation across the turbine. The multi-tube PDC was operated at a range of conditions with three firing patterns: a single tube firing, all tubes simultaneous and all tubes sequential. Analysis of these data reveal wave interactions in the transition plenum between the PDC exit and turbine inlet plane can affect the optimum operation of the multi-tube PDC. In addition, there is over 20 dB attenuation of the peak pressure pulse and 10 dB attenuation of the broadband acoustic noise through the single-stage, axial flow turbine.

Operation and Noise Transmission of an Axial Turbine Driven by a Pulse Detonation Combustor

Pulse Detonation Combustors (PDC’s), as part of a hybrid PDC-turbine engine, have potential thermodynamic benefits over existing Brayton-cycle gas turbines. The form of combustion is a cyclic, controlled series of detonations. These systems apply a near or quasi-constant volume combustion process that provides both heat addition and pressure. In a hybrid PDC-turbine engine, the goal of incorporating a pulsed detonation chamber upstream of a turbine is to extract more mechanical energy in a turbine that receives the products of a repeating, pressure-rise detonation process versus the constant pressure, steady-flows available in conventional gas turbines. A rig was built to investigate PDC-turbine interactions and was operated to gather data on performance, operability, and noise levels. The rig consists of a single pulsed detonation combustor firing into a partial-admission, two-stage axial turbine. This paper reports findings of critical risk areas including turbine response to PDC operation, mechanical robustness, noise and system control. At a PDC operating frequency of 5 Hz, the acoustic level near the rig was approximately 3 dB higher than from the turbine operating at the same speed with steady flow input. The noise level is 28 dB lower than a PDC with no turbine downstream operating at the same frequency and discharging directly into the room. Insights into the mechanism for noise reduction were gained via imaging experiments and CFD simulation. High speed video imaging in a 2D PDC-turbine cascade configuration showed significant shock reflection from the cascade. An unsteady, reacting flow computational study showed similar shock reflection as well as a shock system that forms downstream of the cascade. Together, these results show that shock waves are both transmitted and reflected by the turbine stages in proportions that are dependent upon turbine stage design.Copyright

Wave Interactions in a Multi-tube Pulsed Detonation Combustor-Turbine Hybrid System

A large-scale, multi-tube pulsed detonation combustor (PDC) with eight tubes arranged in a can-annular configuration was integrated with a single-stage axial turbine nominally rated for 10 lbm/s, 25000 RPM and 1000 hp. This PDC-turbine system was operated using ethylene-air mixtures with each tube firing at 20 Hz. High frequency pressure transducers were installed throughout the flow path to investigate the wave interactions at a range of conditions with three firing patterns: a single tube firing, all tubes firing simultaneously and all tubes firing sequentially. Analysis of these data revealed that wave interactions in the transition plenum between the PDC exit and turbine inlet plane can affect the optimum operation of the multi-tube PDC.

Performance of a Pulse Detonation Combustor-Based Hybrid Engine

A key concept envisioned for Pulse Detonation Engine (PDE) technology is a hybrid engine, where a Pulse Detonation Combustor (PDC) replaces the combustor in a conventional gas turbine. A systems level performance estimation model for a PDC-based hybrid engine cycle was presented. A variable property formulation was used to estimate the cycle performance parameters namely the thermal efficiency (ηth ) and the net specific work (Wnet ). Performance estimations were obtained using a one-step finite-rate chemistry to simulate reactions, and the frozen reactions assumption to model the products of combustion. Two specific parametric studies are performed in which the compression ratio (CR) and the purge fraction (pf) were systematically varied. The predicted variations of ηth and Wnet with varying compression ratio and purge fraction are in agreement with the trends reported in the literature. For a range of values of CR (1-40), performance (ηth ) advantage of a PDC-based hybrid engine is predicted, when compared to a conventional gas turbine engine. The present calculations show that the assumed unsteady turbine component efficiency (ηT ) for the case of a PDC-based hybrid engine has a large effect on ηth . An experimental study investigating the operation of a multi-tube PDC-turbine hybrid system was performed to understand the effect of unsteady flows entering the turbine on the turbine component performance (ηT ). An eight-tube PDC can-annular configuration was integrated with a single-stage axial turbine nominally rated for 10 lbm/s, 25000 RPM and 1000 hp. The system accumulated a total of 144 minutes of operation with long duration runs of approximately 5 minutes, in order for the rig to achieve thermal steady state and for the turbine to attain constant speed. The turbine component efficiency was found to be similar under PDC-fired operation and steady flow operation within the uncertainty of the measurement.Copyright

Investigations of Thrust Generated by a Valved, Multitube PDE with Exit Nozzles

A thrust stand capable of measuring the time averaged thrust produced by a PDE was designed and tested. The study quantified the effect of exit nozzles on a valved multi-tube PDE system with three tubes arranged in a close-packed configuration. Three nozzle geometries were considered; straight, converging, and converging-diverging. Numerical simulations were also performed of a single detonation chamber with the same nozzle exit geometries. The test measurements are used to validate the predictions of time-resolved pressure, and the thrust predictions from the limit cycle model of PDE. Comparisons of trends in test measurements and model predictions of pressure-time traces and thrust show good agreement, for all the nozzles considered in the present study.

Experimental and Numerical Investigation of a Valved Multi-Tube PDE

A valved multi-tube PDE system with three tubes arranged in a close-packed configuration has been tested using three different exit nozzle geometries (straight, converging and converging-diverging). The PDE was operated using ethylene-air mixtures with each tube firing at 20 Hz in a sequential pattern. High frequency pressure transducers were installed throughout the flow path to investigate the time-resolved chamber pressure. Numerical simulations were also performed of a single detonation chamber with the same nozzle exit geometries. Analysis of the experimental data revealed that the converging and converging-diverging nozzles had similar behavior and increased the average pressure rise by 40% when compared to the straight nozzle. Although the numerical simulations matched the experimental trends, they over-predicted the static pressure-rise. The numerical results represent an upper bound, in guiding the design of an optimized PDE system.

Measurement of Pressure -Rise in a Pulse Detonation Engine

*† ‡ § Experimental measurements of static pressure -rise and thrust are made in a pulse deton ation engine (PDE). The research employs a valved ethylene -air 3 -tube PDE at an operating frequency of 20 Hz per tube. New terminology and definitions for pressure -rise in a PDE are introduced. The mean PDE fired static pressure was quantified with both low and high frequency pressure sensors. Pressure measurements were obtained at the combustor head -end and tail -end for configurations with straight and converging exit nozzles. It was determined that the high frequency pressure measurements provided a more accurate measure of the mean PDE fired pressure. The test measurements are used to validate computational predictions obtained using a quasi -1D limit cycle code. The overall PDE pressure -rise was found to be sensitive to system flow losses and the e xit nozzle geometry. It was observed that in an un -optimized configuration, with straight exit nozzle, the PDE operated with an overall pressure loss instead of a pressure gain. However, an overall pressure gain was observed when the PDE included a conve rging exit nozzle. The PDE thrust was quantified using a thrust stand to measure thrust directly, and by using tube pressure measurements to calculate thrust. The thrust calculated by pressure measurements was found to over -predict both the thrust stand measured thrust and computational thrust predictions.

Mechanical Response of a Thin -Walled Pulsed Detonation Tube Under Cyclic Operation

An experimental study was performed to investigate the mecha nical response of pulsed detonation engine (PDE) tubes to cyclic detonation loading using tubes of different material and thickness. A mechanical response rig was designed and tested at frequencies of 4, 5, 10, and 18 Hz using stainless steel, aluminum, a nd titanium tubes with outside diameters of 2” wall thicknesses of 0.120” and 0.035”. The effects of the propagation of a pure detonation wave versus the deflagration -to -detonation transition (DDT) process on tube strain were studied. Results show that hoo p strain was the most dominant response. A 30 -35 kHz resonance response was also measured, correlated with finite element simulation. and supported by literature findings. The present data suggest that cyclic detonation loading up to 40 Hz has no effect o n mechanical response when compared with single -shot detonation loading. Additionally, the strain during DDT was found to be an average of 50% higher than strain resulting from the propagation of a pure detonation wave.