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43rd Aerospace Sciences Meeting and Exhibit AIAA 2005-1345 10-13 January 2005, Reno, NV

American Institute of Aeronautics and Astronautics

Experimental Investigation into the Acoustic Performance of a Pulse Detonation Engine with Ejector

Aaron Glaser, Nicholas Caldwell, and Ephraim Gutmark

University of Cincinnati, Aerospace Engineering and Engineering Mechanics Cincinnati, OH 45221

Abstract Acoustic performance of a Pulse Detonation Engine (PDE) driven ejector system was experimentally investigated. The addition of an ejector at the PDE exhaust was seen to drastically alter the far-field acoustic signature. Several straight cylindrical ejector geometries were evaluated by varying the ejector to PDE diameter ratio (DR) from 2 to 4. For these experiments the axial placement of the ejector relative to the PDE exhaust was varied from an x/DPDE of –2 to 2. The PDE was run at fill-fractions of 0.6 and 1. Results from the tests performed showed that the ejectors had a significant impact on the sound measured in the far-field. For the current PDE test conditions an optimum ejector (based on sound attenuation) was found to be the DR=3, at a downstream axial placement of x/DPDE = 1. A global reduction in OASPL was observed to take place at all directivity angles, with the maximum reduction being approximately 6.8 dB.

Nomenclature DPDE detonation tube diameter DEJECT ejector diameter DR ejector-to-PDE diameter ratio (DR=DEJECT/DPDE) LEJECT ejector length x ejector position (distance from

PDE exit to ejector inlet) ff fill-fraction

Introduction Pulse detonation engines (PDEs) are currently the focus of much research. The PDE is an innovative propulsion technology that could potentially provide significant

advantages over more traditional gas turbine-based engines. Compared to these current engine technologies, the PDE offers mechanical simplicity, higher thrust-to-weight ratios, lower cost, and higher efficiency. As PDE research progresses towards practical applications, many new concepts have been proposed. It has been suggested that the use of ejectors on PDEs may be an effective way to increase the thrust being generated. An ejector is a coaxial duct placed around the exhaust of an engine and performs like a fluidic pump. The surrounding ambient air is entrained by the primary exhaust flow and directed into the ejector. Recent experimental work has shown promising results for ejectors in enhancing the thrust of PDEs. In a study performed using rounded-inlet, straight cylindrical ejectors, a maximum thrust augmentation level of 16% was seen [1]. An experimental study by Allgood et al [2] was carried out using a contoured bell-mouth inlet with several ejector configurations. Results from those experiments showed maximum thrust augmentation levels of 28% for straight ejectors and 65% for diverging ejector geometries. By placing an ejector duct at the exhaust of a PDE, the detonation tube exit condition will be changed as compared to a PDE with no ejector installed. Due to this, it is expected that ejectors will have an impact on the far-field acoustic signature of PDEs. The motivation for the current study is to quantify the impact that ejectors have on the acoustic behavior of PDEs. There is a large need to study the noise generated by PDEs as very little data in this important area has been published.

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

AIAA 2005-1345

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

2 American Institute of Aeronautics and Astronautics

Experimental Setup

Description of PDE System Acoustic testing of a pulse detonation engine (PDE) was carried out in the University of Cincinnati anechoic test facility. The PDE tube is constructed of 316 stainless steel and the geometry tested consisted of a 1” inner diameter with 25” length. Detonations were obtained in this small diameter tube due to the small cell size achieved by using ethylene and nitrogen-diluted oxygen as the fuel and oxidizer. The oxidizer used during testing was 20% nitrogen and 80% oxygen by mole fraction. This nitrogen dilution ratio allowed repeatable performance of the PDE at high frequencies while keeping the fuel-oxidizer mixture within the detonability limits of the tube geometry. The system is non pre-mixed with the fuel, oxygen, and nitrogen being stored in separate regulated high pressure supply tanks. The ethylene, oxygen, and nitrogen were injected into the PDE through the use of high-speed computer-controlled electromagnetic valves. These injectors were specifically designed to deliver gaseous fuels for automotive applications and hence do not have liquid lubrication requirements as do common automotive liquid fuel injectors. To achieve the required flow rates, oxygen was delivered using three valves mounted in a common manifold. Fuel and nitrogen were delivered through one valve each, with these valves mounted into a common manifold. When the valves are open they operate choked, therefore the gas flow rates can be adjusted by changing the upstream supply pressures. Mixing of the fuel/nitrogen and oxygen streams was achieved inside the detonation tube due to the opposing impingement of the two streams on each other. Spark ignition was accomplished using a stock automotive spark system delivering approximately 35mJ of energy. An orifice plate obstacle with blockage ratio of 0.6 was located 4 tube diameters from the headwall to accelerate the deflagration-to-detonation transition process. The current

PDE configuration was able to run at a maximum operating frequency of 20 Hz. Operation of the PDE was computer controlled and synchronized using a LabView interface program written at UC. The PDE LabView interface provided the flexibility of specifying engine operating parameters such as fill-fraction, spark timing, number of detonations desired and frequency. Dynamic pressure sensors (PCB model M102A) were mounted along the detonation tube length to verify that Chapman-Jouguet detonations were obtained. Data was acquired using a National Instruments PCI-6110 DAQ board able to sample at 5 MHz simultaneously on 4 channels. This fast sampling rate was adequate to accurately resolve the detonation wave speed. Acoustic Test Facility The acoustic test facility at UC was specifically designed for accurate jet noise testing. The chamber has previously been validated against much proven data [3]. The PDE was installed in the anechoic test chamber parallel to an existing jet flow test rig. A radial microphone array of eight microphones at an average radius of 9-½ feet (114 jet diameters) from the PDE exhaust provided the means for collecting far-field acoustic data at directivity angles ranging from 62° to 167° (see Figure 1). Directivity angle was defined as the angle from the PDE tube centerline to the microphone, where the downstream flow direction was 180° and directly upstream of the PDE was 0°. Placed along the array were 8 Bruël and Kjaer model 4939 ¼” condenser microphones. The microphone data was acquired at a sample rate of 204.8 KHz using a pair of synchronized National Instruments model 4452 dynamic signal acquisition boards. The raw data was recorded as a binary time record. This raw data was then processed using custom written LabView codes developed at UC for jet noise studies. Narrowband spectral data was obtained from the data processing. In


addition to spectral data, calculation of the overall sound pressure level (OASPL) was computed. For the purposes of this report, OASPL was defined as the rms of the fluctuating component of the microphone signals. Prior to the OASPL calculation, the captured time signal was passed through a 4th order Butterworth high pass filter to remove frequencies below the anechoic cutoff limit of the chamber. For this study a filter cutoff frequency of 500Hz was used for the OASPL calculations. PDE Ejector Test Setup The key PDE operating parameters that could be varied were the following: 1) PDE frequency, 2) fill-fraction, 3) equivalence ratio, and 4) nitrogen dilution ratio. The fill-fraction is defined as the ratio of the tube volume filled with a detonable mixture to the total tube volume. The nitrogen dilution ratio is the molar ratio of nitrogen to oxygen in the oxidizer. Unless otherwise noted, all tests for the current experiments were performed with a stoichiometric fuel/oxidizer mixture, a nitrogen dilution ratio of 20%, fill-fraction of 1, and a PDE operating frequency of 20 Hz. During the acoustic testing the PDE was run for a time duration of 5 seconds, or 100 detonation cycles, for each data point collected. As shown in Table 1, four ejector geometries were tested. The ejectors were all straight cylindrical tube ejectors with square edged inlets, and mounted coaxially to the detonation tube. The axial placement of the ejector inlet compared to the PDE exit plane was varied from –2 to +2 inches. A negative position corresponds to the ejector inlet placed upstream of the PDE exit, with the ejector overlapping the detonation tube, while for a positive placement the ejector is mounted downstream of the detonation tube. The ejector-to-PDE diameter ratio (DR) was varied from 2 to 4, while the ejector length-to-diameter ratio (LEJECT/DEJECT) was varied from 2 to 3.

DEJECT LEJECT DR L/D (inch) (inch)

D2L6 2 6 2 3 D3L6 3 6 3 2 D4L6 4 6 4 1.5

Results and Discussion

Baseline PDE Testing The baseline PDE system for these tests consisted of the detonation tube with no ejector installed. This baseline configuration was characterized extensively to allow for an accurate determination of the effect of the ejector on the far-field acoustics of the PDE. Several PDE operating parameters were varied for the baseline testing. Fill-fraction has been seen to be an effective way of throttling a PDE. It is interesting to note that as well as a simple scheme to control the thrust output, significant performance efficiency gains can be attained by decreasing the fill fraction of the PDE tube due to partial fill effects [4]. For these baseline tests the fill-fraction was varied over a range of 0.6 to 1. Figure 2 shows a directivity plot comparing two separate PDE fill conditions, fill-fractions of 0.6 and 1, and their effect on OASPL. It can be seen that for both fill-fractions higher acoustic levels were measured at the more downstream directivity angles. This behavior can be attributed to the directional variation in shock strength of the exiting detonation wave. A qualitative measure of this variation in shock strength can be determined from Figure 3a, a shadowgraph image showing a detonation wave exiting the PDE tube. The leading shock wave is visible as the thick black band, although it should be noted that the actual shock wave thickness is much smaller than this black band. The band is actually an artifact caused by the shadowgraph technique, which is a

Table 1: Ejector Geometries Tested


method of visualizing density gradients using a collimated beam of light. The large density gradients associated with a shock wave cause excessive bending of the light and appear as the think band in the visualization. The thickness of the black band can be correlated to the strength of the shock wave as a stronger shock wave causes a thicker band. From the shadowgraph it can be seen that the strength of the shock traveling downstream along the tube axis is greater than the strength of the shock traveling in the upstream and sideline angles by comparing the thickness of the black band. There is a large amount of expansion taking place as the detonation diffracts from the confined PDE tube to an unconfined space that weakens the shock strength perpendicular to the tube axis. Figure 2 shows that a decrease in fill-fraction shifts the directivity curve down to lower acoustic levels. This shift is caused by the large effect that fill-fraction has on the strength of the blast wave exiting from the PDE tube. The detonation wave speed, measured near the end of the PDE tube at a fill-fraction of 1 was 2258 m/s, which is the Chapman-Jouguet detonation wave speed for the mixture used. However, for a fill-fraction of 0.6 the measured detonation wave speed was 1112 m/s. The shadowgraph visualization in Figure 3b shows the structure of a detonation wave produced by an under-filled PDE tube. From comparison of this image to Figure 3a, it can be seen that the shock wave for the under-filled case is weaker then that for the case of a fill-fraction of 1. Previous research has shown a logarithmic increase in OASPL with PDE operating frequency [5]. The trend from that study was based on data collected for PDE operating frequencies from 1 to 6 Hz, which was the maximum achievable frequency of the earlier PDE system. The acoustic levels measured increased with frequency due to a greater number of detonations within the PDE run time. The current baseline PDE system was run over an operating frequency range from 1 to 20 Hz at a fill-fraction of 1. As before, the OASPL was observed to

increase logarithmically with operating frequency, thus verifying this trend, as shown in Figure 4. The logarithmic trend with frequency is a result of the OASPL being defined as the rms of the fluctuating microphone signal. For the remainder of the current testing the PDE operating frequency was kept at 20 Hz. PDE Ejector Testing The exit geometry at the PDE exhaust is modified by the addition of an ejector duct. Results from the current testing show that the ejector duct has a significant impact on the acoustic signature of the PDE. For the ejector geometry D3L6, the reduction in OASPL caused by the ejector is shown in Figure 5. It can be seen from this directivity plot that OASPL is reduced at all angles with more attenuation occurring at the downstream angles. The maximum reduction was 6.8 dB taking place at an angle of 167 degrees. The directivity angle at which the peak noise occurred for each configuration was shifted from 156 degrees on the baseline system to 132 degrees with the ejector duct installed. A representative plot of the acoustic spectrum, generated by performing a Fourier analysis, at a microphone angle of 144 degrees is shown in Figure 6. Curves on the plot correspond to the baseline case with no ejector and another with the D3L6 ejector installed. The spectral distribution of acoustic energy shown is consistent with that from a shock wave. As the detonation wave generated by the PDE exits the tube into the free atmosphere it essentially transitions into a decaying spherical shock wave as described by ideal blast wave theory. The dominant mode of sound generation by a PDE is through shock noise. It can be seen that the most energetic frequencies are in the range from 200 to 800 Hz, and it is within this low frequency range that the majority of the acoustic energy is concentrated. There is also a substantial amount of energy contained in the high frequency range, above 1000 Hz. The higher frequency components were observed to roll off at approximately -20.6 dB per decade.


From a comparison of the two curves shown in Figure 6, it is seen that the ejector geometry significantly altered the low frequency acoustic spectrum by reducing the SPL. At frequencies of 8000 Hz and above the spectrum is not changed much by addition of the ejector, and the two curves are nearly identical. The ejector-to-PDE diameter ratio (DR) was varied from 2 to 4. Figure 7 is a directivity plot showing how OASPL changes with DR. It can be seen that at the upstream angles the ejector with DR=2 has the lowest OASPL, while at the downstream angles it exhibits the highest peak OASPL. The largest diameter ejector, with a DR=4, shows amplification at the upstream angles, increasing the OASPL above the baseline levels. This same ejector shows attenuation of the noise at the downstream angles. The ejector with a DR=3 shows the best overall acoustic attenuation. To better visualize the change in acoustic performance with diameter ratio, a directivity angle of 144 degrees was chosen to be representative of the peak noise generated for each ejector configuration. The OASPL at an angle of 144 degrees for the ejector geometries tested is shown in Figure 8. From this figure the effect of DR on acoustic performance is easily seen. For the current PDE operating conditions, a DR=3 is the optimum sized ejector, thus giving the lowest sound levels. Another important parameter of the PDE ejector system is the axial placement of the ejector relative to the PDE exhaust. The ejector placement was varied from an x/DPDE of –2 to +2. Initial testing was performed at a constant DR=3 using the D3L6 ejector duct. Results from this testing are given in Figure 9, a directivity plot comparing four different axial positions with the baseline PDE configuration. When the ejector is overlapping the PDE tube at an x/DPDE of –2 the ejector performed the worst with respect to acoustic effects. It can be seen that by moving the ejector from the most overlapped position towards a downstream placement at x/DPDE equal to +1, there is a

reduction in OASPL at nearly all directivity angles. For ejector positions in the range of –2 to 0, there was amplification of the sound generated at the upstream directivity angles as compared to the baseline case with no ejector. These axial positions did however provide some attenuation at the downstream directivity angles. A downstream ejector placement of x/DPDE equal to +1 was the only axial position that showed a global reduction in OASPL as compared to the baseline case. These findings imply that there is an optimum ejector position that would be most beneficial in reducing noise from the PDE. The OASPL data given in Figure 10 clearly shows the effect of both ejector diameter and axial placement. It can be seen that the optimum axial placement changes as a function of the ejector diameter. For all ejector geometries tested the optimum ejector placement was either inline with the PDE exhaust or downstream. The ejector geometry that gave the most acoustic attenuation was the D3L6 ejector. This ejector had an optimum axial position at x/DPDE of +1, while moving the ejector either upstream or downstream from this position caused an increase in OASPL. Recent PDE thrust performance measurements have shown that the thrust augmentation generated by using PDE driven ejectors is a strong function of axial placement [2]. Similar to the acoustic performance trends, the highest thrust augmentation levels were produced at either the inline or downstream ejector positions. It is advantageous that the thrust augmentation and acoustic attenuation trends are similar in that when the ejector is producing a large thrust augmentation, a large acoustic attenuation would also be provided. A set of experiments was run in order to determine how the PDE fill-fraction affects the sound generated by the PDE ejector system. Measurements were made at fill-fractions of 0.6 and 1.0, and results are shown Figure 11. Values of OASPL for both fill cases, taken at the representative directivity angle of 144 degrees, are plotted at different axial placements of the ejector.


At both fill-fractions, the optimum ejector placement is at an x/DPDE of +1. The low fill case is seen to be more sensitive to downstream placement as evidenced by the sharp increase in OASPL moving from an x/DPDE of 1 to 2. With the ejector placed at the optimum position, the high fill case (ff=1.0) showed a 2.91 dB reduction from the baseline configuration with no ejector. At the lower fill condition, a reduction of 2.1 dB from the (ff=0.6) baseline case was achieved. The OASPL level at a fill-fraction of 0.6 is less than that at a fill-fraction of 1.0, but the higher fill case saw a larger attenuation from its respective baseline.

Conclusion Far-field acoustic measurements were performed on a PDE-ejector system. The effect of a straight cylindrical ejector on the acoustic signature of a PDE was quantified. Baseline acoustic testing on a PDE without ejector was also performed. The baseline testing showed that OASPL decreases with fill-fraction, essentially shifting the directivity curve to lower sound levels. It was verified that baseline OASPL increases logarithmically with the PDE operating frequency. It was found that the effect of the ejector on the PDE acoustics was a function of the diameter ratio of the ejector. For the PDE configurations tested, a diameter ratio of 3 gave the most acoustic attenuation. The acoustics generated by the PDE-ejector system were seen to be sensitive to the axial placement of the ejector. The overlapping ejector configurations (x/DPDE < 0) were in general worse for sound reduction than the downstream configurations (x/DPDE > 0). The optimum ejector position was found to be at x/DPDE=1. At this position the PDE-ejector system experienced a global reduction of OASPL at all directivity angles. The downstream angles experienced a larger sound attenuation than did the upstream angles. The largest reduction in OASPL was measured to be 6.8 dB. Fill-fraction effects were also investigated, showing that larger

acoustic attenuation occurred at a fill-fraction of 1 as compared to a fill-fraction of 0.6. Results from the current work show that ejectors can have a significant effect on the acoustic signature of a PDE. Experimental thrust performance measurements done on PDEs with straight ejectors have shown that maximum thrust augmentation generally occurs with an inline or downstream axial placement of the ejector [2]. This trend of increased thrust augmentation at the downstream axial positions is similar to the trend observed for the acoustic attenuation performance of ejectors at the downstream positions. Due to the similarity of these trends it should be possible to design an optimized system that produces both high thrust augmentation and a significant reduction in noise.

Acknowledgements The authors would like to extend our appreciation to Richard Deloof from NASA Glenn Research Center for sponsoring this research under grant NAG3-2669

References 1. Rasheed, A., Tangirala, V., Pinard, P.F., Dean, A.J., “Experimental and Numerical Investigations of Ejectors for PDE Applications”, AIAA 2003-4971, 39th AIAA Jet Propulsion Conference, Huntsville, AL, July 21-23, 2003. 2. Allgood, D., Gutmark, E., “Performance Measurements of Pulse Detonation Engine Ejectors”, AIAA 2005-0223, 43rd AIAA Aerospace Sciences Meeting and Exhibit, Reno, NV, January 10-13, 2005. 3. Callender, B., Gutmark, E., and DiMicco, R., “Design and Validation of a Coaxial Nozzle Acoustic Test Facility”, AIAA 2002-0369, 40th Aerospace Sciences Meetings and Exhibit, Reno NV, Jan. 14-17.


4. Schauer, F. Stutrud, J., and Bradley, R., “Detonation Initiation Studies and Performance results for Pulse Detonation Engine Applications,” 39th AIAA Aerospace Sciences Meeting, AIAA 2001-1129 5. Allgood, D., Glaser, A., Caldwell, N., and Gutmark, E., “ Acoustic Measurements of a Pulse Detonation Engine,” 39th AIAA Aerospace Sciences Meeting, AIAA 2001-1129


PDE

90º

Average Radius: 9.5 ft

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Figure 1: Description of Microphone Array

Figure 2: Effect of fill-fraction on OASPL directivity

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Figure 3a: Directional variation of shock strength Figure 3b: Effect of PDE under-fill on shock wave structure

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Figure 5: Alteration of the acoustic signature of a PDE by the addition of an ejector

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Figure 11: Variation of OASPL with ejector axial position at two fill-fractions

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