[american institute of aeronautics and astronautics 44th aiaa aerospace sciences meeting and exhibit...

American Institute of Aeronautics and Astronautics

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Performance Measurements of a Pulse Detonation Combustor Array Integrated with an Axial Flow Turbine

Aaron J. Glaser*, Nicholas Caldwell†, and Ephraim Gutmark‡ University of Cincinnati, Cincinnati, Ohio, 45221

Experimental studies were performed to investigate the performance of a hybrid propulsion system integrating an axial flow turbine with multiple pulse detonation combustors. The integrated system consisted of a circular array of 6 pulse detonation combustor tubes exhausting through an axial flow turbine. Turbine power extraction measurements were made to quantify the integrated system performance. The isentropic efficiency of the turbine section was also determined. The effects of the PDE operating parameters of fill-fraction and equivalence ratio were studied. It was found that system performance was sensitive to the turbine inlet temperature. The specific power output of the turbine as well as the turbine efficiency were found to increase with both fill-fraction and equivalence ratio.

Nomenclature PDC = pulse detonation combustor ff = fill-fraction φ = equivalence ratio β = bypass ratio η = isentropic turbine efficiency

I. Introduction ULSE detonation engines (PDEs) have generated much interest as a possible future high efficiency propulsion device. So far, the majority of the work performed on PDEs has been focused on using the detonation tube to

directly generate an impulsive thrust for aerospace propulsion applications1. However various other detonation based applications and concepts have been proposed. One such innovative concept being studied is a hybrid propulsion system which incorporates detonative mode combustion within a conventional turbofan engine. The modern gas turbine engine has matured to a level where the efficiency is very high, but additional gains have virtually reached a plateau. A radical departure from the current steady-flow gas turbine engine design would be needed to create a large increase in efficiency. It has been proposed that the incorporation of pulse detonation combustors (PDCs) into the gas turbine engine could provide significant advantages. The hybrid PDC-Turbine system would build on the strengths of both engines to increase the operating efficiency of the overall system. Compared to the traditional turbofan engine, the hybrid PDC-Turbine could offer mechanical simplicity, lower cost, and competitive performance. A PDC based hybrid engine system could show performance gains in both propulsion and stationary power generation applications. For the latter application, the flow would be additionally expanded through a power turbine. There are many unknowns and challenges involved with creating a successful hybrid PDC-Turbine system. The current work seeks to address some of these issues.

A PDE is an inherently cyclic device that operates in the detonative mode of combustion. A combustion process is characterized as a detonation if the combustion wave travels at supersonic speeds. Detonation wave speed is a function of the initial mixture composition, with typical values ranging from a Mach number of 3 to 7. Due to these high combustion wave speeds the detonation event closely approximates a constant volume combustion process. From a thermodynamic analysis, a constant volume heat addition is more efficient than the constant pressure heat addition associated with traditional gas turbine combustors. Additionally, the leading shock of the detonation is a

* Graduate Research Assistant, Department of Aerospace Engineering, ML0070, Student Member AIAA. † Graduate Research Assistant, Department of Aerospace Engineering, ML0070, Student Member AIAA. ‡ Professor and Ohio Eminent Scholar, Department of Aerospace Engineering, ML0070, Associate Fellow AIAA.

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44th AIAA Aerospace Sciences Meeting and Exhibit9 - 12 January 2006, Reno, Nevada

AIAA 2006-1232

Copyright © 2006 by Aaron J. Glaser. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.


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strong compression wave: a detonation wave traveling at Mach 7 has a pressure ratio across the shock of nearly 30 atmospheres. This compression due to the detonation wave eliminates the need for the separate high pressure inlet compression cycle that exists in a standard gas turbine engine. In the proposed hybrid PDE concept, the entire high-pressure core of a gas turbine engine would be replaced by a detonative mode, pressure rise combustion system. This hybrid configuration would take advantage of the increased combustion efficiency of the near constant volume combustion (CVC), while getting rid of the high pressure compressor and turbine associated with a conventional constant pressure combustion based gas turbine engine. A low pressure compressor would still be necessary to provide sufficient air flow needed to fill and purge the detonation combustion chambers. In order to power the compressor and other accessories, a low pressure turbine would also be needed. The integration of the detonation tubes with the turbine is a key area that must be studied as this integration is a significant challenge that must be overcome in order to achieve a successful hybrid PDE system.

The limited amount of previous work performed on integrated PDC gas turbine systems has shown potential performance benefits, as well as highlighted areas where more work needs to be done. A theoretical analysis was performed using the Numerical Propulsion Systems Simulation (NPSS) tool2, to investigate a hybrid PDE cycle. For the analysis, a detonation tube was incorporated into the core of a turbofan engine operating at an altitude of 35,000 feet and Mach number of 0.85. Operating with equal amounts of inlet airflow, the hybrid PDE system produced 2% higher thrust and showed 8% to 10% lower specific fuel consumption than the baseline turbofan engine. Experimental work performed by Schauer et Al.3. investigated a detonation driven turbine. The PDE system was exhausted into an automotive based centrifugal turbine. The automotive turbine was tested over a wide range of operating conditions and reached rotational speeds of 130,000 RPM. A comparison of the experimental data to theoretical calculations showed high losses during work extraction by the turbine. Although poor performance was observed, it should be noted that the centrifugal turbine tested was not designed to efficiently operate in a detonation environment. A properly designed axial flow turbine would be more suitable, however more research needs to be performed to determine the optimum turbine configuration. Designing an integrated PDE turbine system is not a straight forward matter, as the interaction between components is a complex problem. Turbomachinery designed to work in current gas turbine based engines is designed to function in a steady manner while a PDE is a highly unsteady device. The exhaust from a PDE system varies from low subsonic to high supersonic speeds throughout the engine operating cycle. Strong compression waves are also generated during the detonation event. Therefore the integrated turbine would experience a wide range of inlet conditions. A successful hybrid PDE system would have to operate efficiently under these transient flow conditions.

The current work focused on investigating the performance of multiple PDCs integrated with an axial flow turbine. Integrated system performance is quantified by measuring the amount of power extraction available from the turbine section. A turbine efficiency analysis was carried out to further asses the system performance. The axial flow turbine was mapped over a complete range of PDC operating conditions. This research serves to increase the understanding of how detonations interact with axial flow turbines. This understanding provides insight in how to properly design and optimize a hybrid PDC-Turbine system.

II. Experimental Setup

A. University of Cincinnati Integrated PDC-Turbine Test Facility A facility to experimentally investigate an integrated PDC-turbine system has been designed and fabricated at

UC4. The PDC tubes are constructed of 316 stainless steel and their geometry consists 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 40% nitrogen and 60% 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 premixed 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


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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 is able to run at a maximum operating frequency of 20 Hz.

Operation of the PDE is computer controlled and synchronized using a LabView interface program written at UC. The PDE LabView interface provides the flexibility of specifying engine operating parameters such as fill-fraction, spark timing, number of detonations desired, frequency, number of detonation tubes running, and tube firing pattern. Dynamic pressure sensors were mounted along the detonation tube length to verify that Chapman-Jouguet detonations are obtained. With all 6 detonation tubes running the total combined exhaust flow rate is approximately 13.2% of the 1.6 lbm/s turbine design point flow rate. Significant amounts of supplemental bypass air is used to increase the total flow through the turbine to near the design levels. The current PDE configuration is able to run at a maximum operating frequency of 20 Hz in each tube.

The integrated system consists of a circular array of 6 PDC tubes exhausting into an axial flow turbine. A diagram of the PDC tube layout is shown in Fig. 1. The system incorporates supplemental bypass air that is mixed with the PDC exhaust before expanding through the turbine section. Flexibility was designed into the integration rig in order to be able to vary the following key design parameters: 1) incorporation of detonation tube nozzles, 2) distance from PDC tube exit plane to turbine inlet, 3) amount of bypass air, 4) straight or swirling injection of bypass air, and 5) incorporation of stator blades. The turbine section used for the current testing is from a model JFS-100-13A gas turbine engine manufactured by Allied Signal. For the integrated system testing, only the single stage free turbine section, designed to extract shaft power from the flow is used. The turbine wheel shaft is attached to a planetary type gear train with a reduction ratio of 18.1:1. At design operating conditions the turbine is rated to generate 90 shaft horsepower at a rotational speed of 60,400 RPM, and a mass flow rate of 1.6 lbm/s. The power output shaft would be running at around 3,000 RPM under these conditions. The blade height from hub to tip is approximately 1.15 inches, which is close to the size of the 1 inch diameter detonation tubes being used in the system. The 6 tube PDC array is set on a 5 inch diameter bolt circle, which is close to the mean diameter of the turbine blades. As can be seen from Fig. 1, all of the detonation tubes exhaust through individual nozzles before flowing around a centerbody, and into a common annular chamber. Any nozzle geometry can be used including converging, diverging, and straight nozzles. Bypass air is injected through the outer wall of this annulus where it mixes with the PDC exhaust flow. The bypass air can be injected with a varying degree of swirl, including the baseline of no swirl case. The length of the common chamber can be varied to bring the PDC exit ports closer to the turbine. A set of stator blades are located upstream of the turbine section to properly direct the flow into the turbine. Although the stator blades give the correct flow angles into the turbine blades, it is thought that they will cause significant flow losses due to interaction with the detonation waves. A qualitative study was performed on the stator section which showed the complicated interaction and possible flow energy loss mechanisms. From Figure 2, a shadowgraph image of the stator section, transmitted and reflected shocks can be seen. In order to determine the effect of the stator blades on integrated system performance, the stator section is made to be removable. With the stator blades removed the correct rotor blade flow angles can be achieved by introducing swirl into the bypass air and/or using properly contoured PDE nozzles to give the detonation exhaust a swirl component. After expanding through the turbine section the flow is turned 90 degrees to avoid the reduction gear train and exhausted. The turbine power output shaft is connected to a torque cell and water dynamometer to load the system and obtain power extraction measurements. The torque cell used is a Lebow model 1604-2K, and records a dynamic measurement of the torque 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. 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.

III. Results and Discussion

A. System Operability For the current work, the PDC-turbine rig was operated with 6 PDCs firing. A diagram of the PDC layout is

shown in Fig. 3. The PDC tubes are designated by numbers for ease of reference. Results reported in the current work were obtained by running the PDC tubes in sequence with equal time delays between each tube firing event. In order to avoid confusion, a clockwise firing sequence, with reference to Fig. 3, was termed co-rotation. This term indicates that the tube firing pattern progresses in the same direction as the turbine rotation. A tube firing sequence running counter-clockwise is thus termed counter-rotation, as the PDC sequence is in the opposite direction of the turbine rotation. Unless otherwise stated, all results shown were operated in a sequential co-rotating pattern. Each


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individual PDC was operated at a frequency of 10 Hz for the current work, therefore an aggregate frequency of 60 Hz was obtained with all 6 tubes firing.

The PDC operating parameters of fill-fraction and equivalence ratio were varied for these tests. The fill-fraction is defined as the ratio of the tube volume filled containing a detonable mixture to the total tube volume prior to ignition. Detonation wave speed measurements obtained with high speed dynamic pressure sensors verified that detonations were obtained within a PDC. Fig. 4 shows the measured change in wave speed with equivalence ratio. A theoretical detonation curve generated using a thermodynamic chemical equilibrium program5 is also shown on this figure. It can be seen that detonations are achieved for equivalence ratios above Ф=0.69, while lower equivalence ratios lead to a failed detonation. Detonation failure occurs due to the increasing cell size associated with a lean mixture. When the cell size length scale exceeds that of the detonation tube diameter, a stable CJ detonation will not form. For the current PDC system, operating equivalence ratios lower than Ф=0.69 correspond to high speed combustion events. These failed detonations still have supersonic wave speeds and are accompanied by a sizable pressure rise.

As can be seen from Fig. 5, the integrated system takes a given amount of time to achieve a steady-state operation. The time required to obtain steady measurements is a function of the system inertia, due to the turbine and powertrain, and from thermal effects as the PDCs heat up to thermal equilibrium. For the data shown, the turbine speed increases to approximately 12000 RPM when driven by the bypass air alone. Operation of the 6 PDC tubes starts at 35 seconds into the test, and ends at 95 seconds. Once the tubes begin firing, the turbine speed smoothly rises until a steady turbine speed is achieved. For the case shown the steady turbine speed is 25300 RPM. The average turbine inlet temperature is also plotted. The temperature shown is the average between two thermocouples mounted as described by Fig. 4. Thermocouple number 1 is directly inline with PDC tube 6, while thermocouple 2 is mounted in between tubes 4 and 5. Due to a strong pattern factor associated with the PDC tubes, the turbine inlet temperature has been shown to be extremely sensitive to measurement location6. The steady value of the average turbine inlet temperature for the test shown is 660°F. All experiments reported in this work were run for 45 seconds which was seen to be a sufficient duration to achieve steady measurements.

The bypass ratio, β, of the PDC-turbine system is defined as the ratio of the mass flow rate of cold air injected into the swirl chamber to the mass flow rate of fuel and oxidizer through the PDC tubes. For the tests reported in this work, the bypass ratio was kept at β=7. Due to the high bypass ratio used, the majority of the mass flow passing through the turbine is from the bypass air. Modification of the PDC operating parameters showed an insignificant effect on the total turbine mass flow. Therefore the mass flow passing through the turbine was considered to be constant for the current work, and was approximately 47% of its rated capacity.

B. Effect of Fill-Fraction on System Performance Tests were performed in order to determine the effect of fill-fraction on performance of the PDC-turbine system.

Fill-fraction was varied from 0.6 to 1.0. It has been shown that the fill-fraction is 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 due to partial fill effects. At the under-filled conditions tested, sufficient tube length was available so that a detonation wave formed within the volume of the tube containing fuel and oxidizer. After the detonation wave passed into the un-filled region of the tube, a decaying shock wave continued to propagate towards the tube exit.

The specific power is a non-dimensional quantity commonly used to characterize turbine performance. It is defined as the change in enthalpy through the turbine divided by the turbine inlet temperature. Fig 6. shows how the specific power extracted from the turbine section varied with the fill-fraction parameter. As fill-fraction is increased, specific power is also increased. This trend is to be expected as more chemical energy content is being added into the system through the fuel flow. In order to better understand the observed behavior, turbine inlet temperature is shown in Fig. 7 as a function of fill-fraction. Fill-fraction has the effect of increasing the turbine inlet temperature. Approaching a fill-fraction of 1.0, turbine inlet temperature starts to level off. Presumably for even higher fills, temperature would continue to level off as the excess fuel is sent into the mixing annulus upstream of the turbine. Though fuel will be present outside of the detonation tube in an over-filled case, most likely the detonation wave will not transition to an external detonation due to the lean nature of the mixture external to the tube. The lean mixture condition is caused by the large amounts of bypass air being injected into the swirl chamber. The turbine inlet temperature is an important quantity; the amount of available turbine power production is directly linked to it. For the simple case of an isentropic turbine, operating at a constant mass flow and fixed pressure ratio, increasing the turbine inlet temperature alone enables more power to be produced by the turbine. The integrated PDC-turbine is following this same trend.


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An efficiency analysis was carried out in order to determine how well the available flow energy is being extracted through the turbine. Isentropic turbine efficiency, η, is defined as the ratio of the actual work output to the ideal work output. The ideal work output for these tests was estimated by using the measured turbine inlet temperature and pressure ratio across the turbine section. The turbine efficiency data in this work is presented as a change in efficiency, with a reference efficiency, ηref, subtracted from the actual turbine efficiency. Data plotted in Fig. 8 describes how turbine efficiency varies with fill-fraction. As did specific power, the turbine efficiency increased with fill-fraction. The change in efficiency seen over the range of fill-fractions tested was approximately 2.5%. It is proposed that turbine efficiency is increasing with fill-fraction due to more favorable turbine inlet conditions. As turbine inlet temperature is increased, the average turbine inlet conditions more closely match those at which the turbine was designed to operate. Or in other words, the gas flow angles entering the rotor section will be shifted closer to what the designed rotor inlet angles are.

It is worth noting that these tests were not performed at a constant turbine speed. Fig. 9 reports the steady turbine rotational speed achieved at each fill-fraction. A change of 5000 RPM was measured by going from a ff=0.6 to a ff=1.0. Ideally all of these tests would be performed at a constant turbine speed to minimize frictional effects.

C. Effect of Equivalence Ratio on System Performance The equivalence ratio of the initial mixture was varied by changing the fuel flow rate. In this way, the system

performance as a function of the equivalence ratio parameter could be determined. Equivalence ratio was varied from Ф=0.54 to 1.08. Similar to the fill-fraction, by decreasing the equivalence ratio, the system chemical energy content can be decreased. As previously discussed, for the current PDC system, equivalence ratios greater than Ф=0.69 produced CJ detonations. Leaner equivalence ratios led to strong high speed combustion waves.

The effect of equivalence ratio on specific power output of the turbine section is shown in Fig. 10. The data shows that increasing the equivalence ratio increases the turbine power output. This follows the same trend as the turbine inlet temperature, shown in Fig. 11. The turbine inlet temperature increases with equivalence ratio. Instead of leveling off at Ф=1.0, the temperature appears to be still climbing. This can be explained by the behavior of the CJ temperature with equivalence ratio. The CJ temperature continually increases over the range of equivalence ratios studied, and continues to climb until reaching a peak temperature near Ф=1.3. It would be expected that the turbine inlet temperature would continue to rise until that point, after which it should start to drop off. Turbine efficiency as a function of equivalence ratio is shown in Fig.12. Efficiency follows the same trend as that of the turbine inlet temperature. Going from a lean to rich mixture increases the turbine efficiency by approximately 2%.

D. Turbine Performance Mapping It is Typical to plot turbine performance data against the turbine pressure ratio. This pressure ratio is defined as

the total pressure at the turbine inlet to the static pressure at turbine outlet. For the current work, static pressure measurements were made upstream of the turbine. The total pressure was then estimated by use of the turbine inlet temperature, mass flow rate, and cross-sectional flow area. It was found that for the PDC operating parameters investigated, the pressure ratio was nearly constant. This is shown in Fig. 13 where data from the equivalence ratio and fill-fraction studies is plotted. In order to present the operating parameter data on a single plot, the turbine efficiency was plotted against specific power in Fig. 14. Data for both of the operating parameters is plotted. For both fill-fraction and equivalence ratio, the turbine efficiency increases with increasing values of specific power.

IV. Conclusion Performance measurements were made on an integrated PDC-turbine system. The turbine section performance

was investigated over the PDC operating parameters of fill-fraction and equivalence ratio. Measurements of the turbine inlet temperature showed that temperature increased with both fill-fraction and equivalence ratio. It is suggested that the temperature increased due to increasing the chemical energy input to the system. Specific power was seen to increase with both operating parameters studied. The power increase can be explained by the higher turbine inlet temperatures. For a constant mass flow rate through the turbine, increasing the turbine inlet temperature allows the turbine to produce more power. Turbine efficiency was observed to increase with fill-fraction and equivalence ratio. It is proposed that efficiency is also increasing due to the rising turbine inlet temperature. As the turbine inlet temperature increases, the turbine inlet flow conditions are getting closer to design conditions, and thus more favorable for the turbine operation.


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Acknowledgments The authors would like to thank both NASA Glenn Research Center and General Electric Global Research

(GEGR) for providing financial support for this and other related work. The collaboration and guidance by Dr. Tony Dean and Dr. Adam Rasheed of GEGR has been greatly appreciated.

References 1Schauer, 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. 2Petters, D.P. and Felder, J.L., “Engine System Performance of Pulse Detonation Concepts Using the NPSS Program, AIAA

2002-3910, 2002. 3Schauer, 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. 4Caldwell, N., Glaser, A., Dimicco, R., Gutmark, E.,

“Acoustic Measurements of an Integrated Pulse Detonation Engine with Gas Turbine System”, 43rd AIAA Aerospace Sciences Meeting and Exhibit, January 9-13, Reno, NV, AIAA 2005-0413.

5Gordan, S., Mcbride, B., “Computer Program for Calculation

of Complex Chemical Equilibrium Compositions and Applications”, NASA Reference Publication, #1311, 1994

6Rasheed, A., Furman, A., and Dean, A.J. “Experimental

Investigations of an Axial Turbine Driven by a Multi-tube Pulsed Detonation Combustor System”, 41st AIAA/ASME/SAE/ASEE Joint Propulsion Conference, July 10-13, Tucson, AZ, AIAA 2005-4209.

7Rasheed, 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.

Direction of Flow

Figure 1. Cross-section view of the PDC-Turbine system.

Figure 2. Shadowgraph image showing the interaction of a detonation wave with the stator section.

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Figure 3. Diagram of PDC designations. Looking from the headwall towards the turbine section. Thermocouple locations used to measure the turbine inlet temperature are shown.

Turbine Inlet Temperature and Pressure Measurement Locations

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Turbine Inlet Temperature and Pressure Measurement Locations

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Figure 5. Time history of a power extraction test. Turbine rotational speed and the average turbine inlet temperature are shown. Test duration was 45 seconds.


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Figure 7. Effect of fill-fraction on turbine inlet temperature. Temperature values reported correspond to the steady temperature achieved at each run condition.


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Figure 9. Effect of fill-fraction on turbine rotational speed. Values reported for the turbine speed correspond to the steady speed achieved at each run condition.


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Figure 11. Effect of equivalence ratio on turbine inlet temperature. Temperature values reported correspond to the steady temperature achieved at each run condition.


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Figure 13. Effect of fill-fraction and equivalence ratio on the turbine pressure ratio. Triangle symbols represent the fill-fraction data. Square symbols represent the equivalence ratio data. Pressure ratio was nearly constant for all conditions tested


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Figure 14. Plot showing the change in turbine efficiency with the specific power. Triangle symbols represent the fill-fraction data. Square symbols represent the equivalence ratio data.

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