NASA/TM--2002-211997

Overview of NASA Multi-Dimensional

Stirling Convertor Code Development

and Validation Effort

Roy C. Tew and James E. Cairelli

Glenn Research Center, Cleveland, Ohio

Mounir B. Ibrahim

Cleveland State University, Cleveland, Ohio

Terrence W. Simon

University of Minnesota, Minneapolis, Minnesota

David Gedeon

Gedeon Associates, Athens, Ohio

December 2002
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NASA/TM--2002-211997

Overview of NASA Multi-Dimensional

Stirling Convertor Code Development

and Validation Effort

Roy C. Tew and James E. Cairelli

Glenn Research Center, Cleveland, Ohio

Mounir B. Ibrahim

Cleveland State University, Cleveland, Ohio

Terrence W. Simon

University of Minnesota, Minneapolis, Minnesota

David Gedeon

Gedeon Associates, Athens, Ohio

Prepared for the

Space Technology and Applications International Forum (STAIF-2003)

cosponsored by the American Institute of Aeronautics and Astronautics

and the University of New Mexico

Albuquerque, New Mexico, February 2-6, 2003

National Aeronautics and

Space Administration

Glenn Research Center

December 2002
Acknowledgments

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Aeronautics and Space Administration.

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Available electronically at http://gltrs.grc.nasa.gov
Overview of NASA Multi-Dimensional Stirling Convertor Code Development and Validation Effort

Roy C. Tew and James E. Cairelli

National Aeronautics and Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Phone: 216 433 8471, Email: Roy.Tew@grc.nasa.gov

Mounir B. Ibrahim

Department of Mechanical Engineering

Cleveland State University

Cleveland, Ohio 44115

Terrence W. Simon

Department of Mechanical Engineering

University of Minnesota

Minneapolis, Minnesota 55455

David Gedeon

Gedeon Associates

Athens, Ohio 45701

Abstract. A NASA grant has been awarded to Cleveland State University (CSU) to develop a multi-dimensional (multiD) Stirling computer code with the goals of improving loss predictions and identifying component areas for

improvements. The University of Minnesota (UMN) and Gedeon Associates are teamed with CSU. Development of test

rigs at UMN and CSU and validation of the code against test data are part of the effort. The one-dimensional (l-D)

Stirling codes used for design and performance prediction do not rigorously model regions of the working space where

abrupt changes in flow- area occur (such as manifolds and other transitions between components). Certain hardware

experiences have demonstrated large performance gains by varying manifolds and heat exchanger designs to improve

flow- distributions in the heat exchangers. 1-D codes were not able to predict these performance gains. An accurate multiD code should improve understanding of the effects of area changes along the main flow-axis, sen sitivity of performance

to slight changes in internal geometry, and, in general, the understanding of various internal thermodynamic losses. The

commercial CFD-ACE code has been chosen for development of the multi-D code. This 2-D/3-D code has highly

developed pre- and post-processors, and moving boundary capability. Preliminary attempts at validation of CFD-ACE

models of MIT gas spring and "two space" test rigs were encouraging. Also, CSU's simulations of the UMN oscillatingflow- rig compare well with flow- visualization results from UMN. A complementary Department of Energy (DOE)

Regenerator Research effort is aiding in development of regenerator matrix models that will be used in the multi-D

Stirling code. This paper reports on the progress and challenges of this multi-D code development effort.

INTRODUCTION

The Department of Energy (DOE), Lockheed Martin (LM), NASA Glenn Research Center (GRC) and the Stirling

Technology Co. (STC) are developing a high-efficiency Stirling Radioisotope Generator (SRG) for NASA Space

Science Missions. The SRG is being developed for potential multi-mission use, including the provision of electric

power for unmanned Mars rovers and deep space missions. The SRG is being evaluated as a high-efficiency power

source alternative to replace Radioisotope Thermoelectric Generators (RTG's). The SRG system efficiency of

greater than 20 percent will reduce the required amount of radioisotope by a factor of 4 or more compared to RTG's.

This significantly reduces radioisotope cost, radiological inventory, and system cost.

STC previously developed the Stirling convertor under contract to DOE and is now providing further development

as a subcontractor to LM. Technology Demonstration Convertors (TDC's), operating as single units and in

dynamically balanced opposed pairs, are now being tested by STC and GRC. Power outputs of 55 to 65 We and

efficiencies or 25 to 28 percent have been demonstrated for TDC operating conditions of 650 ° C hot-end

temperature and cold-end temperatures varying from 80 ° to 120 ° C. Lockheed Martin has been selected as the SRG

System Integration Contractor. GRC is conducting an in-house technology project to assist in developing the

convertor for space qualification and mission implementation (Thieme, 2002), has also initiated several advanced

Stirling technology efforts and is evaluating Stirling convertors for other applications. The multi-D code

development effort reported on here is one of GRC's advanced Stirling technology efforts.

NASA/TM--2002-211997 1
Theone-dimensional (I-D) Stirlingcodesusedfordesignandperformancepredictiondonotrigorouslymodel

regionsof theworkingspacewhereabruptchangesinflowareaoccur(suchasmanifoldsandothertransitions

betweencomponents). Certainhardwareexperiences havedemonstrated largeperformancegainsby varying

manifoldsandheatexchanger designstoimproveflowdistributionsintheheatexchangers. 1-Dcodeswerenotable

topredicttheseperformance gains.Anaccuratemulti-Dcodeshouldimproveunderstanding oftheeffectofarea

changesalongthemainflowaxis,sensitivityofperformance toslightchangesininternal geometry, and,ingeneral,

theunderstanding ofvariousinternal thermodynamic losses.

MULTI-DIMENSIONAL STIRL1NG CODE DEVELOPMENT AND VALIDATION

A two-year grant to Cleveland State University (CSU) for Stirling multi-D code development and validation was

awarded effective August 1, 2000 and a two year grant renewal was awarded effective August 1, 2002. The

University of Minnesota (UMN) and Gedeon Associates are subcontractors. The principal investigator (PI), Mounir

Ibrahim of CSU, has directed the overall effort, the multi-D code development, and preparation of CSU's Stirling

Laboratory Research Engine (SLRE) test facility for code validation tests. Terrence Simon of UMN has directed the

building and testing of a large scale piston/cylinder test section that allows oscillating air flow to make a 90 degree

turn as it passes into a disk-shaped volume which opens to the room at its outer edge. He has also acted as a

consultant to CSU in CSU's development of multi-D models. David Gedeon has acted as consultant to CSU and

UMN: He used the 1-D Sage code (Gedeon, 1999) to model UMN test sections and CSU's SLRE test section to aid

in their design, and designed the SLRE test section and its flow inserts. He is now working on design of a new UMN

expansion head (or "180 degree turn") test section, that will roughly model the flow path from the displacer/cylinder

through the heater of a "TDC like" Stirling convertor. He is helping CSU with plans for SLRE instrumentation and

data acquisition, and has also guided plans for measurements and calculations of regenerator "enhanced

conductivity" resulting from flow eddies within the matrix.

Much of the regenerator matrix modeling effort is being done under a complementary DOE regenerator research

effort. This three year effort began Sept. 1, 2000. Ibrahim of CSU is also the PI for this effort. Other members of the

team are Simon of UMN, Gedeon Associates, and GRC. A major part of this eflbrt is large-scale, oscillatory-flow,

low-frequency testing of a piston/cylinder-cooler-regenerator-heater test section at UMN. Dimensionless variables

for the test were chosen based upon those of one of STC's Stirling engines. The test section is sufficiently large

scale and low frequency that hot-wire anemometer measurements of dynamic temperature are being made (1) within

the gas plenum between the cooler tubes and the regenerator, (2) within the matrix gas, and (3) on the matrix wires.

CSU is providing CFD modeling support for the test work. Several progress reports about this DOE work have been

or are being published (Niu, 2002; 2003; Ibrahim, 2002b). Results of the NASA multi-D Stirling code grant effort

have been published in Ibrahim (2002a) and Adolfson (2002).

The commercial CFD-ACE code was chosen for multi-D code development. This 2-D/3-D code has highly

developed pre- and post-processors and moving boundary capability. CFD-ACE's developer is the CFD Research

Corporation (CFDRC) of Huntsville, AL.

CFD-ACE Modeling of MIT Gas Spring and "Two-Space" Test Rigs

CFD-ACE was used to develop 2-D axisymmetric models of MIT gas spring and "two-space" test rigs (Ibrahim,

2001). Agreement with MIT's measured dimensionless gas spring work (Kornhauser, 1989) was very good. The

two-space test rig was fabricated by mounting an annular heat exchanger on top of the original gas spring (The wall

boundary condition for both rigs was the constant room temperature). Wall heat transfer per unit area at positions

along the annular heat exchanger was calculated and compared with MIT's data and also with previous in-house

CAST (for Computer Aided Simulation of T._urbulent Flow) code calculations (Tew, 2001). The CFD-ACE heat

transfer per unit area over the cycle was in qualitative agreement with the data, but quantitative agreement could

have been better. There was also quantitative disagreement with the CAST code calculations at most heat exchanger

axial positions (But variations of heat transfer over the cycle as indicated by the test data, CFD-ACE and the CAST

code all looked qualitatively similar).

NASA/TM--2002-211997 2
Plansaretore-visittheCFD-ACEtwospacetestrig modelingandcomparisonwithtestrigdata.Also,careful

comparisons willbemadebetweenthe2-Daxisymmetric CFD-ACEmodelsand1-DSagemodelsofthesetwotest

rigs.Preliminarycomparisons betweensuchmodels,indicatedthatoverallenergyflowsascalculatedbythe2-D

and1-Dmodelswerequitecloseforthegasspringtestrigbutsubstantially different forthetwo-spacetestrig.This

isnotunexpected sincetheflowareaisuniforminthegasspringbutismuchdifferentinthecylinderandheat

exchanger ofthetwo-spacetestrig.Sincethe1-Dmodelassumesuniformflowandcannotaccuratelymodelthe

effectsofflowseparationatchangesinarea,it seemslikelythaterrorsinthe1-Dcalculations mightoccur.But,at

thisearlystagein thedevelopment of multi-Dmodelsof enginesand"engine like" test rigs, it can not be

automatically assumed that 2-D models of the two-space rig are more accurate than 1-D models in calculation of

energy flows. One possible reason for this is that the large-eddy turbulence models used contain empirical constants

and have not been "calibrated" for oscillating flows in the relatively complex Stirling geometries. Also, Sage,

together with the earlier GLIMPS code, has about a decade long history of comparison with engine and oscillatingflow rig test data.

Two-Dimensional Model of Stirling Convertor (or Engine/Alternator)

CSU has developed a 2-D model of a "TDC like" CSUmod Stirling engine. The 2-D axisymmetric approximation of

the 3-D geometry of interest became operational in June, 2002. The model includes the oscillating power piston and

displacer, together with the variable volume compression and expansion spaces, and models of the heater, cooler and

regenerator. A schematic of an early version of this model is shown in Figure 1. The initial model did not include the

walls surrounding the working space. The major 2-D simplifications of the 3-D geometry of interest were: (1) In the

3-D geometry of interest, the heater and cooler have alternating rectangular fins and flow passages with long axes

perpendicular to the cylinder axis. In the 2-D approximation, the radial fins and flow passages have been replaced by

concentric fins and flow passages with the same axis as the Stirling cylinder; total flow area and fin cross-sectional

area were maintained. It was possible by choice of the number of concentric fins and flow passages to choose a

hydraulic diameter that was close to that of the original 3-D geometry. (2) Four symmetrically arranged holes in the

base or "spider" supporting the displacer rod, were replaced by an annular opening in the 2-D model.

Cooler Regenerator Heater

iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii

_'omprcssion '_

_-- • ExpansiOn _

Pis, i_NigSpac e ' ,,,,__

ton : Space Displacer Sl_ao¢

----------------,Axis of Sy_mnctry .........................

FIGURE 1. Schmnatic Showing U-Velocity Contours Generated by a 2-D

Axisymmetric CFD-ACE Modei ofa Stirling Convertor Working SDce. Details ofthe

Heat Exchanger Geometries Were Not Included in this E_ly Version of the Model,

A number of modifications to the 2-D

Stirling code are underway or have now

been completed: (1) A model of the full

pressure vessel wall has been added

around the annular heater, regenerator

and cooler. This will be used to study

interactions between the wall

temperature profile and the internal gas

temperatures. (2) The surface layer of

the pressure vessel wall, approximately

equal to the thermal penetration

thickness (the thickness over which the

wall temperature varies over the cycle),

is being modeled around the expansion and compression spaces. Modeling the complete thickness would require

additional computational time. (3) Work is underway to develop a model of the appendix gap and seal regions

between the displacer and its cylinder. Shuttle and appendix gap losses result from the fluid flow and heat transfer

processes that occur in this region.

Progress on Three-Dimensional Model of Stirling Convertor

An initial 3-D model of the CSUmod Stirling convertor design was set-up and run through part of an engine cycle.

This permitted an estimate to be made of computing time. The previous 2-D model, on a 2 GHz Windows

workstation, required about 55 cycles over two weeks to reach a steady periodic cycle. The partial cycle 3-D run

indicated that a complete run would take on the order of 1 year of computational time. This suggests the need to use

a parallelized version of the CFD-ACE code on a cluster of workstations to achieve practical 3-D Stirling model

computing times (a cheaper approach than use of available supercomputers).

NASA/TM--2002-211997 3
ThebasicCFD-ACEcodeisparallelized. CFDRChasbegunparallelization ofthecode'sgriddeformationmodule

thatpermitssimulationofmovingboundaries(pistons) andprojectscompletionbyOct.-Nov. 2002.Also,GRChas

purchasedthefirstof32dualprocessor workstations thataretobepurchasedoverthenexthreeyearstobuilda

dedicatedcluster.BasedonCFDRC'sclusterexperience, it isanticipatedthatthecompletedGRCclustershould

produceacomputational acceleration ofabout50.Thusit ishopedthatacompleted3-DCFD-ACEmodelofaTDC

orsimilarStirlingconvertor willeventuallyrunto"periodicsteadystate"ontheorderof 1week.

CSUisalsoputtingtogether amini-cluster of4dualprocessor computersinFY2003. Useoftheparallelizedversion

ofCFD-ACEshouldpermitthemtospeeduptheirdevelopment runsbyanacceleration factorofroughly7.Asthe

largerGRCclusterdevelopsoverthethreeyearperiodFY2003-2005, it will bepossiblefor CSUto make

development runsontheGRCcluster, if thatprovestobeadvantageous tothecodedevelopmentffort.

"90 DegreeTurn" or "90 Degree Expansion Head" Experiments and CFD Modeling

Flow visualization tests were run using a "90 degree turn" test section that replicates important features of

oscillatory flows. These include large-scale separation and adverse and favorable, spatial and temporal acceleration.

The geometry is a tube perpendicular to a space between two circular discs, one of which the tube penetrates such

that the end of the tube is flush with the disc inner surface. The flow, driven by a piston in the tube, is oscillatory and

acts on the exhaust stroke as a jet impinging on the center of a circular disc and on the intake stroke as a sink flow

drawn into a tube from the space between the two discs (referred to later in the text as the "channel"). Time-varying

recirculation zones and periodic appearances of highly turbulent flow were observed. In addition to a

characterization of oscillatory, separating flow, these tests provided code validation support for Stirling engine

design models. The project goals are to identify the areas where 1-D modeling is especially weak, provide support

for developing more sophisticated design models that can be used with greater confidence, and help validate the

CFD-ACE Stirling model. A more complete description of these experiments is given in Adolfson (2002).

Test Section

F[ywheel_ . Piston _ I

Baler cing Working Flow Delivery

Drive Drive Section

FIGURE 2. UMN Scotch Yoke Drive Facility

separation zones that challenge the capabilities of the code.

from 25 mm to 127 mm (1 inch to 5 inch).

The test facility is shown schematically in Fig. 2.

The facility uses a Scotch yoke drive system to

deliver a sinusoidal piston motion. The rotational

speed is continuously variable from 4.5 to 70

revolutions per minute. The test section mentioned in

Fig. 2 is shown in Fig. 3. The piston diameter is 0.22

m. The bore of the tube shown in Fig. 3 matches the

piston diameter. The two large discs have a major

diameter of 1.2 m. The disc diameter was chosen to

be large to minimize flow momentum in and out of

the test section. The tube attached to the left disc in

Figure 3 has a length of 0.3 m.

The test section turns the flow 90 ° as the piston

travels back and forth. The geometry of Fig. 3 was

chosen to provide a mathematically "simple"

situation for first-cut modeling and code

development, yet still complex enough to provide

The distance between the two large discs is variable

The primary measurement technique in this study was photographic documentation of streaks made with laser light

reflected from helium bubbles. Very briefly, in a sample procedure, a signal processing box is programmed to open

the shutter over a 15 degree segment (e.g. 0 ° _<0 _< 15 ° after top dead center (ATDC)) and to open it at the same

time each cycle over 100 cycles. A 35mm camera is set up to view the laser light sheet continuously. After 100

cycles, the processing box stops opening the laser shutter and the camera shutter is manually closed. The result is an

ensemble-averaged photograph of a 15 ° segment of crank angle duration. An example of such a result is presented

in Fig. 4 on the right side, for the "channel" field of view defined in Fig. 3.

NASA/TM--2002-211997 4
CSUuseda2-Daxisymmetric CFD-ACEmodeltocalculateurbulent andlaminarflowcasesforcomparisonwith

theflowvisualizationresults.ThesearealsoshowninFig.4.Itcanbeseenthatheturbulent calculationdidabetter

jobthanthelaminarindeterminingthepositionofthecenterofthevortex.

%hannel"

fieldof

view

Turbulent Flow- Laminar Flow- Flow-Vis.

Wall Wall

FIGURE4.Computed Results(CSU)end Flow- Visualization

(UMN) over 0 ° _<0_<15 °

A significant feature of this test data, also captured by the CFD calculations, is a

large standing vortex (or eddy) between the discs (see Fig. 4). The exhaust stroke

serves to energize the eddy whereas the intake stroke simply causes it to move

slightly. This standing vortex translates within the test section, but never

disappears. The implication is that there is large eddy transport within the space

between the two discs. This eddy augments heat and momentum transport within

the regions of Stirling engines simulated by this experiment (e.g., the expansion

FIGURE 3_Cross, Section of space volume "above" the displacer of the TDC). So, this vortex is of great interest.

Test Section (above), Solid

Model _below) When the geometry is changed to a narrower disc spacing of 54 mm, the dynamics

of the vortex change dramatically. A vortex appears on the exhaust stroke, but it is

ingested and destroyed on the intake stroke. Thus, for the aforementioned case, the presumed enhanced momentum

and heat transfer due to the standing eddy is active only for one half of the cycle. Thus the geometry of the

expansion and compression volumes in a Stirling convertor would have a significant effect on the nature of the

vortices that occur in these spaces, and could significantly affect heat and momentum transfer in these variable

volumes, and the performance of the convertor. The thermodynamic hysteresis loss in these variable volumes would

appear to be the loss most likely to be affected by the characteristics of these vortices. It's also possible that designs

calculated to "harness" the heat transfer enhancement effects of these vortices might offer improved performance via

design changes that reduce dead volume of a convertor (suggested by Berchowitz, 2002).

There are also interesting phenomena that occur in the "tube" or "cylinder" field of view defined in Fig. 3. For

example Fig. 5, taken from Ibrahim (2002), shows a comparison between laminar CFD calculations and flow

visualization data for UMN test Case IIIa also discussed in Adolfson (2002) (54 mm disc spacing, 70 RPM, and

72 ° < 0 < 126°ATDC ). It should be noted that the CFD calculations are shown from the edge of the tube to the

piston head, while the flow visualization is only for a portion of the tube. The flow visualization and CFD results

show a separation bubble created downstream of the sharp tube inlet during the suction stroke

(0 ° < 0 < 180°ATDC ). More illustrations of the test results are in Adolfson (2002) and more comparisons with

CFD results are in Ibrahim (2002a). These results show that the separation bubble grows in the streamwise and

cross-streamwise directions as the suction stroke proceeds. It was noted that the laminar CFD calculations did a

reasonable job of predicting the center and size of the original separation bubble. However, some other features of

NASA/TM--2002-211997 5
thedatawerenotpredicted, suchas(1)thedifferenceinflowdirectionduringdifferentcycles,indicatedbythe

"streaklines"inthelowervisualizationdataofFig.5,and(2)turbulencephenomena ithelatterpartofthesuction

stroke,sincealaminarcalculationwasrun.

AxesofSymmetry f

\

FIGURE 5. Comparison between CSU CFD-ACE Calculations (Top) and UMN

Flow Visualization Data for Test Case IIIa (54 mm Disc Spacing, 70 RFIVl) at

72 ° < O< 126°ATDC

Hot-wire anemometry is now

being used to make velocity

measurements within the UMN

rig. These measurements will

allow for a more severe test of

the CSU CFD-ACE model

calculations. Also, a "180 degree

turn" or "180 degree expansion

head" test section is being

designed for attachment to the

UMN scotch yoke drive facility.

This new test section will

roughly simulate the 180 degree

turn flow path from the displacer

to, and including, the heater that

occurs in current Stirling

convertors of interest, like the

TDC. Tests with this new test

section will include the effects of heat transfer, rather than just being limited to flow phenomena as in the current

"90 degree turn" test module. Fabrication of the new test module is expected to be completed, and flow visualization

tests are to begin, during the first quarter of 2003.

CSU Stirling Laboratory Research Engine (SLRE) Test Rig

The CSU SLRE derives from a two-piston Stirling engine developed by NASA Jet Propulsion Laboratory (Hoehn,

1982). It is a higher frequency device than the UMN test rig. The phase angle between the two opposed SLRE

pistons is adjustable. A special test section has been designed and fabricated for bolting between the two existing

piston-cylinder flanges on the rig. The pistons will drive atmospheric-pressure air or helium through the test section

at Reynolds and Valensi numbers representative of Stirling engine practice. Several flow inserts have been designed

and fabricated to be representative of regions of Stirling engines where flow area changes abruptly or flow velocity

changes direction. The SLRE test section contains a window to permit LDV (laser doppler velocimeter)

measurements of velocity. Before making velocity measurements, temperature and pressure measurements will be

taken within the test section; these tests are to be completed at CSU by the first quarter of 2003. Also, CFD-ACE

simulations of the SLRE tests with various flow inserts will be made for comparison with the data.

COMPLEMENTARY DOE REGENERATOR RESEARCH EFFORT

A complementary DOE Regenerator Research effort is also providing oscillating-flow and heat-transfer data for

validation of CFD-ACE. This work is also aiding in development of regenerator matrix models for the multi-D

Stirling code. The purpose of this three year effort (9/1/00-8/31/03) is to do fundamental regenerator research which

will lead to improvements in Stirling regenerator, and convertor, performance. It is funded by DOE's Office of

Energy Efficiency and Renewable Energy (Goldin, Co.) to advance solar dish-Stirling technology; Lizana Pierce is

contract monitor. The team effort is being carried out by CSU (the lead organization, responsible for CFD), UMN

(responsible for testing), Gedeon Associates and GRC. Results of the UMN test effort have been reported by Niu

(2002; 2003). Results of the corresponding CSU CFD effort have been reported by Ibrahim (2002b).

Fig. 6 shows a schematic of the facility and the test section. The facility consists of an oscillating flow generator, a

flow distributor, an air-water cooler, a regenerator, an electrical heater and an isolation duct. The piston is driven by

the same scotch yoke driver shown in Fig. 2. The present experiments are run at a frequency of 0.4 Hz, a stroke of

356 mm and a piston diameter of 356 mm. A detailed description of the test section, including the large-scale

regenerator mesh dimensions and arrangement, is given in Niu (2002). The three cooler tubes shown are part of a

NASA/TM--2002-211997 6
J

FIGURE 6. Schematic of the UMN Regenerator Experimental Fadlity and Test Section

3x3 rectangular array. Note the plenum between the cooler tubes and regenerator matrix whose length can be varied

to study the effect on jetting of flow from the cooler tubes into the matrix. The regenerator porosity is 90%, similar

to that of a number of modern small free-piston Stirling engines. Hot-wire anemometry has been used to measure

velocities at the axial center of the plenum and temperatures both within the plenum and within the regenerator

matrix. A representative Stirling engine was chosen for definition of test section design dimensionless parameters

(e.g., maximum Reynolds number and Valensi number).

Fluid velocities at the axial mid-point of the plenum, taken across the flow paths of the three central cooler tubes, are

shown in Fig. 7. The plenum diameter is 190 mm and width is 6.35 mm. The peaks in velocities are due to jetting

flows out of the cooler tubes in Fig. 7a and are due to flow "sinks" into the cooler tubes in Fig. 7b. Thus the flow

peaks in Figs. 7a and 7b are actually in opposite directions. The flow at the edges in Fig. 7a (3.5 and -3.5 inches) is

thought to be in the opposite direction from the peak flow velocities (hot-wire anemometry velocity measurements

do not define the flow direction). This assumption is consistent with CFD-ACE flow computations and also with

previous 2-D regenerator/plenum modeling done by Gedeon (1989). This suggests that, as fluid from the cooler

tubes "jets" into the matrix, there is flow recirculation so that flow is actually exiting the matrix at the wall. Niu

(2003) addresses the fraction of the matrix that is "inactive" due to the jetting of fluid into the matrix. Velocity

measurements have been taken for different plenum lengths at UMN, but these are not yet available for reporting.

Also, the permeability and the inertial flow coefficient for the test matrix have been measured.

iii¢' ,_0o i _- a00° i

> i i i !

6i ; !

35 3 25 2 15 1 --05 0 05 1 15

Location (inch)

i i

2 25 3 35

-3.5 -3 -2.5 -2 -1.5 -1 -05 0 0.5 1 1.5 2 2.5 3 3.5

Loc_tion (inch)

(a) Blowing Half of Cycle (b) Suction Half of Cycle

FIGURE 7. Velocities at Axial Mid-Point of the Plenum. Angles Shown are After Top-Dead-Center or ATDC.

(Suction-Half Velocities are in Opposite Direclion from Blowing-Half)

NASA/TM--2002-211997 7
An initial 2-D CFD model of the UMN flow test section predicted plenum velocities that were in reasonable

agreement with the data. A porous media model of the regenerator is being used to simulate velocities in the

regenerator. CFD-ACE results (plenum velocities and initial regenerator velocities), generated with a model of the

UMN test section for comparison with data, were reported by Ibrahim (2002b).

CONCLUDING REMARKS

Multi-dimensional Stirling models provide an alternative loss characterization strategy to attempting detailed

measurements within small, high-frequency engines. Validation of these models can be approached by use of test

sections that are simpler than a complete engine but that incorporate "engine like" flow and heat transfer processes.

Multi-D models of these test sections can then be used for CFD-ACE code validation. Addition of thermodynamic

2 nd law analysis (entropy, available energy, exergy, etc.), not currently available in CFD-ACE, to post process the

multi-D computations would enhance the ability of these multi-D codes to characterize and separate thermodynamic

losses.

There are some multi-D code development problems. 2-D models require considerable computer time. 3-D model

time requirements are much greater. Assuming these time requirements can be practically addressed by using a

parallelized version of CFD-ACE with a cluster of workstations, there are still problems in modeling the fluid-flow

and heat-transfer processes that need solution. For example, (1) accurate prediction of turbulence phenomena in an

oscillating-flow, oscillating-pressure level, complex geometry environment is not state-of-the-art. This includes the

problems of predicting when transitions between laminar and turbulent flow occur (Earlier loss understanding

testing at UMN did address laminar-turbulent transition for oscillating flow in a pipe). (2) Even in a nominally

axisymmetric geometry, there are usually/frequently small asymmetries that produce non-axisymmetric phenomena.

These will not be tracked by an axisymmetric 2-D or a 3-D simulation. (3) In general, the porous media that are used

for regenerator matrices have such small flow passages and solid metal dimensions that it is not practical to simulate

them exactly; therefore, macroscopic models of the porous media must be used. (4) CSU and CFDRC are working

together to solve apparent problems found when the CFD-ACE porous media module was used with one of the

available CFD-ACE turbulence models to model oscillating flow in a porous matrix. (5) Thermal relaxation of solid

materials is a problem. For example, thermal relaxation time of the regenerator matrix, or wall, can be several orders

of magnitude greater than the cycle period. This is a problem unless special means can be devised to accelerate

convergence of the solid material temperatures.

REFERENCES

Adolfson, D.A., Simon, T.W., Ibrahim, M.B., and Gedeon, D., "Measurements in Oscillatory Flow-s with Separation in Support

of Stirling Engine Model Development," Proceedings of the 37 th Intersociety Energy Conversion Engineering Conference,

Washington, D.C., IECEC 2002 Paper No. 20142, (2002).

Berchowitz, D., Private Communication, 2002.

Gedeon, D., "Modeling 2-D Jets Impinging on Stirling Regenerators," Proceedings of the 24 th Intersociety Energy Conversion

Engineering Conference, Washington, D.C., Paper No. 899202, (1989).

rd

Gedeon, D., "Sage User's Guide," 3 Edition, Gedeon Associates, 16922 South Canaan Road, Athens, OH 45701, (1999).

Hoehn, F.W., "Stirling Laboratory Research Engine," JPL Publication 82-13, (1982).

Ibrahim, M.B., Tew, R.C., Zhang, Z., Gedeon, D., and Simon, T.W., "CFD Modeling of Free-Piston Stirling Engines,"

Proceedings of the 36 thIntersociety Energy Conversion Engineering Conference, Savannah, GA, 2001 (IECEC2001-CT-38),

NASA/TM _001-211132, 2001.

Ibrahim, M.B., Zhang, Z., Kembhavi, S., Simon, T.W., and Gedeon, D., "A 2-D Axisymmetric CFD Model of Oscillatory Flow

with Separation," Proceedings of the 37 th Intersociety Energy Conversion Engineering Conference, Washington, D.C.,

IECEC 2002 Paper No. 20121, (2002a).

Ibrahim, M.B., Zhaaag, Z., Wei, R., Simon, T.W. and Gedeon, D., "A 2-D CFD Model of Oscillatory Flow-with Jets Impinging on

a Random Wire Regenerator Matrix," Proceedings of the 37 th Intersociety Energy Conversion Engineering Conference,

Washington, D.C., IECEC 2002 Paper No. 20144, (2002b).

Kornhauser, A.A., Gas-Wall Heat Transfer During Compression and Expansion, Massachusetts Institute of Technology, S.D.

Thesis, 1989.

Niu, Y., Simon, T.W., Ibrahim, M.B., and Gedeon, D., "Oscillatory Flow- and Thermal Field Measurements at the Interface

between a Heat Exchanger and a Regenerator of a Stirling Engine," Proceedings of the 37 thIntersociety Energy Conversion

Engineering Conference, Washington, D.C., IECEC 2002 Paper No. 20141, (2002).

NASA/TM--2002-211997 8
Niu,Y.,Simon, T.W.,Ibrahim, M.B.,andGedeon, D.,"Thermal Dispersion ofDiscreteJtsuponEntrance toaStirlingEngine

Regenerator underOscillatoryFlow-Conditions," Proceedings' of the 6th ASME-JSME Thermal Engineering Joint

Conference, Hapuna Beach, Hawaii Island, Hawaii, TED-A J03-641, 2003.

Tew, R.C., "Study of Two-Dimensional Compressible Non-Acoustic Modeling of Stirling Machine Type Components,"

Proceedings of the 36 thIntersociety Energy Conversion Engineering Conference, Savannah, GA, 2001 (IECEC2001-CT-27),

NASA/TM _001-211066, 2001.

Thieme, L.G., Schreiber, J.G., and Mason, L.S., "Stirling Technology Development at NASA GRC," Proceedings of the Space

Technology Applications International Forum (STAIF-2002), edited by MS. El-Genk, AIP Conf. No. 608, AIP, NY, 2002 ,

NASA/TM_001-211315/REV1, 2002.

NASA/TM--2002-211997 9
Form Approved REPORT DOCUMENTATION PAGE

OMB No. 0704-0188

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1. AGENCY USE ONLY (Leave blank) 2. REPORT DATE 3. REPORT TYPE AND DATES COVERED

December 2002 Technical Memorandum

4. TITLE AND SUBTITLE 5. FUNDING NUMBERS

Overview of NASA Multi-Dimensional Stirling Convertor Code Development

and Validation Effort

6. AUTHOR(S)

Roy C. Tew, James E. Cairelli, Mounir B. lbrahim, Terrence W. Simon,

and David Gedeon

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration

John H. Glenn Research Center at Lewis Field

Cleveland, Ohio 44135-3191

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration

Washington, DC 20546-0001

WU-755-1A-14-00

8. PERFORMING ORGANIZATION

REPORT NUMBER

E-13669

10. SPONSORING/MONITORING

AGENCY REPORT NUMBER

NASA TM--2002-211997

11. SUPPLEMENTARY NOTES

Prepared for the Space Technology and Applications International Fomm (STAIF-2003) cosponsored by the American Institute of

Aeronautics and Astronautics and the University of New Mexico, Albuquerque, New Mexico February 2-6, 2003. Roy C. Tew and

James E. Cairelli, NASA Glenn Research Center; Mounir B. Ibrahim, Cleveland State University, Cleveland, Ohio 44115; Terrence W.

Simon, University of Minnesota, Minneapolis, Minnesota 55455; and David Gedeon, Gedeon Associates, Athens, Ohio 45701.

Responsible person, Roy C. Tew, organization code 5490, 216-433-8471.

12a. DISTRIBUTION/AVAILABILITY STATEMENT

Unclassified -Unlimited

Subject Categories: 34 and 20 Distribution: Nonstandard

Available electronically at http://gltrs.arcmasa.gov

This publication is available from the NASA Center for AeroSpace Information, 301-621-0390.

12b. DISTRIBUTION CODE

13. ABSTRACT (Maximum 200 words)

A NASA grant has been awarded to Cleveland State University (CSU) to develop a multi-dimensional (multi-D) Stirling computer

code with the goals of improving loss predictions and identifying component areas for improvements. The University of Minnesota

(UMN) and Gedeon Associates are teamed with CSU. Development of test rigs at UMN and CSU and validation of the code against

test data are part of the effort. The one-dimensional (l-D) Stirling codes used for design and performance prediction do not rigorously

model regions of the working space where abrupt changes in flow area occur (such as manifolds and other transitions between

components). Certain hardware experiences have demonstrated large performance gains by varying manifolds and heat exchanger

designs to improve flow distributions in the heat exchangers. 1-D codes were not able to predict these performance gains. An accurate

multi-D code should improve understanding of the effects of area changes along the main flow axis, sensitivity of performance to

slight changes in internal geometry, and, in general, the understanding of various intemal thermodynamic losses. The commercial

CFD-ACE code has been chosen for development of the multi-D code. This 2-D/3-D code has highly developed pre- and postprocessors, and moving boundary capability. Preliminary attempts at validation of CFD-ACE models of MIT gas spring and "two

space" test rigs were encouraging. Also, CSU's simulations of the UMN oscillating-flow rig compare well with flow visualization

results from UMN. A complementary Department of Energy (DOE) Regenerator Research effort is aiding in development of

regenerator matrix models that will be used in the multi-D Stirling code. This paper reports on the progress and challenges of this

multi-D code development effort.

14. SUBJECT TERMS

Stirling cycle; Stirling engines; Simulation; Fluid mechanics; Heat transfer

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15

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