NASA Technical Memorandum 102097 

Initial Characterization of a Modular Heat 

Exchanger With an Integral Heat Pipe 

/NASA :,?I- 10 20 9 7) IN I TI AL CHARACTER I2 A ?I Off OF A HODULAR HEAT EXCHAIGEB WITH A1 INTEGRAL 

HEAT PIPE (NASA. Lewis Research Center) 

8P CSCL 05A Unclas 

N89-25078 

G.3/85 0217635 

Jeffrey G. Schreiber 

Lewis Research Center 

Cleveland, Ohio 

Prepared for the 

24th Intersociety Energy Conversion Engineering Conference 

cosponsored by the IEEE, AIAA, ANS, ASME, SAE, ACS, and AIChE 

Washington, D . C., August 6- 1 1, 1989 
INITIAL CHARACTERIZ4TION CF A MODULAR HEAT EXCHANGER NITH AN INTEGRAL HEAT P!PE 

jeffrey G. Schreiber 

National Ae:mautics and Space Administration 

Lewis Research Center 

?levelarid, Ohic 43135 

. ABSTRACT 

As part of the Civil Space Technology Initia- I 

tive (CSTI) fidvanced Technology program, a conceptual aesign of the Stirling space dngine (SSE) was 

gefierated. The overall gcal of tnd CSTI high 

capacity power element is to develop the technology 

base needed to meet the long duration, high 

0-l U') capacity power requirements for future PIASA space 

d co missions. The free-piston Stirling engine (FPSE) 1 has been chosen as the grcwth option in thc CSTI W 

program. A major goal dui ing the conceptual design 

of the SSE was to r?duce the number of critical 

joints. One area of colicern was the heat exchanger 

assemtlies that typically have the majoritj of 

critical joints. The solution proposed in the SSE 

conceptual design used 45 modular heat exchangers. 

Each module nas its own integral heat pipe to 

transport heat from the heat source to the engine. 

P demonstration of the modular conceDt was 

undertaken before committing to the detailed design 

of the SSE heat exchangers. An existing FPSE was 

modified as a test bed for modular heat exchanger 

evaluation. The engine incorporated three heat 

exchanger modules, each having a sodium filled heat 

pipe. The thermal loading of these modules was 

intended to be similar to the conditions projected 

for the SSE modules. The engine was assembled and 

tests are underway. 

This paper briefly describes the design and 

fabrication Gf the heat exchanger modules and the 

engine used for these tests. Evaluation of the 

individual heat pipes before installation in the 

engine is described. The initial test results 

with the modules in operation on the engine are 

presented. Future tests involving the engine are 

out1 i ned. 

INTRODUCTION 

The free-piston Stirling engine (FPSE) has 

been chosen as the growth optlon for the Civil 

Space Technology Initiative (CSTI) Advanced Technology program. The goal of this program is to 

develop the technology needed to allow for the 

development of a 100 kN electric power system for 

use in future space missions. Heat will be generated by a nuclear reactor and the Stirling engine 

Nil1 then be used to convert that heat to electricity through a free-piston Stirling enginell inear 

alternator configuration. 

4s d part sf this Advariied T?chc?iogy Zrogram. 

the coiceptual design cf a free-ais:on Sti-ling 

s3a-e engine (SSE) vios gcnei'ated. -hi; Nas perf\j.-me: ,.noer a NASA cfi;lrr'act and ?.ai seen :epor;ezi 

in kef. I. Ihc i7i "C !'";50 K; s11pe:diIo:; 5clE ij 

ai- intetmediate test rjndition oetviee'i thc 3!7 "C 

(653 K) space power aenonstrator engine (SPCE) ana 

the everttuai IO27 OC (1300 K) refrac:ory version 

of the SSE. For nuclear space power applications, 

sj';tt.m cptimization bi.jed cn minimum nass :ndicates 

thdt the,:e 2ngines shc..ild operate with the remce-ature ratio at 2.0. 

The SSE is a single cylinder free-pis;on 

Stirling engine (FPSE) designed to produce 25 kW 

of electric power. The engine must satisfy both 

efficiency and mass goais while operating at a temperature ratio of 2.0. The difficulties in creating a highly efficierit and low mass FPSE while 

being limited to a temperature ratio of 2.0, was 

first investigated with the SPDE. The SPDE program 

has been reported in Refs. 2 to 4. 

Future missions utilizing the CSTI power system require both long life and reliability. These 

requirements caused the SSE designers to propose a 

unique heat exchanger module conceDt that would 

drastically reduce the number of critical joints. 

This modular heat exchanger was proposed as a 

replacement for the previously used tube and shell 

heat exchanger. 

Only 40 heat exchanger modules are required 

to power the 25 kW SSE. This was considered an 

improvement over previous heat exchanger designs 

such as used in the SPDE which had approximately 

3200 brazed tubes in the heater and 3800 brazed 

tubes in the cooler. The larger number of brazed 

joints in the SPDE was a result of the decision to 

use standard tube and shell heat exchangers. Each 

braze joint represents a potential single point 

failure. The preliminary design of the SSE is currently being performed by Mechanical Technology 

Incorporated (MTI) of Latharn, NY, under contract 

to NASA Lewis Research Center. 

This report presents a brlef description of 

the design, fabrication. and operation of the heat 

exchanger modules. Evaluation of the individual 

heat pipes before being installed on the engine is 

also described. Results from the initial test runs 

are presented and discussed. 

1 
MODULAR HEAT EXCHANGER CONCEPT 

The modular heat exchanger concept was proposed for use in the SSE in an effort to reduce the 

number of critical joints that form the pressurized 

boundary of the engine. Each module contains (1) a 

heat exchanger used to transfer heat to the working 

fluid of the engine, (2) a regenerator, and (3) a 

cooler to remove waste heat from the cycle. A heat 

pipe was proposed as the heat transport device used 

to deliver heat to the module. Evaluation of the 

modular cooler was not included in this project. 

Figure 1 shows a model of one heat exchanger 

modul e. 

A decision regarding the general type of heat 

exchanger to be used in the SSE was required early 

in the preliminary design effort. The tube and 

shell concept, the modular heat exchangers, along 

with other candidates were being considered. 

Because the SSE preliminary design effort could 

lead to a long term commitment to a particular heat 

exchanger, a prototype test program involving modular heat exchangers would be in the best interest 

of the overall SSE project. A test using an existing facility along with existing engine hardware, 

to demonstrate the modular heat exchanger concept, 

was developed. This test was designed to provide 

modular heat exchanger test data at a very small 

cost in a timely manner, long before a commitment 

to a particular heat exchanger was made. 

HEAT EXCHANGER MODULE TEST ENGINE 

Existing hardware from the RE-1000 FPSE was 

used as the basis for the heat exchanger module 

test rig. Sensitivity tests involving the RE-1000 

were described in Ref. 5. As was the case with the 

RE-1000. this engine was designed to operate at 

7.0 MPa helium mean pressure, and at 30 Hz engine 

frequency. 

The design and fabrication of the heat 

exchanger modules and engine components were 

performed by Sunpower Inc., of Athens, OH, under 

contract to NASA Lewis. Thermacore Inc., of 

Lancaster. PA, performed the heat pipe design, fabrication, and processing. While the SSE conceptual 

design contained 40 modules around the engine cylinder, three modules around this smaller engine 

simulate the SSE module thermal load within acceptable limits. A cutaway of the engine is shown in 

Fig. 2. 

Slots embedded in the wall of the condensers 

of the heat pipes act as the helium passages forming the heater of the engine. Adjacent to the 

engine heater section of the module is the regenerator cavity. Any one of several different types of 

regenerators can be installed in this cavity.. The 

cavity can be completely filled with the regenerator matrix, or as was the case in the NASA Lewis 

tests, an annular regenerator can be used. The 

annular regenerator was chosen because it was less 

susceptible to flow maldistribution for this parti cul ar geometry. 

in the sensitivity test results were at 600 OC 

The highest temperature data points reported 

average heater tube temperature. 3ecause ssdium 

is used for the liquid metal neat transpo1.T system 

in the SSE, it was also used in the test modules. 

Most tests will be conducted at temperatures 

between 527 OC (800 K) and 677 OC: !950 K). Special 

tests have been planned ri th :he evaporaror :emperature ranging as high as 777 "C (1050 K) for snort 

periods. During the sensitivity tests the maxirrium 

power output was approximately 1500 W. Because tr:e 

modular heat exchangers were designed to operate at 

higher temperatures than in pas: RE-1900 tests, the 

engine should be capable of producing nearly 2000 W 

of indicated power. 

Due to the nature of this research and the 

test facility environment, the design philosophy 

used in this project required the heat pipes and 

heat exchangers to be made more rugged than the 

eventual space flight hardware. The heat pipes and 

the modules in general do not represent a design 

optimized for high performance or minimum rna:s, but 

rather a design with a high probability of successful and safe operation. The walls of the heat 

pipes were sized such that they could withstand 

full engine pressure in the event of a leak in the 

heat exchanger that allowed the pressurized helium 

into the heat pipe. These somewhat thicker walls 

require a higher temperature drop to drive the heat 

into the sodium along with another temperature drop 

as the heat is transferred from the sodium to the 

engine working fluid. 

excess working fluid. This excess is in the form 

of a liquid pool. aecause of the variation in the 

density of the vapor, the amount of excess liquid 

will change as the vapor temperature changes. In 

a zero gravity environment the excess fluid has a 

tendency to accumulate at the condenser of the heat 

pipe. 

form of inventory control, however this was not 

attempted in this test program. 

The heat exchanger module test engine is 

shown in the test cell in Fig. 3. and a closer 

view of the modules is shown in Fig. 4. The evaporators of the three heat pipes are located above 

the condensers and therefore, due to the fluid 

forces and gravitational forces, the excess liquid 

will accumulate at the end of the condensers of 

the heat pipes. In this case the liquid pool will 

be adjacent to the regenerator. Liquid sodium has 

approximately the same thermal conductivity as the 

stainless steel heat exchanger body. The presence 

of relatively low thermal conductivity liquid 

sodium pool at the end of the engine heater 

increases the overall thermal resistance of the 

heat exchanger and hence compromises its 

effectiveness. 

The test stand and mounting brackets for :he 

engine were designed so that the entire engine 

could be inverted and operated with the condensers 

of the heat pipes above the evaporators. In this 

gravity assisted mode of operation, the excess liauid sodium inventory should collect as a pool in 

the evaporator. The condenser will therefore have 

no excess liquid to inhibit heat transfer to the 

engine working fluid, and should allow heat to be 

Heat pipes typically contain some amount of 

This could potentially be altered by some 

2 
transferred effectively to the helium working fluid 

of the engine along the entire length of the helium 

flow passages. A noticeable effect on the temperature gradient of the heater near the regenerator is 

anticipated. A series of thermocouples have been 

located in the wall of the heater near the regenerator to allow this change to be measured. 

The heat pipes were designed, fabricated, and 

processed by Thermacore. These heat oipes use a 

sintered powder metal wick with two arteries per 

pipe. Figure 5 shows a cutaway view of one of the 

heat pipes. ,Although the pipes are capable of 

operating with only one artery, the second artery 

was incorporated to provide a degree of redundancy. 

The heat pipes were evaluated several times during 

the fabrication process to verify their performance relative to the design specifications. The 

evaluation was done with the evaporators above the 

condensers in order to verify the ability of the 

wick and arteries to operate against gravity. A 

water cooled gas gap calorimeter was mounted around 

the condenser and used as the thermal load. The 

temperature of each heat pipe was varied as the 

test proceeded. These tests verified the heat flux 

capability of the heat pipes and the experimental 

sonic limit of the heat pipes agreed with the anal yt i ca l pred i c t i ons . 

TEST PLANS 

Current plans are to characterize the heat 

exchangers with evaporators above the condensers 

as the baseline case, and then invert the entire 

engine and repeat the test matrix. The test matrix 

has been kept relatively small in an effort to make 

these tests more timely. Engine performance will 

be studied and compared to computer predictions. 

More specifically, the temperature gradient of the 

regenerator end of the heater will be measured for 

both orientations and compared to predictions from 

several different computer simulations. 

Other tests have been proposed for this engine 

that are not directly related to the use of heat 

pipes or modular heat exchangers. Effects of seal 

clearances, heat exchanger manifolding, center port 

systems, and other parameters may be studied.' If 

the need arises, alternate heat pipe and modular 

heat exchanger designs can be tested. The present 

set of modular heat exchangers are mechanically 

attached to the engine and can be removed and 

reinstalled. 

TEST RESULTS 

The engine was initially operated in September 

1988. Limited testing has been performed with the 

heat pipe evaporator temperatures ranging from 875 

to 1050 K. All of the testing to date has been 

with the evaporators physically located above the 

condensers. 

The initial goals of these tests were to 

demonstrate the feasibility of the modular heat 

exchanger and to provide operational experience 

with modular heat exchangers on an engine using 

liquid metal heat pipes. These goals have been 

accomplished and operation of the modular heat 

exchanger has been demonstrated. Performanc? r,f 

the engine with the modules is currently being 

s tudi ed. 

During these tests. the engine ha; not Seen 

operated at conditions necessary to produce Taximcm 

power, yet the thermal loading in the heat pipes 

and heat exchangers has verified feasibility of the 

modular design and agreed well with predicted performance. The reduced Dower output of the engine 

has been caused by a short displacer stroke. This 

reduced displacer stroke causes the Dressure phase 

angle to decrease. Excessive leakage from the displacer gas spring or improper damping within the 

heat exchanger flow path may be the cause of the 

reduced stroke. Because this displacer utilizes 

wear couples instead of gas bearings, a small 

amount of wear is expected in the displacer gas 

spring with use. Engine performance will improve 

as the test program continues with components 

being upgraded as needed. 

Measured and calculated temperature profiles 

along the heat pipe are in agreement. Figure 6 

shows a comparison of the measured and calculated 

temperatures for one of the data points recorded. 

Based on the thermal energy being transported 

through the heat pipe, temperature drops at both 

the evaporator and condenser were calculated. The 

temperature drops through the stainless steel wall 

and the saturated wick have been taken into 

account. A correlation published in Ref. 6 was 

used for the thermal resistance of the sintered 

powder metal wick. This correlation is a function 

of resistivity of the sodium and the wick material. 

The summation of the evaporator and condenser temperature drops were compared to the measured value. 

drop while the measured value was 79 "C. The calculation assumed that vapor within the heat pipe 

acted isothermally. In reality there was some 

small pressure and temperature drop in the sodium 

vapor. The data point chosen for this example was 

not near the sonic limit, therefore a vapor temperature drop of 10 OC is quite plausible [61. 

Although this assumption was reasonable at the conditions of the sample data point, it would not be 

appropriate were the heat pipe operating at the 

sonic limit. 

The calculation predicts a 69 OC temperature 

Analysis of the data also suggests that the 

measured temperatures may be in error by up to 

20 OC due to aging effects of the type K thermocouples. The integrity of the thermocouples and the 

installation techniques used are being evaluated. 

Appropriate corrections will be made and quality 

measurements of the temperature profiles of the 

heat pipe, the heat exchanger, and the engine working gas will be made in future runs. 

With these results, the performance of similar 

heat transport systems can be evaluated. This Comparison was based on bulk temperature at each end 

of the heat pipe and does not look at the temperature gradient anticipated at the condenser, causeO 

by the nonuniform thermal draw of the engine. 

3 
SUMMARY 

Conceptual design of an advanced FPSE for 

space power application, the SSE. proposed the use 

of a new concept in heat exchanger design which 

replaced conventional tube and shell heat exchangers. The SSE represents an advance in technology 

from the 377 OC (650 K) SPDE toward the technology 

needed for the 1027 "C (1300 K) engine. The new 

heat exchanger module concept was offered as a 

means of drastically reducing the number of critical joints in the heat exchanger assembly. 

the SSE preliminary design task requires choice of 

a heat exchanger concept that will have long lasting implications, a relatively low cost, near term 

demonstration of the heat exchanger concept was 

undertaken to benefit the overall program. 

An engine was designed with modular heat 

exchangers which simulated the operating conditions of the SSE. The test engine used only three 

heat exchanger modules instead of the 40 that would 

be required for the proposed SSE design; however, 

the operating condition of each module was the same 

as on the SSE. The three modules, along with other 

necessary hardware, were built and engine testing 

started. 

Since 

The concept of modular heat exchangers with 

integral heat pipes has operated successfully on 

the engine. 

pipes agree well with the analytical models. 

perature drops and the conditions at which the 

Measured characteristics of the heat 

Temsonic limit is encountered also agree with models. 

These tests have established feasibility of the 

modular heat exchanger concept for use in future 

FPSE designs. The engine is currently operat'ng 

at reduced power levels. Testing will continue as 

improvements are made to the hardware :o increase 

the performance. 

REFERENCES 

1. L.B. Penswick. "1050 K Stirling Space Engine 

Design," NASA CR-182149, 1988. 

2. J.G. Slaby, "Overview of the 1985 NASA Lewis 

Research Center SP-100 Free-Piston Stirling 

Engine Activities," NASA TM-87028, 1985. 

3. J.G. Slaby, "1988 Overview of Free-Piston 

Stirling Technology for Space Power at the 

NASA-Lewi s Research Center," NASA TM-100795, 

1988. 

4. D.L. Alger, "Progress Toward the Evolution of 

a Stirling Space Engine," SAE Paper 880545, 

NASA TM-100221, 1988. 

5. J.G. Schreiber, S.M. Geng, G.V. Lorenz, 

"RE-1000 Free-Piston Stirling Engine Test 

Results," DOE/NASA/1005-11. NASA TM-88846, 

1986. 

6. P.D. Dunn, and D.A. Reay, Heat Pipes, 3rd 

edition, Pergamon Press, New York. 1982. 

..""I--. 

COOLER-REGENERATORFIGURE 1. - STIRLING MDULAR HEAT EXCHANGER ASSEMBLY. 

- . 

ORIGINAL PAGE 

BLACK AND WHITE PHOTOGRAPH 

4 
FIGURE 2. - CUTAWAY OF HEAT EXCHANGER TEST ENGINE. 

I 

C-88-0672 

FIGURE 3. - HEAT EXCHANGER TEST ENGINE BEING READIED FOR TESTING. 

5 BLACK AND OR!GfNAC WHITE PAGE PHOTOGRAPH 
FIGIIRE 4. - HEAT lXIWi€R MOWLES ATTACHED TO THE ENCIIIE. 

KASURED 

1759 WATTS 

THROUGHPUT 

/ 

702 OC J' 

1 

CONNECTION TO r HEAT PIPE EVAPORATOR 

EXPANSION SPACE7 

r GAS MANIFOLD 

- REGENERATOR 

F16uIE 5. - C(WPOWUIT LAYOUT OF A HEAT O(CHAWGER IWIWLE. 

PREDICTED 

11 OC AT (WALL) 

702 OC c 

6 OC AT (WICK) 

1759 

WATTS 

12 OC AT (WICK) 

40 OC AT (WALL) 1 

69 OC AT (TOTAL) 

623 OC 

HEAT EXCHANGER MODULE ELECTRIC ANALOG 

FIGURE 6. - COMPARISON OF THE CALCULATED AND MEASURED TERPERATURE DROP ALONG THE HEAT PIPE. 

6 BLACK AND ORIGINAL WHITE PAGE PHOTOGRAPH 
National Aeronautics and l- Space Administration 

1. Report No. 

NASA TM- 102097 

Report Documentation Page 

2. Government Accession No. 

17. Key Words (Suggested by Author@)) 

Stirling engine 

Heat engine 

Stirling cycle 

Heat pipe engine 

7. Author(s) 

Jeffrey G. Schreiber 

9. Performing Organization Name and Address 

National Aeronautics and Space Administration 

Lewis Research Center 

Cleveland, Ohio 44135-3191 

18. Distribution Statement 

Unclassified -Unlimited 

Subject Category 85 

12. Sponsoring Agency Name and Address 

National Aeronautics and Space Administration 

Washington, D.C. 20546-0001 

15. Supplementary Notes 

19. Security Classif. (of this report) 20. Security Classif. (of this page) 21. No of pages 

Unclassified Unclassified 8 

3. Recipient's Catalog No. 

22. Price' 

A02 

5. Report Date 

6. Performing Organization Code 

8. Performing Organization Report No. 

E-4859 

10. Work Unit No. 

586-01-1 1 

11. Contract or Grant No. 

13. Type of Report and Period Covered 

Technical Memorandum 

14. Sponsoring Agency Code 

Prepared for the 24th Intersociety Energy Conversion Engineering Conference cosponsored by the IEEE, AIAA, 

ANS, ASME, SAE, ACS, and AIChE, Washington, D.C., August 6-11, 1989. 

16. Abstract 

As part of the Civil Space Technology Initiative (CSTI) Advanced Technology program, a conceptual design of 

the Stirling space engine (SSE) was generated. The overall goal of the CSTI high capacity power element is to 

develop the technology base needed to meet the long duration, high capacity power requirements for future 

NASA space missions. The free-piston Stirling engine (FPSE) has been chosen as the growth option in the CSTI 

program. A major goal during the conceptual design of the SSE was to reduce the number of critical joints. One 

area of concern was the heat exchanger assemblies that typically have the majority of critical joints. The solution 

proposed in the SSE conceptual design used 40 modular heat exchangers. Each module has its own integral heat 

pipe to transport heat from the heat source to the engine. A demonstration of the modular concept was 

undertaken before committing to the detailed design of the SSE heat exchangers. An existing FPSE was modified 

as a test bed for modular heat exchanger evaluation. The engine incorporated three heat exchanger modules, each 

having a sodium filled heat pipe. The thermal loading of these modules was intended to be similar to the 

conditions projected for the SSE modules. The engine was assembled and tests are underway. This paper briefly 

describes the design and fabrication of the heat exchanger modules and the engine used for these tests. Evaluation 

of the individual heat pipes before installation in the engine is described. The initial test results with the modules 

in operation on the engine are presented. Future tests involving the engine are outlined. 

*For sale by the National Technical Information Service, Springfield, Virginia 221 61 NASA FORM 1626 OCT 86