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High-Temperature Guarded Hot Plate Apparatus: Control of Edge Heat Loss.


pdf icon High-Temperature Guarded Hot Plate Apparatus: Control of Edge Heat Loss. (1895 K)
Flynn, D. R.; Healy, W. M.; Zarr, R. R.

International Thermal Conductivity 28th Conference/International Thermal Expansion 16th Symposium. Proceedings. June 26-29, 2005, New Brunswick, Canada, DEStech Publications, Inc., Dinwiddie, R.; White, M. A.; McElroy, D. L., Editor(s)(s), 208-223 pp, 2005.

Keywords:

guarded-hot-plate apparatus; heat loss; high tempeature; transmission; insulation; equations; finite element analysis; heat flow

Abstract:

The guarded hot plate (GHP) apparatus is the most common type of absolute apparatus for measurement of the thermal transmission properties of thermal insulation. As the name implies, the hot meter plate is surrounded by a coplanar guard plate, separated by a narrow guard gap, that is held at a temperature close (e.g., 0.01 K) to that of the meter plate so as to promote one-dimensional heat flow through the test specimen(s). If the apparatus is located in an environmental chamber, that chamber can be controlled at approximately the mean temperature of the test specimens so that heat gains or losses at the edges of the specimen and the outer edge of the guard plate can be kept acceptably small, affecting the measured properties by less than 0.2 percent. However, for high-temperature apparatus, environmental chambers are normally not used and some form of edge guarding is used with the intention of controlling excessive extraneous heat flows. Most commonly, for a high-temperature circular GHP apparatus, the edge guard is a heated cylinder located coaxially with the hot and cold plates, with edge insulation filling the annulus between the outer edges of the plates and the inner diameter of the edge guard. The major objective of this paper is to examine the effectiveness of this type of edge guarding. First, an analytical solution based on an effective heat transfer coefficient at the edge of the specimen(s) is summarized. Second, analysis is made and computations are carried out to illustrate that, for the type of high-temperature edge guarding that is most commonly used, there can be very significant heat flows in the edge insulation that are not predicted by previous analytical models but that can lead to serious errors in the measured thermal transmission properties. Third, computations based on finite element analyses are presented to show the effectiveness of edge guarding for geometries that are more complex than can readily be handled with analytical solutions. For existing apparatus, the presence of significant shunting heat flows can be confirmed by running tests on specimens of the largest thickness of interest at the same mean temperature, but with very different temperature drops across the specimens.