NATIONAL BUREAU OF STANDARDS REPORT

68^2

THERMAL CONDUCTIVITY OF A SPECIMEN

OF POROUS SINTERED 316 STAINLESS STEEL

by

Thomas W. Watson and Frank J. Powell

Report to the

U.S. Naval Ordnance LaboratoryWhite Oak

Silver Spring, Maryland

<NBp>

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
THE NATIONAL BUREAU OF STANDARDS

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3, 1901, as amended by (^ngre>is in Public Law 619, 1950. These include the development and

maintenance of the national standards of measurement and the provision of means and methods for 'making measurements consistent with these standards; the determination of physical constants and

properties of materials; the development of methods and instruments for testing materials, devices,

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of Documents, Government Printing Office, Washington 25, D.C.
NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

1006-30-10642 May 5, I960 6842

THERMAL CONDUCTIVITY OF A SPECIMEN

OF POROUS SINTERED 316 STAINLESS STEEL

by

Thomas W. Watson and Frank J. Powell

Heat Transfer Section

Building Technology Division

Report to the

U.S. Naval Ordnance Laboratory

White Oak

Silver Spring, Maryland

IMPORTANT NOTICE

NATIONAL BUREAU OF

intended for use within

to additional evaluation ; listing of this Repoft, eit the Office of the Directoi however, by the Governn

to reproduce additional (

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST)

on October 9, 2015.

or progress accounting documents

s formally published it is subjected

ing, reproduction, or open-literature mission is obtained in writing from

C. Such permission is not needed,

ally prepared if that agency wishes

<NBS>

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

Thermal Conductivity of a Specimen

of Porous Sintered 316 Stainless Steel

by

Thomas W. Watson and Frank J. Powell

1.

INTRODUCTION

A specimen of porous sintered 316 stainless steel was

submitted by the U.S. Naval Ordnance Laboratory at White

Oak, Maryland, for measurement of its thermal conductivity

in the temperature range -150° to 6bO°C.

2.

SAMPLE

The sample was a bar of 2.35 cm square cross-section

and 37*0 cm long. It was stated to be 5 micron sintered

spherical 316 stainless steel powder with binder. The

porous bar had a 2.35 cm cylindrical solid stainless steel

end (with a 1.35 cm drill-hole) fastened by threads for the

coolant well. The porous material was described as having

a density ratio 0.527 of that of stainless steel. No

chemical analysis of the metal powder composition was furnished, or made at the National Bureau of Standards.

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

(coolant) 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 end provides a liquid-tight connection for circulating

a coolant through the top drill-hole.
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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 coppertube 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 1.2? 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 cons tant- 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 0.10 cm in diameter. Junctions

in the specimen were inserted into radially-drilled holes

0.11 cm in diameter and 0.17 cm 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
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powder insulation, which also was used to insulate the

space surrounding the guard tube.

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 crosssectional 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 insignificant 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:

AMI + fS = Q Ax

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 its bottom end at the heater to the midpoint

of the given span, expressed as the product of S,

which is a close approximation of the integral

J(^(tbar - tguard)dx, and an average heat transfer
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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-70^ digital computer, suitably programmed to compute the thermal conductivity and corresponding

mean temperature for each of the six spans. The computer was

used to fit a third-power equation to the thermal conductivity

and temperature results, by method of least squares. Much

time is saved by machine computation, and accidental errors

in the considerable calculations are avoided.

4. RESULTS

The results of the thermal conductivity determinations

are shown in Figure 2 and the table below. The 36 individual

values of thermal conductivity plotted represent six sets of

tests, each with values for the six spans. The solid line

represents the trinomial fitting the data, as determined by

the computer and from which values are also tabulated below.

°c watt/cm °C °C watt/cm

I4o 0.010 300 0.019

100 .012 340 .019

-60 .013 380 .019

-20 .014 420 .020

20 .015 460 .020

60 .016 500 .021

100 .017 540 .022

l40 .017 580 .023

180 .018 620 .024

220 .018 64o .025

260 .018

5 . DISCUSSION OF RESULTS

The individual values of thermal conductivity plotted

in Figure 2 show considerable scattering from the smooth

curve in the temperature range 125° to A25°C. Little scattering is shown over the temperature ranges -1^5° to 100°C

and 450 ° to 645°C, The average departure from a smooth

curve in the temperature range 125° to 425°C is about ± 13

percent', for other temperature ranges, about ± 1.7 percent.

Due to the extremely low conductivity of this material

as compared to other metals, the inaccuracy of the results

over the temperature range given is estimated to be within

10 to 15 percent of the absolute value.

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APPARATUS FOR MEASURING THE THERMAL

CONDUCTIVITY OF METALS

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U<S. DEPARTMENT OF COMMERCE

Frederick H. Mueller, Secretary NATIONAL BUREAU OF STANDARDS

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

The Hcopu of activities of the National Bureau of Standards at its major laboratories in Washington, D.C., and

Boulder, Colorado, is suggested in the following listing of the divisions and sections engaged in technical Work.

In general, each section carries out specialized research, development, and engineering in the held indicated by

its title. A brief description of the activities, and of the resultant publications, appears on the inside of the front cover, WASHINGTON, D,C.

Electricity and Electronics. Resistance and Reactance. Electron Devices. Electrical Instruments. Mag*

netic Measurements. Dielectrics. Engineering Electronics, Electronic Instrumentation. Electrochemistry.

Optics and Metrology, Photometry and Colorimetry. Photographic Technology, Length. Engineering

Metrology, .

'

Heat, Temperature Physics. Thermodynamics. Cryogenic. Physics. Rheology. Molecular Kinetics. Free

Radicals Research. Atomic and Radiation Physics. Spectroscopy. Radiometry. Mass Spectrometry. Solid State Physics, Electron Physics. Atomic Physics. Neutron Physics. Radiation Theory. Radioactivity. X-rays. High

Energy Radiation. Nucleonic Instrumentation. Radiological Equipment.

Chemiatry. Organic Coatings. Surface Chemistry. Organic Ghemiatry, Analytical Chemistry. Inorganic

Chemistry. Electrodeposjtion. Molecular Structure and Properties of (iases. Physical Chemistry, Therm<»-

chemistry. Spectrochemistry. Pure Substances.

Mechanics, Sound. Mechanical Instruments, Fluid Mechanics. Engineering Mechanics. Mass and Scale. Capacity, Density, and Fluid Meters. Combustion Controls. Organic and Fibrous Materials. Rubber. Textiles. Paper. Leather. 'I'csting and Specifications. Poly- mer Strncture. Plastics, Dental Research.

Metallurgy, Thermal Metallurgy. Chemical Metallurgy, Mechanical Metallurgy. Corrosion. Metal

.Physics,

Mineral Products, Engineering Ceramics. Glass. Refractories. Enameled Metals, (jonsUluliort and Microstructure.

Building Technology. Structural Engineering, Fire Protection, Air Conditioning, Heating, and Refrigeration. Floor, Roof, and Wall Coverings. Codes and Safety Standards. Heat Transfer. Concreting Materials. Applied Mathematics, Numerical Analysis, Computation. Statistical Engineering. Mathematical Physics.

Data Processing Systems. SEAC Engineering Group, Components and Techniques. Digital Circuitry.

Digital Systems. Analog Systems, Application Engineering.

# Office of Basic Instrumentation. • Office of Weights and Measures. BOULDER, COLORADO

Cryogenic Engineering. Cryogenic Equipment. Cryogenic Processes. Properties of Materials. Gas Liquefaction. Radio Propagation Physics. Upper Atmosphere Research. Ionospheric Research. Regular Propagation

Services. Sun-Earth Relationships. VHF Research. Radio Warning Services. Airglow and Aurora. .Radio

Astronomy and Arctic Propagation.

Radio Propagation Engineering. Data Reduction Instrumentation. Modulation Research. Radio Noise. Tropospheric Measurements. Tropospheric Analysis. Propagation Obstacles Engineering. Radio-Meteorology. Lower Atmosphere Physics.

Radio Standards. High Frequency Electrical Standards. Radio Broadcast Service. High Frequency Impedance Standards- Electronic Calibration Center. Microwave Physics. Microwave Circuit Standards. Radio Commienicallion and Systems. jH»w Frequency and Very'l.,ow Frequency Rctwarcli. High Fre- quency and Very High Frequency R(!searrh. Ultra High Frequency and Super High Frequency Research. Mothilntlon Res«;arch. Antenna Research. Navigation Systems. Systems Analysis. Field Operations.