NATIONAL BUREAU OF STANDARDS REPORT

5859

THERMAL CONDUCTIVITY OF A SPECIMEN

OF TYPE 522 ALLOY

by

Henry E. Robinson and Frank J. Powell

Report to the

Champion Spark Plug Company

Toledo 1, Ohio

<Sb§>

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
THE NATIONAL BUREAU OF STANDARDS

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NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

1000-30-4890 April 15, 1958 5859

THERMAL CONDUCTIVITY OF A SPECIMEN

OF TYPE 522 ALLOY

by

Henry E. Robinson and Frank J. Powell

Heat Transfer Section

Building Technology Division

Report to the

Champion Spark Plug Company

Toledo 1, Ohio

IMPORTANT NOTICE

NATIONAL BUREAU OF STA

Mended For use within the G

tc additional evaluation and re listing of this Report, either if the Oidce of the Director, Nat

however, by the Government j tc reproduce additional copiei

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST)

on October 9, 2015.

rogress accounnng

mally published it is subjected

reproduction, or open-literature

ion is obtained in writing from Such permission is not needed,

prepared if that agency wishes

<NBS>

II. $. DEPARTMENT OF COMMERCE

Thermal Conductivity of a Specimen

of Type 522 Alloy

by

Henry E. Robinson 1. and Frank J. Powell

INTRODUCTION

A specimen of Type 522 Alloy was submitted by the

Champion Spark Plug Company, Toledo 1, Ohio, for measurement of its thermal conductivity in the temperature range

from room temperature to 1300°F. The sample was shipped

by the Champion Spark Plug Company, Toledo 1, Ohio.

2.

SAMPLE

The sample was a bar of metal machined to form a specimen 2.53 cm in diameter and 36.98 cm long. It was

stated to be a sample of Type 522 Alloy. No analysis of

the metal composition was furnished, or made here.

3.

TEST APPARATUS AND METHOD

The thermal conductivity of the sample was determined

by means of a steady-state flow of heat longitudinally in a

bar specimen, with measurements of the temperatures existing

at the ends of six consecutive approximately 3*51 cm spans

along the central length of the bar. Each determination

required a pair of tests at moderately different temperature

conditions, and yielded values of thermal conductivity at

six different mean temperatures.

The test apparatus is shown schematically in Figure 1.

The specimen, a bar approximately 37 cm long and of

uniform external dimensions, is supported at the top (cool)

end concentrically within a stainless steel guard tube of

1-cm wall thickness, which in turn is held concentrically

within a cylindrical outer container. The specimen is

drilled at each end with a 1.35 cm hole 5*5 cm deep. An

electrical heater is inserted and secured in the hole at

the bottom (hot) end, and the supporting fixture at the top

provides a liquid-tight connection for circulating a coolant

through the top drill-hole.
.
53.0

COOLANT

A

,

o

N-‘ ro

REMOVABLE TRIVET

SUPPORT

CONTAINER

GUARD CYLINDER

SPECIMEN

INSULATION

BAR HEATER

GUARD HEATER

. DIMENSIONS IN CENTIMETERS

2. X INDICATES A

THERMOCOUPLE

POSITION

3. MAIN THERMOCOUPLES

ON BAR ARE PLACED

3.51 CM. APART

APPARATUS FOR MEASURING THE THERMAL

CONDUCTIVITY OF METALS

FIGURE I.

- 2 -

Temperatures along the specimen were indicated by seven

thermocouples located symmetrically about the longitudinal

center of the specimen, spaced approximately 3*51 cm apart,

with one additional thermocouple near the bottom end of the

specimen. Thermocouples were similarly located in almost

exactly corresponding longitudinal positions on the guard

tube

.

The guard tube was equipped near its lower end with an

external circumferential electric heater as shown. The guard

tube was cooled at the top by means of a copper-tube coil

soldered circumferentially at a position corresponding in

effect to that of the specimen coolant well. Coolant was

pumped through the guard coil and specimen well in series

connection, as shown.

The electric heater for the specimen consisted of a porcelain cylinder 1.27 cm in diameter and 5*2 cm long threaded

longitudinally with 26-gage nichrome heater wire. Its resistance was approximately 22 ohms. Current was brought to the

heater through relatively large heater leads, to which

separate potential leads were connected at the point where

they entered the porcelain core. The heater was energized

by an adjustable constant-voltage d-c source. Heater current and voltage drop measurements were made using standard

resistors and the high precision manual potentiometer used

for thermocouple observations. The guard was heated with

constant voltage a-c power.

The thermocouples were made from calibrated chromel and

alumel 26-gage wires, electrically welded to form a spherical

junction about 1.0 mm in diameter. Junctions in the specimen

were inserted into radially-drilled holes 1.1 mm in diameter

and 1.7 mm deep in the side of the bar, and tightly secured

by punch-pricking the metal around the hole. The thermocouple wires were individually insulated electrically with

fiberglass sleeving, and were wrapped around the bar (one in

each direction) and tied at the back to secure them in the

transverse plane of the junction. The wires were brought

out through the powder insulation near the guard tube. The

thermocouples in the guard tube were similarly attached to

its exterior surface. The longitudinal positions of the

thermocouple junctions were taken as those of the centers of

the drilled holes, measured to the nearest 0.01 cm with a

laboratory cathetometer

.

After installation of the specimen, the space between

it and the guard tube was filled with diatomaceous earth

powder insulation, which also was used to insulate the space

surrounding the guard tube.

- 3 -

In principle, if there were no heat exchange between the

specimen and its surroundings, the conductivity could be

determined from the measured power input to the specimen and

the average temperature gradient for each of the six spans

along the specimen, all of uniform known cross-sectional

area. In practice, a perfect balance of temperatures between

the bar and guard all along their lengths is not possible

because of differences in their temperature coefficients of

conductivity, and the effect of the outward heat losses of

the guard. In addition to heat exchanges between the bar and

guard from this cause, a not-insignif icant longitudinal flow

of heat occurs in the powder insulation surrounding the specimen, and the contribution of the specimen to this heat flow

must depend somewhat on the bar-to-guard temperature unbalance.

In order to evaluate the heat flow in the bar at the

center points of each of the six spans, a partly empirical

procedure was used. Two steady-state test-runs were made

with slightly different bar and guard temperatures and

power inputs. In the two tests, the heat flow and the

observed temperature drop from end to end of a given span

differed, as did also the approximate integral with respect

to length of the observed temperature differences between

bar and guard, summed from the hot end of the bar to the

span center point. It is thus possible to write for each

span two equations (one for each test-run) of the form:

AkAt

Ax + fS Q

where A is the cross-sectional area of the specimen.

k is the specimen conductivity at the mean temperature

of the span.

At is the temperature drop from end to end of the span.

Ax is the length of the span.

fS represents the total net heat loss from the bar

(from the far end at the heater to the span center

point) expressed as the product of S, which is an

approximation of the integral Jox (tbar " tguardMx,

and an average heat transfer coefficient f, which

lumps the thermal conductance of the path from bar

to guard and the contribution of the specimen to the

longitudinal heat flow in the insulation up to the

span center point.

-4-

Q is the measured power input to the specimen heater.

By simultaneous solution of the two equations, the value

of k for each span, corresponding to the mean span mean temperature, is obtained. All of the computation of results is

effected by use of an IBM-704 digital computer, suitably programmed to compute the value of k for each of the six spans

from input data consisting of the average observed temperature values at the several bar and guard positions, and the

power input to the bar, for each of the two steady-state

test-runs constituting a pair at nearly the same temperatures.

Much time is saved by machine computation, and accidental

errors in the considerable calculations are avoided.

4. RESULTS

The results of the thermal conductivity measurements

are shown in Figure 2. The points plotted on the figure

represent the individual values obtained in four runs on

the specimen, each run yielding six values. A curve is

drawn to represent the trend of the data. Values of conductivity indicated by the curve at particular values of

mean temperature are tabulated below

°C Watt/cm °C °C Watt/cm °C

150 0.300 450 0.371

200 .310 500 .386

250 .320 550 .4-02

300 .331 600 .4-18

350 .343 650 -4-34-

4-00 .357 700 .4-50

5. DISCUSSION OF RESULTS

The individual values of thermal conductivity plotted

in Figure 2 show moderate scattering from the smooth curve.

The extreme departure (at 600 and 700°C) is about 1.2 percent; most departures are less than 1 percent.

The scattering is believed due chiefly to small inaccuracies in the temperatures at positions on the specimen,

as indicated by its thermocouples. Such inaccuracies could

arise from departure of a thermocouple from the temperatureemf calibration of the thermocouple wires, or from imprecision in ascertaining the exact longitudinal position of

the thermocouple junction, or possibly from heat conduction

in the thermocouple wires near the junction. Although the

wires encircled the specimen as nearly as possible in the

Oo WO/M - AllAllOnaNOO IVIAIdBHl

MEAN

TEMPERATURE

-5-

transverse plane of the junction, the temperature gradients

in the specimen were from 17*5 to 33-0 deg C/cm, and a slight

displacement of the wires from that plane would cause a temperature gradient in them near the junction. However, an

error in the indicated temperature at a point intermediate

between two spans of the specimen would reduce the observed

temperature drop for one span by as much as it would increase

that for the other. Thus, the values of thermal conductivity

for the two spans would be approximately equally, but

oppositely, affected, and the position of a smooth curve

drawn amongst a number of such individual points would be

affected little.

USCOMM-NBS-DC

u. s. department of commerce

Sinclair Weeks. Secretary

NATIONAL HUH KAll OF STANDARDS

A. V. Astin, Director THE NATIONAL BUREAU OF STANDARDS

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