IAIAA-89-1658 An Experimental and Computational Analysis of the Thermal Interface Filler Material Calgraph J. W. Welch and L. E. Ruttner The Aerospace Corporation El Segundo, CA

AlAA 24th Thermophysics Conference Buffalo, New York / June 12-14, 1989

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For permission to copy or republish, contact the American Institute of Aeronautics and Astronautics 370 L’Enfant Promenade, S.W., Washington, D.C. 20024

AN EXPERIMENTAL AND COKPDTATIONAL ANALYSIS OF THE THERMAL INTERFACE FILLER MATERIAL CALGRAPH

John W. Welch and Les E . Ruttner

The Aerospace Corporation, El Segundo, California

Abstract

The heat transfer capability of Calgraph was determined experimentally using a physical configuration that represented an electronic box in a vacuum environment. The heat transfer across two plates with a sheet of Calgraph between them and compressed with screws around the periphery was examined at hot and cold extreme temperatures and different screw torques. A thermal model was developed and used with the test data to gain insight into the heat transfer process in four regions that characterize the plates. It was found that Calqraph increased the heat transfer coefficients in and adjacent to the flange region by the mounting screws by factors of two to three over those for a bare interface. In the center reuion where the heat transfer can be very poo; for a bare interface, Calqraph provided no improvement.

Nomenclature

A nominal area of contact, in2 -_ h heat transfer coefficient, Btu/hr-ft*-

Q steady state heat flux crossing the OF

interface. Btu/hr AT steady state temperature drop across

the interface, OF

Abbreviations

K L I Kultilaycr insulation R 1 V Roan teaperature vulcanizing

Introduction

This study is a result of the interest in the aerospace industry in improving the heat transfer between electronic boxes and their mounting surfaces. Although heat transfer coefficients are used by thermal analysts to predict the temperature drop across such interfaces with various filler materials, the actual interface heat transfer process and the assumptions associated with using these coefficients are many times not clearly understood. An experiment was designed to determine the heat transfer coefficient across a sheet of the filler material Calgraph and to obtain heat transfer data for a typical flange mounting configuration. Two plates were used to simulate the physical configuration of an electronic box bolted to a mounting surface.

.- The purpose of this work was to provide a better understanding of how heat is transferred across an electronic box

CoP?riehi American lnilitufe of Aeronautics and Arlr~nail l i tr . 1°C.. 1989. All riehir iercrved.

interface and to assess the effectiveness of Calgraph in this type of application.

When heat flows across the interface of two surfaces pressed together, a large thermal resistance may exist at the region of contact. Surface roughness causes only a small fraction of area between the two surfaces to actually touch. This can be less Fhan one percent of the total surface area. No matter how well polished, every surface has inherent irregularities consisting of peaks and valleys. Heat transferred by conduction is thereby constrained to flow through the reduced areas of contact and not over the entire surface area.

In applications where the thermal resistance at an interface must be kept low, compressible interface materials are placed between the two surfaces. Filling in the valleys where contact would otherwise not occur, the interface material increases the contact area and reduces the thermal resistance. For the purpose of analysis, a heat transfer coefficient must be determined to characterize the interface. Although coefficients are specified for numerous filler materials, there are difficulties in establishing such values. The contact between surfaces with an interface filler material is rarely perfect. The effectiveness of a filler-material is many times not a result of the material's thermal conductivity, but a result of the material's ability to flow into the valleys that enable conduction heat transfer. A second problem with determining the heat transfer coefficient across an interface concerns the unknown thickness of the filler material after it is compressed. AS a result of the peaks and valleys in both surfaces, the thickness of the compressed filler material can vary from its uncompressed thickness to very little thickness at all.

A third problem is associated with mounting screws or bolts used to compress a filler material between two surfaces. The pressure distribution across the surfaces will vary greatly with location. Near the mounting screws, good contact is made, the interface thickness is small and a high interface conductance results. Away from the mounting screws. one or both surfaces mav bow outward and contact between the surfaces and the filler material may be lost.

Despite these difficulties in esrablishing heat transfer coefficients, thermal analysts use average coefficient values to predict the ten.peratute drop across the interface of

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an electronic box and a mounting surface. This is often done in concert with the assumption that the heat flux across the interface is uniform and independent of location. Common heat transfer coefficients range from 5 Btu/hr-ft2-"F for large boxes to 20 Btu/hr-ft'-OF for small boxes when no filler material is used and 25 to 100 Btu/hr-ft'-"F when a filler material is used.

Nevertheless, heat transfer coefficients higher than those associated with a bare interface are needed to improve the transport of the large heat fluxes seen in many of today's applications. With the continual reduction in the size of electronics, high heat flux densities are common, while performance and reliability considerations necessitate low junction temperatures. Weight and packaging requirements for electronic boxes for space vehicles impose practical constraints on heat sinking provisions. Therefore tremendous importance is placed on the numerous thermal resistances between the electronic part and mounting surface. As spacecraft must operate in a vacuum environment, the high heat fluxes are transferred to the mounting surfaces primarily by conduction with radiation usually providing only a secondary effect. It is therefore essential that heat flow paths from the electronic part to the mounting surface provide a low enough resistance so as to keep part junction temperatures below their maximum allowable temperature.

The use of an interface material between the box baseplate and the mounting surface on space vehicles and satellites is a common practice in the effort to minimize the interface resistance. There are two types of fillers used by aerospace contractors - those that are viscous and those that come in sheets. RTV, a room-temperature vulcanizing silicone compound, is an example of the former. Applied wet, it cures to a soft, rubbery material. The mixing procedure and application process is not a simple task, but instructions are well documented. RTV is usually used in thicknesses of five to ten thousandths of an inch although it is often difficult to measure the thickness of the final gap. Heat transfer coefficients commonly range from high values resulting from experimental tests to conservatively low values in actual design, indicating a lack of true understanding as to how effective RTV is in its applications. In addition to problems encountered in mixing and applying, the cure forms a bond that makes the box difficult to remove should it be necessary. To avoid this, thermal greases have been used as substitutes for RTV. As these greases do not cure, they remain in a paste-like condition, facilitating box removal. However, they tend to flow quite.readily making them messy and increasing the likelihood that they will contaminate other parts. For this reason, they are not widely used in space flight applications.

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The second type of interface filler material is that which comes in sheets and is cut to size and placed between the two surfaces. One such material is CHO-THERM, manufactured by CHROMERIC~. CHO-THERM is a composite consisting of a silicone elastomer binder with either boron nitride or alumina as the thermally conductive filler. It is easy to cut and has a medium hard consistency. Calgraph, with other trade names such as Sigraflex and Grafoil, is another thermal filler material manufactured in sheets. Unlike CHO-THERM, Calgraph is a flexible graphite sheet that does not appear to be as compressible as CHO-THERM. Currently used bv British AerOSDace at mountins surfaces in spacecraft applications, it i5 similar to CHO-THERM in that it is relatively easy to handle.

Yet despite the advantages, there are several problems associated with sheet-like filler materials. In some applications, the pressure required to compress the filler material may exceed stress allowances in the screws or the mounting structure. Sheet- like filler materials also do not have the space experience of RTV or the thorough documentation of effective thermal conductance values for electronic box applications. Heat transfer coefficients commonly range from very poor to extremely good. As these new filler materials are introduced, it is important that reliable thermal conductivity data be available to determine in which application each of these new materials would prove beneficial. Despite high heat transfer coefficients reported by British Aerospace, Calgraph has neither the extensive data characterizing its thermal properties nor the appearance of adequately dissipating large quantities of heat. Its paper-like thickness and texture are not easily compressed so it appears unable to fill the small valleys at the interface of two surfaces. It was therefore a goal of this study to experimentally gather the necessary data to assess whether Calgraph could transfer heat across an interface in a vacuum environment.

Several experimental studies have been performed to determine the heat transfer coefficient of Calqraph. The most complete work to date is that of P. F. Taylor. An aluminum block, 2 inches x 2.75 inches x 1.5 inches high, was mounted to a 0 . 5 inch thick aluminum plate with four 8-32 screws. The heat transfer coefficient reported, 2609 Btu/hr-ft'-"F, was comparable to the RTV value obtained during the same test, 2685 Btu/hr-ft'-OF, and several times greater than the bare interface value, 8 7 6 Btu/hr- ft2-"F. The small plate area Taylor used, not very representative of an electronic box baseplate, best explains the high Calgraph value.

Polycarbon, Inc., a manufacturer of Calgraph, has provided a typical tQermal conductivity value of 4 Btu/hr-ft-'F. For a 0.008 inch thick sheet, the heat transfer coefficient would be 6000 Btu/hr-ft'-'F. As in Taylor's investigation, this high value

most probably resulted from a test where a high pressure and uniform contact was placed across the Calgraph. This is different from an electronic box where the pressure varies with distance from the mounting screws. Unlike these two investigations, the goal of this study was to experimentally measure the heat transfer coefficient of Calgraph in an electronic box mounting application.

An additional goal of this study was to better understand interfacial heat transfer between plates bolted together along their perimeter. The configuration of the test plates is representative of an electronic box bolted along its flange to a mounting surface. It is commonly assumed that the heat flux between such surfaces is uniform for a filled and even for a bare interface. Elementary physical considerations suggest that this assumption is invalid, perhaps grossly so, for many of the configurations for which it is used. If such is the case, heat transfer coefficients so obtained are equally invalid and can result in erroneous temperature predictions. An analytic method was used in conjunction with the temperature measurements to obtain actual interface heat fluxes and heat transfer coefficients.

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Bmerimental analysis

Test SetuD

Two aluminum 6063-T6 plates 11 inches x 6 inches x 5/16 inch high were fastened together with sixteen No. 8-32 stainless steel screws around the periphery. The physical configuration is shown in Figure 1. Stainless steel washers separated the screw heads from the top plate surface. The two metal plate surfaces in contact were machined to a surface finish of 63 pin. Nine thermocouples were attached to the outer surface on one of the plates. The location of the nine thermocouples is depicted in Figure 2 . A Kapton heater, 10 inches x 5 inches, was then cured to the plate. The resistive elements in the heater were closely spaced and uniform heating was expected. At the same corresponding locations, nine thermocouples were attached to the outer surface of the other plate. Two configurations were tested - one with Calgraph between the two plates and one without Calgraph between the two plates. A 0.008 inch thick Calgraph B Graphite sheet, obtained from Polycarbon, Inc. of Valencia, California, was placed between the two plates for the Calgraph interface tests, Holes 5/16 inch in diameter were cut in the Calgraph sheet to allow the screws to freely compress the two plates without pulling on the Calgraph.

A schematic of the test setup is given in Figure 3. A thermal vacuum chamber was used to test the samples. At the base of the chamber, a cold plate provided the

thermocouples were attached to the cold plate to monitor the boundary condition temperature. The temperature was set by

I

temperature boundary condition. TWO

v

I

I

0 0 0

Ti4-d-m (dimensions In lncheri

Figure 1. Top View of Test Plate

SCREW REGiON

-i BETWEfN SCREWS REGION

CENTER LOOP REGION

CENTER REGION

LOCATION

Figure 2 . Test Plate Showing Thermocouple and Region Location

Multilayer Insulation Blanket (10 layers)

\ Strip Heater Top Plate

Calgraph

Bottom Plate

Cold Plate

Thermocouple

Figure 3 . Experimental Test configuration

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liquid nitrogen which flowed through tubing welded to the bottom of the cold plate in a serpentine configuration. A layer of

placed between the lower plate and the cold plate to provide a high thermal conductance - between these surfaces. A ten sheet multilayer insulation (MLI) blanket was constructed to fit over the top test plate and down over the edges of both test plates. To prevent a heat leak from the side of the plates to the cold plate, the edges of the blanket's outermost layer were taped to the underneath surface of the bottom plate.

Test Procedure

A test procedure was established to determine the heat transfer coefficient for a bare and a calgraph interface at two mounting screw torque levels and two temperature levels. Table 1 summarizes the tests performed. Torque levels of 10 in-lbs and 20 in-lbs were chosen, the latter being the design torque for the mounting screws. Tem erature levels on the cold plate of -30 F and 16OOF were chosen as realistic electronic box boundary temperatures. In the remainder of this report, torque levels of 10 in-lbs and 20 in-lbs shall be referred to as the low and high torque levels, respectively, and the temperature levels of -30F and 160°F shall be referred to the cold and hot cases, respectively.

OMEGATHERM thermal conductive paste Was

B

Table 1. Test Matrix

I SCREW TOROUE (IN-LE1 I

INTERFACE TEHPERATLTRE ( F ) TEMPERATIJRE ( F )

BARE CALGRAPH

The first step in the experimental procedure after the test plates were placed on the cold plate and chamber sealed was to evacuate the system. Vacuum was established whep the chamber had been pumped down to 10. Torr or less. Once the system was evacuated the initial heater rate was set. To provide the maximum temperature difference between the two plates, the heater voltage, controlled by a variable voltage Veriac unit, was set so that the resulting heater power was approximately 2 5 0 Btu/hr ( 7 2 Watts). The chamber pressure and voltage were checked throughout the test.

The next step was to adjust the control on the liquid nitrogen so as to establish the desired cold plate temperature. The temperature was established during each test by cycling the liquid nitrogen on and off once the cold plate temperature had reached the desired temperature extreme. Equilibrium was established at both the hot and cold temperature tests when the temperature change on the top plate and bottom plate was fO.l°F or less over a ten minute period of time. Between each test

.+

performed at a different torque level, the plates were removed from the chamber and the screws were retorqued to the desired level in a tightening sequence of torquing opposite screws.

Heat Flow Reuions

AS described by Bevans,4 for the purpose of analysis, an electronic box baseplate can be subdivided into zones or regions that characterize the amount of heat transferring to the mounting surface. The four regions include areas 1) at the mounting screw, 2 ) between the mounting screws, 3 ) between the screws and center of the plate, and 4 ) in the center of the plate. Bevans named these regions zones 1, 2, 3, and 4 , respectively. The same approach was followed in this analysis, and the thermocouples were placed on the plates so as to measure the temperature difference across the interface in each of the four zones. To describe these regions more clearly, the four zones are referred to as 1) Screw Region, 2) Between the Screws Region, 3 ) Center Loop Region, and 4 ) Center Region. The physical location of the four regions is shown in Figure 2 .

Error AnalVSiS

The accuracy of the reported heat transfer coefficients was calculated based on the error in the experimental measurements. A 0.05 Watt error was associated with the 7 2 watts of heater Dower. A 0.05 in2 measurinu error was associ'ated with the 6 6 in2 area 0: the plates. A 0.1% error in the temperature difference between two thermocouples was associated with a 1.9'C area-averaged temperature difference. These values, combined in a linear error analysis, approximate the total measurement accuracy. In calculations of the heat transfer coefficient,

1 -

1 -

- 5 .1%

The accuracy is most dependent of the error associated with the temperature difference across the interface. For heat transfer coefficients reported in individual regions (not area-averaged coefficients), a l0C temperature difference replaces the 1.9OC value and the measurement accuracy in the heat transfer coefficient is 10.0%

Emerimental Results

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The experimental data from the tests performed with a bare interface are summarized in Table 2 and from the tests performed with a Calgraph interface are summarized in Table 3 . The general trends are similar for the two sets of results. The temperature difference in the Center Region is significantly greater than the difference in the Screw Region. The dependance of temperature difference on torque level at the same temperature level is generally small. For the same torque level, the temperature difference at the cold temperature is greater than at the hot temperature.

REGION THERMOCOUPLE WCATION

SCREW TORQUE (IN-LB)

10 20

TEMPERAlWRE (F) TEMPERRT[IRE (F)

The heat transfer coefficients so calculated for a bare interface are summarized in Table 4 . The heat transfer coefficient in the Screw Region was four to five times greater than in the Center Region. At the same temperature level, significantly larger values were found at the higher torque level compared to the lower torque level only in the Screw Region and the Between Screws Region. At the same torque level, the heat transfer coefficients values were approximately 2 5 percent greater at the hot temperature than at the cold temperature.

12.9 13.8

Table 4. Bare Interface He t Transfer Coefficient Values (Btu/hr-ft P -OF) Based on Assumed Uniform Heat Flux

o.* 1.2 0.0

1.2

3.6 2.7 6.0

9.6 10.0

REGION

Assuming a uniform heat flux through the two SCREW TORQUE (IN-LB)

20 plates, a heat transfer coefficient was calculated for each of the four regions by -- ft') by the temperature difference measured -30 160 -30 160 across that region. Temperature differences

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dividing the uniform heat flux (3.8 Btu/hr- REGION TEMPERATURE (F) TEMPERA'I'LRE ( F l

were averaged at thermocouples located in SCREW REGION 170 260 150 260 that particular region. Figure 2 and Tables BETWEEN SCREWS 250 300 90 140

130 110 120 110 2 and 3 list the thermocouples used in each E;;;; 30 50 30 50

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SCREW REGION BETWEEN SCREWS CENTER W O P I I -30 I 160 I -30 1 160 I CENTER REGION

ISCREW REGION 1. 1 ::

BETWEEN SCREWS 4 .

5. 6. 7.

8. 9.

CENTER W O P

CENTER REGION

I 7.0 I 4.8 3.9

8.9 6.9

12.1 13.2 10.5

19.3 14.9 15.3

Table 3 . Temperature Difference ('F) at Thermocouple Locations for a Calgraph Interface -

SCREW TORQUE (IN-LB)

'1' I 2.3

3'9 7.2 I

REGION THERMOCOUPLE WCATION

SCREW REGION 1. 2 . 3.

*. 5. 6. 7.

8. 9

BETWEEN SCREWS

CENTER W O P

CENTER REGION

1.1 2.2 2.6

1.9

4.7 2.k 7.5

1.7 1.2 2.6

2.1

4.8 2.9 7.3

13.1 13.8

1.2 0.6 0.0

1.6

3.7 2.6 5.9

9.6 9.7

1 I I I I I

SCREW TORQUE (IN-LB)

The heat transfer coefficient values for a Calgraph interface are summarized in Table 5. The coefficient values were approximately eight to ten times greater in the Screw Region than in the Center Region. The coefficient values in the Screw Region were larger at the high torque than they were at the low torque at the same temperature level. Finally, at the same torque level, the coefficient values in the hot case were greater than the values in the cold case. Table 6 shows a comparison of Tables 4 and 5. The greatest improvement in the Calgraph heat transfer coefficients were found in the Screw Region and the Between Screws Region at the low torque and in the Screw Region at the high torque. Table 5. Calgraph Interface Heat Transfer Coefficient

Value5 (Btu/hr-ft'-'F) Based on Assumed Uniform Heat Flux

SCREW TORQUE (IN-LB) I REGION TEMPERATURE (F) TEMPERATURE (Fl

SCREW REGION BETWEEN scREns 280 260 3 4 0 CENTER W O P CENTER REGION

Heat Transfer coefficients Based on uniform Table 6. Percent Improvement in the neat Transfer Coefficients Between a Calgraph and a Bare Heat Flux Tn+arf;.rn

gomutational Analysis

TEMPEPATURE SCREW BETWEEN CENTER REGION SCREWS IOOP

A thermal math model was developed using the svetems Imoroved Numerical Differencina

CENTER REGION

- 2 - -- Analyzer,' *to provide further insight inti the heat transfer phenomena between the two plates. Due to symmetry, 1/4 of the plate was modeled. A three-dimensional schematic of-the n.;rdel is depicted in Figure 4 and a top view schematic, showing node locations with respect to individual regions, is given in Figure 5. The model consists of 48 nodes and 146 connectors. The 4 8 nodes were divided into 2 4 top plate nodes and 24 bottom plate nodes. Connectors were chosen to depict the lateral conduction through the two plates and heat transfer between the two plates. The diagonal connectors on the top plate shown in Figure 4 represent paths to the mounting screws. A heat flux was applied to the top plate nodes to simulate the heater and an external boundary condition node was included to represent the cold plate. Heat loss through the MLI blanket was calculated to be less than 2 percent of the heater power input and was therefore neglected in the analysis. Based on themanufacturer's specified conductivity value of the OMEGATHERM thermal paste, conductance values were calculated for the heat path from the bottom plate to the cold plate.

INTERFACE CONNECTORS ,101 ciallly. O n l y t o w a m I h O W " 1

THERMAL P A V E CONNECTORS ,101 d i i i l y , only tour 111 s h o r n )

cot0 P U l E

Figure 4. Thermal Model of Test Assembly (1/4 of Total Plate Area)

The purpose of the computational analysis was to calculate the actual heat transfer distribution across the interface by assuming a variable, and not uniform, heat flux between the two plates. The heat transfer coefficients were determined for each region using the equation,

Q = h A A T

Figure 5. Top View of Thermal Model Showing Individual Regions

where Q is the heat flux across the interface in a particular region, h is the heat transfer coefficient across the interface in that region, A is the area of the region and AT is the experimentally measured temperature difference across that region. With area and temperature difference known for each region, it was then a matter of choosing four heat transfer coefficients, one for each region, running the thermal model with the coefficients and checking the resulting temperature distributions on the top and bottom plates. Adjustments were made to the heat transfer coefficients and the model rerun until the temperature distributions were in agreement with the experimental data.

Heat Flux Results

The hot and cold cases at the high screw torque were computationally analyzed for a bare interface and for a Calgraph interface. The heat flux results, averaged among the nodes that comprised each region, are presented in Table 7 . As shown, the heat flux distribution across the plate was clearly nonuniform. For both interfaces, the heat flux trends are similar. Most of the heat passed between the plates in the Screw Region, and only a small f l u x crossed the plates in the Center Region. The heat flux in the Screw Region was approximately twice the uniform heat flux value. In the Center Region, the heat flux was considerably less than the other regions. Table 7 . Average Heat Flux (Btu/hr-in2) ReSUltS Based

On the Unconstrained Heat Flux for 20 in-lb Data

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Heat Tran sfer Coeff icients

The heat transfer coefficients obtained in the unconstrained heat flux analysis are shown in Table 8 . The largest coefficient values were in the Screw Region and the

c smallest coefficient values were in the Center Region. The values in the Calgraph interface test were at least twice the value as those in the corresponding bare interface tests. Except in the Center Region, where the heat transfer coefficient values are very small, the Screw Region had the greatest increase. In the Screw Region, the hot case coefficient values were approximately twice the cold case coefficient values for both interfaces. In the other two regions, there was virtually no difference between the hot and cold cases.

Table 8. Unconstrained Heat Flux Heat Transfer Coefficients (Btu/hr-ft*-°FJ that Hatch Experimental Data for the 20 in-lb Test

BARE

ANALYSIS XETHOD

UNCONST. UNIFORM

450 190 150 140 100 70

1 4 0

INTERFACE

CAXRAPH

REGION

CAWRAPH

ANALYSIS UETHOD

UNCONST. UNIFORM

1400 680 200 340 200 150

6 60

SCREW REGION BETWEEN SCREWS CENTER LOOP CENTER REGION

A comparison between the unconstrained heat flux and the uniform heat flux heat transfer coefficients (Tables 4 , 5 and 8 ) at the high torque level is given for the cold temperature case in Table 9 and for the hot temperature case in Table 10. The contrasts are similar at both temperature levels. It is seen that the heat transfer coefficients for the unconstrained heat flux analysis are significantly greater in the Screw Region and appreciably smaller in the Center Region than those obtained assuming a uniform heat flux. The coefficient values in the Between Screws Region and the Center Loop Region did not vary significantly between the two analyses. This occurs because the actual heat flux values in these regions did not differ greatly from the uniform heat flux value, 3 . 8 Btu/hr-inz.

-30 160 -30 160

250 150 700 1400 150 150 250 200 100 100 200 200 0.1 1 0 . 5 6

Table 9. Comparison Between the Unconstrained Reat Flux and the Uniform Heat Flux Heat Transfer Coefficients (Btu/hr-ft'-'F) for t h e 20 in-lb, -3O'F Test Case

INTERFACE I ULERAPH

REGION

CENTER L W P 200 CENTER REGION 0.1 0.5 40

Although it was shown that the heat flux distribution across the plate was clearly nonuniform, average heat transfer coefficients values were determined by area- averaging the heat transfer coefficients of Table 8 for each region. As shown in Table

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Table 10. Comparison Between the Unconstrained Heat Flux and the Unifom Heat Flux Heat Transfer Coefficients (BtU/hr.-ft'-'F] for the 20 in-lb, 160'F Test Case

REGION

SCREW REGION BETWEEN SCREW! CENTER IDOP CENTER REGION

ZNTERfACE

11, the use of Calgraph resulted in values 2-1/2 times the value of a bare interface. In addition, the hot results gave coefficient values that were 1-1/2 times the cold case results.

Table 11. Area Averaqed Heat Transfer Coefficients (Btu/hr-ft -*F) Based on Unconstrained Heat Flux IIl1aIYEis for the 20 in- lb Data

TEMPERI\TLTRE INTERFACE

COLD I HOT

I 280

atemretation of Results

For comparison, area-averaged heat transfer coefficients were determined based on the uniform heat flux analysis. These values, obtained from the coefficients in Tables 4 and 5 , are shown in Table 12. The values depicted for a Calgraph interface were nearly twice the values for a bare interface. For the bare interface, there was a 20 to 30 percent increase in the heat transfer coefficient in the hot case over the cold case. For the Calgraph interface, the increase was nearly 4 0 percent. For both interfaces the coefficient values were nearly identical for the different screw torques at the same temperature level.

Table 12. Area-Avera ed Heat Transfer Coefficients 9 (Btu/hr-ft-'F] Based on Uniform Heat Flux Assumption

SCREW TORQUE (IN-LBJ

1 1 1 124 124

CAWRAPH

A comparison of the region heat fluxes and heat transfer coefficients between the unconstrained heat f l u x analysis and uniform heat flux analysis (Tables 7, 9 and 10) revealed large differences in the Screw Region and the Center Region. Allowing unconstrained heat fluxes across the plate showed that the heat transfer near the

screws was much better than the uniform heat flux analysis predicted. on the other hand, the heat transfer in the center of the two plates was much worse. This trend is expected, given the discussion regarding difficulties in establishing heat transfer ~- coefficients. Near the screws, where the interface contact is good and the interface filler material is greatly compressed, the heat transfer should be greater than other areas. The use of an area-averaged heat transfer coefficient hides these localized effects. Calgraph resulted in larger heat transfer coefficients in each region of the plate as compared to a bare interface. The greatest increase was found in the Screw Region, and a negligible increase was found in the Center Region. A comparison of the area- averaged heat transfer coefficients calculated from the unconstrained heat flux analysis and the uniform heat flux analysis can be made from Table 11 and the last two columns of Table 12. Accounting for localized effects in the unconstrained heat flux analysis resulted in coefficient values that were twice as high for the bare interface and over three times as great for the Calgraph interface.

Conclusion

Average heat transfer coefficients based on a uniform heat flux. such as those Dresented ~~

in Table 12, ~ are used extensively to characterize a thernal interface and to calculate the temperature drop across a

v mounting surface interface. These values are used for convenience, since rarely will the heat flux distribution across the interface be known. For a flange bolted interface, as studied here, this approach can at best approximate area-averaged temperature differences across the interface. It is grossly misleading in regard to the actual heat transfer process, for bare and for filled interfaces, and cannot predict the actual temperature distribution across the interface. As found here, the heat flux profile across such an interface is decidedly non-uniform. Regional heat transfer coefficients determined from the uniform flux assumption are incorrect. Area-averaging these coefficients further removes the results from reality.

Calgraph was found to reduce the temperature drop across an interface by increasing the heat transfer in the regions at the mounting screws. Heat transfer coefficients, as determined from actual interfacial heat fluxes, were three times as great in the Screw Region and from the uniform heat fluxes, twice as great as for a bare interface. As compared to an RTV material that exhibits &an improvement about five times as great, expectations that Calgraph would be a suitable replacement for large heat dissipating applications were not satisfied. From the results of this investigation, it is clear that Calgraph

adequately dissipates heat at and adjacent to the bolt flange region and should find success in amlications where the area is ~ ~~ - _ small and the interface pressure is predictable.

Acknowledsements

The author wishes to acknowledge Dr. Donald Gluck for initiating and guiding this study and Mr. Richard Arrieta and Mr. Fritz Mauritz for preparing the test equipment and performing the experiments.

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References

Snaith, B., O'Callaghan, P. w., and Probert, S. D., "Use of Interstitial Materials for Thermal Contact Conductance Control," A I A A conference Paper 84-0145, January 1984, p. 1.

Taylor, P. F., "TWT Collector Thermal I n t e r f a c e T e s t , " ~ u g h e s Interdepartmental Correspondence to Hagemeyer, W. A . , dated December 4 .

"Polycarbon Flexible Graphite Calgraph," Polycarbon Inc., Valencia, CA. private communications.

Bevans, J. T., Ishimoto, T., Loya, B. R., and Luedke, E. E., "Prediction of SpaceVehicleThermal Characteristics," Air Force Flight Dynamics Laboratory, Wright-Patterson Air Force Base, Ohio, August 1965.

1985.

SINDA 1987/Ah'SI. Gaski, J., Network Analysis Associates, Inc.

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