An Analysis of the Strayton Engine, a Brayton and Stirling Cycle Recuperating Engine

National Aeronautics and Space Administration

An Analysis of the Strayton Engine, a Brayton

and Stirling Cycle Recuperating Engine

Jeffryes W. Chapman Donald L. Simon Ezra O. McNichols NASA Glenn Research Center AIAA Propulsion and Energy Forum, Indianapolis, IN Aug 19-22, 2019 www.nasa.gov

National Aeronautics and Space Administration Introduction • What is the Strayton Engine?

• A hybrid Brayton cycle / Stirling cycle engine concept • Concept developed by Rodger Dyson of NASA Glenn Research Center • Why investigate the Strayton?

• Benefits in efficiency and specific power may be realized by utilizing the synergies between the two cycles.

• Task : • Two week micro seeding was funded to investigate the Strayton cycle advantages and identify key technologies and challenges The purpose of this paper is to disseminate task findings www.nasa.gov

National Aeronautics and Space Administration Thermodynamic Brayton Cycle https://www.grc.nasa.gov/www/k-12/airplane/Animation/turbpar/Images/engslo.gif (1-2) : Isentropic compression on a gas (2-3) : Heat is added to the system, with no loss in pressure (3-4) : Isentropic decompression occurs where energy can be taken from the system as work (4-1) : Waste heat is rejected www.nasa.gov

National Aeronautics and Space Administration Thermodynamic Stirling Cycle (1-2) : Isothermal volume expansion making use of an external heat source (2-3) : Constant volume heat transfer heating up the regenerator (3-4) : Isothermal compression with waste heat rejected to an external cold source (4-1) : Constant volume heat transfer occurs to cool the regenerator.

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Strayton Engine

• Single shaft turboshaft with stirling engine embedded within the shaft

– Thermal acoustic Stirling engine – Energy moves from the Brayton cycle gas path into the Stirling through turbine blade heat transfer – Stirling waste heat is reintroduced to the Brayton cycle before the combustor – Work is gathered from a dual-axis generator (rotational work from the Brayton engine and axial work from the Stirling engine) www.nasa.gov

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Simulating the Strayton Cycle

• Simulation created within the numerical propulsion system (NPSS)

– NPSS native turbomachinery elements used for Brayton cycle engine components – Stirling cycle engine assumed to be a heat engine with an efficiency of 50% Carnot

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𝐶𝐶𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎 𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸 𝑎𝑎 𝐸𝐸𝐸𝐸 = 1 −

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𝐻𝐻 – Interaction between two components managed through power movement into or out of the NPSS duct component www.nasa.gov

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Thermal circuit modeling

• Thermal modeled as: – Brayton hot side heat transfer through turbine blades • Modeled as convection over flat plates – Heat was transferred to and from Brayton and Stirling engines using oscillating heat pipes (OHP) • Modeled using an assumed heat transfer coefficient – Stirling hot and cold side heat transfer – Brayton cold side heat transfer applied into stage 3 • Modeled as a heat exchanger with constant effectiveness www.nasa.gov

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Effect of Brayton cycle engine design criteria

• Designed turbine blade • Overall Pressure ratio temperature limit – Thermal efficiency rises as – Thermal efficiency rises as temperature increases temperature increases – Brayton cycle benefits greater – Strayton benefits larger than than Strayon due to increases Brayton only benefits due to in T which reduce the Stirling system synergies temperature ratio.

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Effect of Stirling design power

• Stirling power fraction : – Increases in power fraction achieved by increasing power flow through the hot section, but only up to a point.

– Inflection occurs when the drop in Stirling efficiency, due to a reduction in Stirling temperature ratio, begins to outweigh the effects of power flow increase – Overall Strayton efficiency drops as the Strayton power ratio continues to reduce www.nasa.gov

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Strayton Temperature profiles

• Temperature vs. power flow to the hot side of the stirling

– Blade cooling • Constant Blade cooling shows increase in Stirling blade cooling effect – Reduction of Stirling temperature ratio • As power flow increases Stirling temperature ratio decreases which reduces the efficiency of the Stirling (Note: there is an assumed required 1.4 temperature ratio for the Stirling to operate) www.nasa.gov

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Optimized systems

• 2 power levels where analyzed for this study, 200 HP and 670 HP

• Baseline Brayton cycle and Strayton cycle concepts were developed for comparison

°

purposes, T = 2750 R

blade

• Efficiency gain of ~10% and ~3% at 200 HP and 670 HP power points respectively

Brayton Cycle Engines

Total Brayton OPR T liner Efficiency SFC Air Mass Flow Power Power (°R) (lbm/HPh) (lbm/s) (HP) (HP) 200 200 6.5 2750 0.24 0.566 1.11 670 670 10 2750 0.29 0.433 3.23

Strayton Cycle Engines

Total Brayton Stirling OPR T T (°R) T liner Efficiency SFC Air Mass Stirling st,H st,C Power Power Power (°R) (°R) (lbm/HPh) Flow Temperature (HP) (HP) (HP) (lbm/s) Ratio 200 166.62 33.38 6.5 1598 1141 3167 0.33 0.402 0.87 1.4 670 640 30.01 10 1761 1246 3117 0.32 0.394 2.57 1.41 www.nasa.gov

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SFC comparison

Comparison of IC, Turbine, and Strayton Engines SFC Vs Shaft Power 0.850 0.750 0.650 0.550 0.450 0.350

SFC - lbm/HPh

0.250 0.150 0 250 500 750 1000 1250 1500 1750 2000

Shaft Power - HP

IC Engines Turbine Engines Brayton200 Strayton200 Brayton670 Strayton670

Strayton engine shows SFC similar to internal combustion engines at low power

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Specific power comparison

Comparison of IC, Turbine, and Strayton Engines Specific Power Vs Shaft Power 5.00 4.50 4.00 3.50 3.00 2.50 2.00 1.50 1.00

Specific Power - HP/lbm

0.50 0.00 0 250 500 750 1000 1250 1500 1750 2000

Shaft Power - HP

IC Engines Turbine Engines Brayton200 Strayton200 Brayton670 Strayton670 Estimated weights of Strayton similar to that of an unmodified gas turbine.

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Stirling controls model

• Dynamic Stirling controls model (DSCM) based on Ohio University

Stirling Engine Analysis (SEA) software, within MATLAB

• Design parameters of compression and expansion swept volume and

clearance volume, cooler, heater, and regenerator size, Stirling frequency,

operating fluid, and pressure were tuned to meet predicted power, Stirling

temperature ratios, and efficiency taken from the NPSS model.

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𝐶𝐶 𝐻𝐻 Cooler Heater Regenerator Compression Expansion Cylinder Cylinder Space Space Compression Compression Expansion Expansion Swept Clearance Clearance Swept Volume Volume Volume Volume www.nasa.gov

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Stirling controls model

• Operation begins at the design point and reduces power and adjusts

stroke length

0.36 100% stroke 95% stroke 90% stroke

• Strayton efficiency

0.35 85% stroke 80% stroke

maximized when

75% stroke 0.34 70% stroke

stroke length reduced

Tblade Limit Most Efficient Operating Line

as stirling power is

Design Point 0.33

reduced

• Efficiency increased

0.32

by up to ~3%

0.31

• Highlights requirement

70% Strayton Efficiency (%) stroke

for a coupled method

0.3

of control and potential

100% 95% 90% 85% 80% 75%

off design efficiency

stroke stroke stroke stroke stroke stroke 0.29 benefit to the system.

0.28 100 110 120 130 140 150 160 170 180 190 200 210 Strayton Power Output (HP)

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National Aeronautics and Space Administration Key technologies High temperature materials • Key to efficient and higher power Strayton is developing higher temperature turbine components.

• Currently, low power turbine engines operate with a T around 2000 ° R for maintenance and cost benefits • This study examined potential turbine blade temperatures of 2750 ° R and liner temperatures of 3200 ° R, which are more in line with large gas turbines with robust blade and liner cooling mechanisms.

• To achieve the required temperatures materials must be developed to allow greater hot side temperatures at a low cost • Research in high temperature ceramics has shown promise, with next-generation ceramic matrix composites expected to reach roughly 3200 ° R • Additional thermal coatings could raise this to 3500 ° R www.nasa.gov

National Aeronautics and Space Administration Key technologies Stirling Engines

• New Stirling technologies will need to be examined

• High power Stirling engines – Thermal acoustic stirling development – Multi-stage Stirling • Reliability of stirling engines (Gas turbines typically run for 1000s of hours) – Rotating Stirling • Installation/manufacturing of Stirling within a gas turbine shaft • Maintainability of Stirling that support line replaceable unit (LRU) access and removal www.nasa.gov

National Aeronautics and Space Administration Key technologies Blade cooling / Heat exchangers / heat pipes • Heat transfer technologies:

Gas turbine to Stirling heat exchanging

• Rotating/ no pressure rise heat exchanger to increase heat transfer on cold side of Stirling • Safe and efficient heat pipes usable at high rotational speeds and temperatures.

– Non-volatile medium materials • Turbine blade thermal transfer materials or coatings to optimize the ratio between turbine blade cooling effect and Stirling power transfer.

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National Aeronautics and Space Administration Key technologies Coordinated control system

• Ability to operate transiently through different

power levels and within the operational envelope

• In a typical gas turbine engine component temperature transients can extend into minutes, while customer power demands may need to be met in seconds.

• Strayton control system must manage this disparity to guarantee power as requirements demand.

– Maintain stall margins and temperature limits during transient operation utilizing potential control effectors, such as fuel flow, and Stirling stroke length – Identifying required control methodology: sensor suite, potential operational schedules.

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National Aeronautics and Space Administration Summary and Conclusions • The Strayton Engine: Brayton cycle, Stirling cycle hybrid engine concept.

– Cycle analysis shows significant efficiency gain over the Brayton cycle only engine with increasing gains as temperature is increased.

• With a 10% increase (over the Brayton cycle only engine) in efficiency at 200 HP engine level – Generally greater efficiency gain for lower power generating engines because Stirling power level scales with temperature ratio, not Brayton power production.

– Controls study demonstrates system sensitivity to design parameters and illustrates Stirling off design operation • Key Technologies – High temperature materials – Stirling Engines (manufacturability, maintainability, high power capability) – Heat transfer (rotating heat exchangers, heat pipes, turbine blades) – Coordinated control system (seamless operation between systems with very different time constants) www.nasa.gov

National Aeronautics and Space Administration Acknowledgments Funding for this work was provided by the Convergent Aeronautics Solutions (CAS) project.

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