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43rd Aerospace Sciences Meeting and Exhibit AIAA 2005-0413 10-13 January 2005, Reno, NV

American Institute of Aeronautics and Astronautics

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Acoustic Measurements of an Integrated Pulse Detonation Engine with Gas Turbine System

Nicholas Caldwell*, Aaron Glaser†, Russell Dimicco‡, and Ephraim Gutmark§

Department of Aerospace Engineering

University of Cincinnati Cincinnati, Ohio 45219

Abstract

A rig has been designed to integrate an annular array of six pulse detonation engines (PDEs) with a common gas turbine engine. The purpose of this rig is to allow for the examination of the acoustic and performance effects of such integration. The overall effect of this combination is a more compact, simplified engine concept whereby the core (the high pressure compressor, combustor, and high pressure turbine) of a typical gas turbine engine is replaced by the PDE tubes, and combustion occurs in an unsteady manner instead of that which comes from typical steady flow combustors. Success in such integration would result in a more efficient form of combustion that would use less fuel while reducing the weight and cost of the engine dramatically. This paper describes the design of the integration rig, and presents some preliminary pressure history results based on the flow inside the PDE tube, inside the rig itself, and the flow downstream of the turbine to provide an understanding of the behavior of the PDE flow passing through the blades of the turbine.

Facility and Equipment

The motivation behind the development of the PDE/turbine rig lies in a cooperative project with NASA, GE Global Research (GEGR), and the Air Force Research Laboratory (AFRL). Similar projects have been undertaken by these groups, and a wide range of studies is under way. These focus largely on the operability, performance, and acoustic behavior of such an integrated system. An initial examination into a combined PDE/turbine system has been published by the AFRL, listed as Reference [1]. In this study, a PDE was connected to a centrifugal turbocharger, and various conditions were tested to determine how the two interacted. The design of the rig used in this study began with the selection of a turbine that could be affected by the flow of a PDE exhaust. Such a turbine would need to be rated for low power, with a small blade diameter, and be readily available. The turbine which was found to best meet these requirements was the Allied Signal model JFS-100-13A power turbine. This turbine has an axial flow free turbine wheel attached to a main gear train pinion and reduction gear; however, the exhaust from the turbine is radial. This feature is clear from the picture in Figure 1. At flight conditions, the turbine is

∗Graduate Student, Student Member AIAA †

Graduate Student, Student Member AIAA ‡Laboratory Coordinator §

Ohio Eminent Scholar, Associate AIAA Fellow

43rd AIAA Aerospace Sciences Meeting and Exhibit10 - 13 January 2005, Reno, Nevada

AIAA 2005-413

Copyright © 2005 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

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rated to output 90 shaft horsepower at 60,400 rpm. After the reduction gearbox this is brought down to around 3000 rpm, a ratio of 18.1:1. The height of the blades is approximately 1.15”, which is one of the key parameters that prompted the selection of this turbine as opposed to others. With this blade height, the exit diameter of the PDE matched very closely the height of the blades. The design mass flow for this system is 1.6 lbm/sec, which is achievable in a lab scenario. The next preliminary design decision was a selection of the PDE systems. The six pulse detonation engines used on this rig consist of 25” long stainless steel pipes with an internal diameter of 1”. The fuel chosen for the engines was ethylene (C2H4), while gaseous oxygen (O2) was selected as the oxidizer. This setup has been extensively studied through experimentation at the University of Cincinnati, and has been found to be detonable without the use of a spiral. At the head walls of the PDEs there are openings that admit the oxygen and ethylene through a system of valves and regulators. The valves used in this system, developed by Quantum Inc, deliver high-pressure jets of these gases that directly impinge upon one another. Originally designed for automotive systems that run on natural gas, these valves require no liquid lubrication, and are thus a good selection for this project. They allow an average mass flow rate of 3 g/sec, allowing quick filling of the PDE tubes. The controller used to operate these valves allows for a maximum frequency of approximately 100 Hz per valve. The ignition system used for the PDEs is based on that used in automotive applications. Prototyping has shown that these PDEs can be

operated to at least 20 Hz. Testing into higher frequency ranges has not been performed, but it is estimated that this system could be fired up to 30 Hz. The PDEs are controlled using National Instruments LabView code. This code allows for the independent controlling of each individual valve in the system, as well as the ignition system. When firing a PDE, a signal is first sent to four valves that first inject a slug of nitrogen and oxygen into the tube for purging and cooling purposes. The fuel valve is then opened for a specified time that determines the fill fraction. All five valves are then simultaneously closed, and the spark plug is fired after a specified time delay. The control code allows for all PDEs to be fired independently, and hence any pattern of PDE firings can be performed by setting the relative time delay between the tubes. A more detailed description of the experimental setup can be found in Reference [2].

Integration Rig Design The main goals set out prior to the design of the rig were to minimize losses in the flow leaving the PDE, and ensuring a modular system for examination of many different concepts. The PDE array diameter was set to 5”, which allowed for a small amount of spacing between the PDE tubes and PDE head walls, while still keeping the PDE array diameter extremely close to the mean diameter of the turbine blades. With this size set, the next decision to be made was how to force the flow into the blades of the turbine with minimal losses. Since the hub of the turbine was basically a flat plate, the inclusion of a contoured centerbody inside the rig was necessary to direct the flow into the

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blades instead of allowing it to impinge on the turbine hub. This centerbody can be seen in the section drawing in Figure 2. Also, in an attempt to match the design mass flow rate for the turbine, there needed to be an addition of bypass air to assist the PDE flow. Since each valve could only pass approximately 3 g/sec, at an operating frequency of 25 Hz this would provide only around 40% of this design mass flow rate. Hence, the design required additional input ports through which this bypass flow could enter the rig. These can be seen with air hoses attached in the photograph of the assembled rig in Figure 3. To increase the modularity of the rig, it was determined that the stator vanes that were included with the turbine should be removable. In the case that they were removed from the system, there would be fewer losses due to the flow passing through the stator. However, the removal of these blades would cause the flow to enter the rotor at an angle far from that for which it is designed. The concept developed to solve this problem was to induce a swirl into the bypass airflow to direct the PDE exhaust into the rotor blades. A description of how this has been done will be given. The assembled rig is pictured below in Figure 3. All parts are built from 1045 carbon steel and black anodized to prevent rusting. The exhaust from the PDE array feeds into the main swirl chamber through a circular array of ports on the rightmost flange of the rig. This can be seen in Figure 4. Once the flow has passed through the detonation tubes, it is forced through a set of nozzles inside the rig itself. This is achieved by a plate which has six nozzles cut into it, spaced to line up with the inlet ports of the PDEs into the integration rig. This

plate can be interchanged to be either a set of diverging, converging, or straight nozzles. Previous studies at the University of Cincinnati have shown positive effects of implementing nozzles at the exit of PDE tubes, including increased thrust and reduced noise ([3], [4], [5], and [6]). The section view given in Figure 2 is presented to better visualize the inside of the rig. In the main swirl chamber the PDE exhaust and bypass air are combined into a single flow. The bypass flow is introduced outside of the main mixing chamber, but enters into it through a series of angled slots after being distributed throughout an outer chamber. A pressure port tapped into the side of the mixing chamber allows for a dynamic measurement of the pressure in this portion of the rig. This is performed at a sample rate of 1 MHz using a PCB model 102A06 dynamic pressure transducer, which has a range of 0-500 psi. Figure 4 shows the location of the three pressure sensors. The hub, in conjunction with contoured walls, directs the mixed bypass air and PDE exhaust into the blades of the turbine stator. This stator can be removed if desired without necessarily losing the swirl generated by the geometry of the stator blades. Its removal would allow for the detonation energy to be transferred directly to the rotor blades, thereby providing a larger source of power to be extracted. A different mixing chamber can be placed inside the rig that can generate the needed component of swirl. The advantage here would be a reduction in losses due to the detonation waves hitting the stator blades, but the disadvantage would be that the exact flow that would have been generated by the stator blades cannot be perfectly duplicated. In fact the shock

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losses that are generated by the stator blades are incredibly large. A shadowgraph image of a detonation passing through the stator section of this turbine is shown in Figure 5. It can be seen here that the flow entering the turbine from the right suffers a large amount of losses through the shock system formed at the exit of the stator. It follows that acoustic transmission through these blades will be minimal, but the potential for useful work extraction will also be greatly reduced. Swirling the flow an appropriate amount to better match the design inlet rotor conditions could lead to a minimization of these losses. The turbine connects directly to the main swirl chamber by means of an additional flange, as shown in Figure 3. This flange has the added use that it helps to direct the flow into the blades of the turbine. The exhaust is directed radially, as mentioned before in the section on the gas turbine. The output shaft can be seen in this picture, which is connected to the torque cell and the dynamometer by means of universal gears. The torque cell used is a Lebow model 1604-2K, and records a dynamic measurement of the horsepower and shaft speed. It is rated up to 2,000 in-lb of torque and can record angular rates of up to 10,000 rpm. This data is transmitted to the computer via a Daytronic System 10 Data Acquisition and Control System and recorded with an in-house LabView program. The turbine is loaded using a water dynamometer from the Go-Power Corporation, model DY-7D. This dynamometer has a range of 10,000 rpm and 30 hp. The bypass ratio can be varied by changing the regulator pressure on the bypass delivery system, thereby changing the mass flow rate of the bypass air into the system.

The system allows for a wide range of parameters to be studied. PDE variables that can be changed include the fill fraction, equivalence ratio, and pulsing frequency. Other factors that can be examined are the number of PDEs used (1-6), the speed of the turbine rotor, the distance of the PDEs from the inlet to the turbine, the inclusion or exclusion of the stator section of the turbine, and whether or not to induce a swirl into the flow entering the turbine. Also, the pattern in which the PDEs are fired is variable, as mentioned before, as well as the nozzle geometries.

Preliminary Results

A brief test matrix was performed on this integrated test rig to determine the effect that the turbine blades have on the flow leaving a single PDE tube. This included a parametric study of fill fraction, equivalence ratio, turbine speed, and frequency. These cases were performed with a single PDE tube with the stator section included in the system and no nozzle plate in place. Figure 6 shows a sample plot of a multi-cycle case in which the PDE was operated at 20 Hz for five detonations. The top plot shows the pressure history inside the PDE tube, the middle plot depicts the pressure inside the swirl chamber and the bottom plot shows the pressure history downstream of the turbine. This case was recorded with no bypass air. It can be seen that the peak pressure of the first detonation drops from near 600 psi to below 100 psi between the PDE tube and the swirl chamber. This peak pressure experiences a drop of 7.1 dB, and results from the expansion of the detonation wave into the swirl chamber. While the

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leading shock survives the expansion, it is greatly weakened due to the sudden volume change. After passing through the turbine blades, the detonation is mostly broken up, and the resulting flow has a peak pressure that is below 5 psi. At this point the energy of the flow has been extracted by the turbine, and the blockage due to both the stator and rotor blades has significantly disrupted the flow. The total peak pressure attenuation in this case is 14.7 dB from the swirl chamber, and a total reduction of 21.8 dB from the PDE tube. Figure 7 shows the effect that the integration has on the attenuation of the peak pressure as the fill fraction of the PDE is varied. There does not seem to be much of a change in the suppression when there is no bypass air being forced through the system. The reduction in the peak pressure of the detonation between the PDE tube and the turbine exhaust seems to hover around 22 dB for fill fractions between 0.4 and 1.4, while the reduction between the swirl chamber and the turbine exhaust is around 14 dB. For the higher fill fractions, this reduction appears to be greater, closer to 15 dB. These higher fill fractions correspond to cases in which the tube is overfilled. An external detonation produces a larger pressure peak inside the swirl chamber, and after passing through the blockage created by the blades is largely disrupted. This could explain why the overfilled cases would experience a greater attenuation. However, a more detailed analysis of this needs to be performed to substantiate this. There is a small enough change in the suppression levels that these explanations remain speculations, and will be studied further in the near future.

Figure 8 shows the same situation, except that bypass air is added to the system. The bypass air is given an upstream pressure of 40 psi, which spins the turbine at approximately 300 rpm. An interesting feature of this plot is that the greatest reduction seems to occur around a fill fraction of 0.8, which corresponds to a slightly under filled tube. The addition of the bypass air seems to cancel out the effect of overfilling the tube. This may result from the bypass air dispersing the spillage into the swirl chamber and preventing the external detonation from occurring. If this were the case, then the most energetic detonation would occur when the tube was filled or slightly under filled. There is a sharp rise at the end of the plot corresponding to a fill fraction of 1.4. This would make sense if the dispersion due to the bypass air is not succeeding in removing a large amount of excess detonable mixture from the rig at this fill fraction. Figure 9 depicts the effect that the equivalence ratio has on the reduction of the peak pressures in the integration rig. The suppression does not seem to be greatly affected by the equivalence ratio when the system is being filled with bypass air. The large amount of air being forced into the system seems to play a large role in dispersing the flow leaving the PDE and eliminating the effects of the PDE parameter variations. The final figure, Figure 10, shows the effect that the amount of bypass air has on the reduction of the detonation pressures. The turbine speed was adjusted from 100 to 10000 rpm, and its effect plotted in this graph. From this plot it looks as though the addition of bypass air succeeds in attenuating the

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peak pressures more than having no bypass air at all. However, these plots suggest that the turbine speed does not greatly affect this attenuation once the turbine is spinning.

Conclusions

This rig provides a test bed that can produce useful information regarding the acoustic and performance characteristics of an integrated pulse detonation combustor with a gas turbine. A brief investigation was performed regarding the effect the turbine blades had on the peak pressures leaving the PDE tube. This preliminary study shows that the peak pressure reduction may in fact be greatest when the PDE tube is completely filled, but not to the point of spillage. The reduction also seems to not be sensitive to the equivalence ratio inside the PDE tube prior to detonation. The final study suggests that the peak pressure reduction between the PDE tube and the turbine increases slightly when the turbine is spinning, but is largely independent of the actual turbine speed. A more detailed investigation of these parameters needs to be made to answer some of these questions and to improve the quality of the data. Also, a larger test matrix will hopefully provide some more insight into the interaction between a PDE and turbine.

Acknowledgements

The authors would like to acknowledge the sponsorship of both National Aeronautics and Space Administration and General Electric Global Research. We would also like to thank Nick Voisard and Ben Miller for their assistance in the construction of the rig.

References

[1] Schauer, F., Bradley, R., and Hoke, J., “Interaction of a Pulsed Detonation Engine with a Turbine”, 41st AIAA Aerospace Sciences Meeting and Exhibit, January 6-9, Reno, NV, AIAA 2003-0891.

[2] Glaser, A., Allgood, D., and

Gutmark, E. “Experimental Investigation into the Off-Design Performance of a Pulse Detonation Engine”, 42nd AIAA Aerospace Sciences Meeting and Exhibit, January 5-8, Reno, NV, AIAA 2004-1208.

[3] Allgood, D., Glaser, A., Caldwell,

N., and Gutmark, E. “Acoustic Measurements of a Pulse Detonation Engine”, 10th AIAA/CEAS Aeroacoustics Conference, Manchester, UK, AIAA 2004-2879.

[4] Allgood, D., Gutmark, E., Hoke, J.,

Bradley, R. and Schauer, F. “Performance Measurements of Multi-Cycle Pulse Detonation Engine Exhaust Nozzles”, 43rd AIAA Aerospace Sciences Meeting and Exhibit, January 9-13, Reno, NV, AIAA 2005-0222.

[5] Allgood, D., and Gutmark, E.

“Effects of Exit Geometry on the Performance of a Pulse Detonation Engine”, 40th AIAA Aerospace Sciences Meeting and Exhibit, January 14-17, Reno, NV, AIAA 2002-0613.

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[6] Yungster, S., “Analysis of Nozzle Effects on Pulse Detonation Engine Performance”, 41st AIAA Aerospace Sciences Meeting and Exhibit, January 6-9, Reno, NV, AIAA 2003-1316.

[7] Rasheed, A., Tangirala, V.E.,

Vandervort, C.L., and Dean, A.J. “Interactions of a Pulsed Detonation Engine with a 2D Blade Cascade”, 42nd AIAA Aerospace Sciences Meeting and Exhibit, January 5-8, Reno, NV, AIAA 2004-1207.

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Figure 1 - Allied Signal Model JFS-100-13A Power Turbine

Figure 2 - Rendition of Section View of Assembled Rig

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Figure 3 - Assembled Rig

Figure 4 - Integration System – PDE Connections – Pressure Transducer Locations Also Shown

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Figure 5 - Shadowgraph Image of Detonation Passing Through Turbine Stator During Blowdown

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Peak Pressure Reduction with Fill Fraction - No Bypass Air

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Peak Pressure Reduction with Equivalence Ratio - With Bypass Air

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