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Research Papers: Conduction

Analysis of the Transient Hot-Wire Method to Measure Thermal Conductivity of Silica Aerogel: Influence of Wire Length, and Radiation Properties

[+] Author and Article Information
Ellann Cohen

Building Technology Program,
Departments of Architecture and
Mechanical Engineering,
Massachusetts Institute of Technology,
Cambridge, MA 02139
e-mail: ellann@alum.mit.edu

Leon Glicksman

Building Technology Program,
Departments of Architecture and
Mechanical Engineering,
Massachusetts Institute of Technology,
Cambridge, MA 02139
e-mail: glicks@mit.edu

1Corresponding author.

Contributed by the Heat Transfer Division of ASME for publication in the JOURNAL OF HEAT TRANSFER. Manuscript received April 4, 2013; final manuscript received October 7, 2013; published online January 30, 2014. Assoc. Editor: Zhixiong Guo.

J. Heat Transfer 136(4), 041301 (Jan 30, 2014) (8 pages) Paper No: HT-13-1186; doi: 10.1115/1.4025921 History: Received April 04, 2013; Revised October 07, 2013

When the transient hot-wire method is used to measure the thermal conductivity of very low thermal conductivity silica aerogel (in the range of 10 mW/m·K at 1 atm) end effects due to the finite wire size and radiation corrections must be considered. An approximate method is presented to account for end effects with realistic boundary conditions. The method was applied to small experimental samples of the aerogel using different wire lengths. Initial conductivity results varied with wire length. This variation was eliminated by the use of the end effect correction. The test method was validated with the NIST (National Institute of Standards and Technology) Standard Reference Material 1459, fumed silica board to within 1 mW/m·K. The aerogel is semitransparent. Due to the small wire radius and short transient, radiation heat transfer may not be fully accounted for. In a full size aerogel panel radiation will augment the phonon conduction by a larger amount.

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Figures

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Fig. 1

Typical plot of hot-wire temperature rise versus the log of time

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Fig. 2

Schematic of hot-wire (Pt wire) embedded in the material

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Fig. 3

Aerogel sample with hot-wire embedded

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Fig. 4

Because the copper wire (254 μm diameter) is an order of magnitude larger than the platinum wire (25 μm) and because the hot-wire test is so short, we assume that the copper wire does not change temperature and is effectively a heat sink

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Fig. 5

Schematic of a fin used to model the hot-wire and correction for end effects

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Fig. 6

Predicted temperature distribution along the length of the hot-wire due to end effects for silica aerogel with a platinum wire at time is 0.5 s and 1.0 s

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Fig. 7

Experimental results for a test between 0.5 s and 1.0 s showing the end effect correction and the linear fit to the test results

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Fig. 8

NIST SRM 1459 experiment and corrected thermal conductivity

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