High-Temperature Guarded Hot Plate Apparatus: Control of Edge Heat Loss.
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.
Building and Fire Research Laboratory
National Institute of Standards and Technology
Gaithersburg, MD 20899