Thermal conductivity of evacuated expanded perlite measured with guarded-hot-plate and transient-hot-wire method at temperatures between 295 K and 1073 K
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.
The Transient Hot Wire method is well established as the most accurate, reliable and robust technique for evaluating the thermal conductivity of fluids and solids [3, 12, 14, 15, 18]. Unfortunately its direct application to dual-phase systems such as solid suspensions in fluids or porous media cannot be supported by the very principles and methodology underlying this method. The derivation of possible ways of rendering the transient hot wire method to dual-phase applications including the development of validity criteria for such applications is proposed and discussed.
A Study on Thermal Conductivity of Mixture of Magnesia Particle and Molten Nitrate by a Transient Hot Wire Method