An Analysis of the Strayton Engine, a Brayton and …...National Aeronautics and Space Administration Introduction 2 • What is the Strayton Engine? • A hybrid Brayton cycle / Stirling

National Aeronautics and Space Administration

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

Jeffryes W. ChapmanDonald L. SimonEzra O. McNicholsNASA Glenn Research Center

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AIAA Propulsion and Energy Forum, Indianapolis, INAug 19-22, 2019

https://ntrs.nasa.gov/search.jsp?R=20190030499 2020-04-22T08:06:26+00:00Z


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Introduction

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• 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


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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


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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)


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• 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

– Interaction between two components managed through power movement into or out of the NPSS duct component

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Simulating the Strayton Cycle

𝐶𝐶𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎 𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝑎𝑎𝐸𝐸𝐸𝐸 = 1 −𝑇𝑇𝐶𝐶𝑇𝑇𝐻𝐻


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• 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

Thermal circuit modeling

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Effect of Brayton cycle engine design criteria

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– Thermal efficiency rises as temperature increases

– Strayton benefits larger than Brayton only benefits due to system synergies

• Designed turbine blade temperature limit

• Overall Pressure ratio– Thermal efficiency rises as

temperature increases– Brayton cycle benefits greater

than Strayon due to increases in T3 which reduce the Stirling temperature ratio.


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• 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

Effect of Stirling design power

9


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• 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)

Strayton Temperature profiles

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Optimized systems

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• 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, Tblade = 2750 °R

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

Total Power (HP)

Brayton Power (HP)

OPR T liner (°R)

Efficiency SFC(lbm/HPh)

Air Mass Flow (lbm/s)

200 200 6.5 2750 0.24 0.566 1.11670 670 10 2750 0.29 0.433 3.23

Total Power (HP)

Brayton Power (HP)

Stirling Power (HP)

OPR Tst,H(°R)

Tst,C (°R) T liner (°R)

Efficiency SFC(lbm/HPh)

Air Mass Flow

(lbm/s)

Stirling Temperature

Ratio200 166.62 33.38 6.5 1598 1141 3167 0.33 0.402 0.87 1.4670 640 30.01 10 1761 1246 3117 0.32 0.394 2.57 1.41

Brayton Cycle Engines

Strayton Cycle Engines


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SFC comparison

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0.150

0.250

0.350

0.450

0.550

0.650

0.750

0.850

0 250 500 750 1000 1250 1500 1750 2000

SFC

-lb

m/H

Ph

Shaft Power - HP

Comparison of IC, Turbine, and Strayton Engines SFC Vs Shaft Power

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

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0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

4.00

4.50

5.00

0 250 500 750 1000 1250 1500 1750 2000

Spec

ific

Pow

er -

HP/lb

m

Shaft Power - HP

Comparison of IC, Turbine, and Strayton EnginesSpecific Power Vs Shaft Power

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

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• 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.

CompressionCylinderSpace

ExpansionCylinder

Space

Regenerator

Cooler Heater

�̇�𝐶𝐶 �̇�𝐻𝐻

Compression Swept Volume

CompressionClearance Volume

Expansion Swept Volume

ExpansionClearanceVolume

Compression

Cylinder

Space

Expansion

Cylinder

Space

Regenerator

Cooler

Heater

Compression Swept Volume

Compression

Clearance Volume

Expansion Swept Volume

ExpansionClearance Volume


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Stirling controls model

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

• Strayton efficiency maximized when stroke length reduced as stirling power is reduced

• Efficiency increased by up to ~3%

• Highlights requirement for a coupled method of control and potential off design efficiency benefit to the system.

Strayton Power Output (HP)

100 110 120 130 140 150 160 170 180 190 200 210

Stra

yton

Effi

cien

cy (%

)

0.28

0.29

0.3

0.31

0.32

0.33

0.34

0.35

0.36

100%

stroke

95%

stroke

90%

stroke

85%

stroke

80%

stroke

75%

stroke

70%

stroke

100% stroke

95% stroke

90% stroke

85% stroke

80% stroke

75% stroke

70% stroke

Tblade Limit

Most Efficient Operating Line

Design Point


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Key technologiesHigh temperature materials

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• Key to efficient and higher power Strayton is developing higher temperature turbine components.• Currently, low power turbine engines operate with a T4 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


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Key technologiesStirling Engines

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• 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


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Key technologiesBlade cooling / Heat exchangers / heat pipes

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• 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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Key technologiesCoordinated control system

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• 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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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

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• 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)


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Acknowledgments

Funding for this work was provided by the Convergent Aeronautics Solutions (CAS) project.

An Analysis of the Strayton Engine, a Brayton and Stirling Cycle Recuperating EngineIntroductionSlide Number 3Slide Number 4Slide Number 5Slide Number 6Thermal circuit modelingEffect of Brayton cycle engine design criteriaEffect of Stirling design powerStrayton Temperature profilesOptimized systems SFC comparisonSpecific power comparisonStirling controls modelStirling controls modelKey technologies�High temperature materialsKey technologies�Stirling EnginesKey technologies�Blade cooling / Heat exchangers / heat pipesKey technologies�Coordinated control systemSummary and ConclusionsSlide Number 21

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An Analysis of the Strayton Engine, a Brayton and …...National Aeronautics and Space Administration Introduction 2 • What is the Strayton Engine? • A hybrid Brayton cycle / Stirling