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NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

1003-30-1015 August 31, 1953 276?

Therr&al Conductivities of No. 430 Stainless

Steel and. 2S Almiinum Specimens

by

H, E. Robinson

S. Katz

Heating and Air Conditioning Section

Building Technology Division

for

Atomic Energy Conimission

Richland , "Washington

<NBS>

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

The publlcatli

unless permis 25,D.C. Su(

cally prepare

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST)

on October 9, 2015.

ole or In part, Is prohibited

I of Standards, Washington

h a report has been speclfijlar report for Its own use.
s

; ...
Thermal Conductivities of Ho. 430 Stainless Steel

and 23 Aluminum Specimens

by

H. E. Robinson

S, Kata

I. IHTRODUCTIOH

Two specimens, designated as Stainless Steel (No. 430)

and Aluminum (2S), were submitted for calibration measurements of thermal conductivity by the Atomic Energy Comciission, Richland, Washington, 5.n connect5,on with a project

undertaken by the rJeneral Electric Company.

Both specimens had a diairiCter of 2.54 cm.

II. PREPARATION OF THE SPECIMENS

The general arrangement of the test apparatus is

shown in Figure 1.

The upper end of each specimen was drilled to provide a well for circulation of the coolant and the lower

end was drilled to accommodate the heater. The internal

heating element was made by passing nichrome wire through

multiple-hole porcelain tubing. Chroiael-alumel theiMiocouples were attached at Intervals of about 4 cm, along

the length of the bar. One thermocouple was attached to

the lower end of the specimen. The completed specimen vras supported on a thin nichrome pin located in the bottom of a

thick-mlled stainless steel guard cylinder. The guard

cylinder was also equipped xvith thermocouples and a heater

element, and at the top mth a copper coil through which

the coolant was circulated. The specimen-guard assembly

was suspended in a large sheet-metal containsi- and the

entire system insulated v/ith a fine granular insulation.
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III. TEST METHOD

Electrical energy was supplied to the heater elements

and adjusted so that a minimum temperature difference

between bar and guard existed at the therraocouples Just

above the heaters. Cooling water at constant temperature was pumped through the well in the specimen and

through the coil on the guard cylinder, \fnen steady temperature conditions had been attained, the emfs of the

thermocouples (reference Junctions at OC) and the current

through and voltage drop across the bar heater were measured

by means of standard resistors and a precision potentiometer.

Temperature conditions were considered satisfactorily steady

when no thermocouple on the bar changed temperature at a rate

greater than O .3 degree C per hour; several readings taken

over a period of three or more hours were averaged for each

test. Thermocouple readings v:ere subsequently converted to

degrees Centigrade, using data determined by a calibration

of the thermocouple wire.

To calculate the thermal conductivity, observed temperatures of the bar and guard were plotted versus position

along the bar as abscissae and smooth curves xvere drawn

through the points along the bar and along the guard. Corrections to the measured heat input to the bar to account

for heat interchange between the bar and guard x-^ere made on

the basis of the temperature differences between them determined from the curves and using the conductivity of the

granular insulation at the appropriate mean temperature,

the corrections were made for the heat interchange (a) between the lower end of the bar and the guar'^ cylinder, (b)

between the bar and guard at the heater region, and (c)

between bar and guard for each thermocouple span. The

average rate of heat flow between any t^/^ thermocouples on

the bar was thus computed and used, ton-ether with the

measured distance and teraperature difference between them,

and the cross-sectional area of the specimen, to calculate

the average thermal conductivity for that span. In the

case of the aluminum specimen, the calculated heat exchange

between bar and guard below the first span was less than

O.S percent (heat gained by bar) and for all six spans was

less than 0,3 percent of the electrical heat input to the

bar. For the stainless steel specimen, theso exchanges xforc ,, respectively, less than 5 percent and 4 percent of the heat

input. It is believed that corrections to account for these

exchanges were evaluated with an uncertainty of not more than
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20 percent, consequently, the uncertainty in the rates of

heat flow used in computing the thermal conductivities was

of the order of not more than 0.2 percent for the aluminum

and 2 percent for the stainless steel.

For each of the separate tests, the thermal conductivity was computed for each of the six 4-cm, thermocouple

spans on the bar, corresponding to the mean temperature

existing in each span.

IV. TEST RESULTS

The values of thermal conductivity obtained for each

test span v/ere plotted against their corresponding mean

temperatures for each of the two specimens.

The results of the measurements on the stainless steel

specimen are shown in Figure 2, and are represented by a

smooth curve faired through the plotted values.

In the case of the aluminum specimen (Fig. 3)} the

data appeared to be best represented by tt<<ro straight lines

of different slope, intersecting at a mean temperature of

about 220® C. The lines were determined by the method of

least squares, using all of the points falling within their

respective mean temperature ranges. Because of faulty

readings of the thermocouple between the third and fourth

spans in the tests of the aluniinum specimen, these two

spans were treated as one span ^ cm. in length, and only

five conductivity values were obtained in each test.

Table 1 lists thermal conductivity values for each

specimen for various mean temperatures, as taken from the

curves.

V, DISCUSSION OF RESULTS

The plotted points show some scatter from the curves

faired through them. Such scatter is an inverse measure of

the precision of the measurements, as affected by such

factors as small random inaccuracies in determining the

thermocouple locations on the bar, slight heat conduction

along the thermocouple wires near the hot Junctions, and

possible slight inhomogenieties in the thermocouple wires.

The latter two possible factors would be of chief importance
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in the case of the aluminum specimen, in which temperature

differences between adjacent thermocouples were relatively

small because of its high thermal conductivity. To minimize heat conduction effects, small thermocouple wires

(No. 26 A.W.G.) were used and were led away from the hot

junction for a few centimeters in the plane of its cross

section, in which the temperature should be fairly uniform.

However, since the temperature gradients along the bar

ranged from 4 to 24 degrees C per centimeter in the several

tests, some conduction effect on individual thermocouple

readings probably could not be avoided.

Since the factors involved in inaccuracies in measurements of thermocouple positions and temperatures were random

in nature, their effect was probably to decrease the precision of the values obtained for each span rather than to

affect the overall results in any one direction.

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Table 1.

Mean teranerature Thermal conductivity. watt/cm ( de^i: C)

°c

I\io. 430 Stainless

steel No. 23 Aluminum

75 0.249 2. IS

100 .251 2.19

150 .255 2.22

200 .259 2.24

250 .261 2.24

300 .264 2.21

350 .266 2.19

400 .267 2.17

450 . 26^ —

500 .269

550 .270

600 .271 —

650 ,272

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February 2 7, 1953
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