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The National Institute of Standards and Technology is building a guarded hot plate apparatus of advanced design to provide very accurate thermal transmission properties for specimens of thermal insulation 500 mm in diameter, with thicknesses up to 110 mm, at mean temperatures from 90 K to 900 K. This paper documents some of the extensive thermal modeling and analyses that were carried out in the course of designing this apparatus and characterizing potential errors and uncertainties. In an idealized guarded hot plate apparatus, the effective thermal conductivity is simply computed from the measured power input to the meter plate heater, the effective area of the metering plate, the temperature of the hot plate and the two cold plates, and the specimen thickness. In an actual apparatus, there may be (a) temperature variations in the hot plate and cold plates, (b) radial heat flow within the specimen, and (c) transient temperature fluctuations that add uncertainty to the measured thermal conductivity. For the new 500 mm apparatus, both analytical solutions and finite element analyses were used to model temperature distributions in critical thermal components, heat flows that might affect the measured thermal conductivity values, and the effects of departures from ideal steady-state conditions on test results. This paper focuses on the results of these analyses and computations.
finite element analysis, guarded hot plate, heat conduction, heat transfer, insulation, R-value, thermal analysis, thermal conductivity, thermal insulation, thermal resistance
Mechanical Engineer, Building and Fire Research Laboratory, National Institute of Standards and Technology, Gaithersburg, MD
President and Chief Scientist, MetSys Corporation, Millwood, VA