Using Finite Element Analysis to Design a New Guarded Hot Plate Apparatus for Measuring the Thermal Conductivity of Insulating Materials

摘要

The National Institute of Standards and Technology (NIST) is developing a guarded hot plate apparatus to measure the thermal conductivity of thermal insulation over a temperature range from 90 K to 900 K. The apparatus will create nominal one-dimensional heat flow through a specimen by placing it between two isothermal plates at different temperatures. The thermal conductivity is then calculated from Fouriers Law using the temperatures of the plates, the heat input to the hotter plate, and the thickness of the specimen. In order for this apparatus to provide accurate results, however, each plate must be maintained at a nearly uniform temperature, and the edge of the specimen must be guarded to prevent radial heat flows. Commercially available finite element analysis software helped detect flaws in the design. Temperature profiles throughout the instrument, unwanted heat gains or losses, and the ability of fluid channels to provide adequate cooling were determined using finite element analysis. The resulting temperature variations are estimated to be approximately 38 mK on the meter plate and less than 5 mK on the cold plate for the worst case scenarios of plate operation. Errors in thermal conductivity from extraneous heat flows out the edge of the plate are estimated to be approximately 0.5 % for thick specimens at approximately 900 K but will be significantly less for thinner specimens and at lower temperatures.