High-Temperature Guarded Hot Plate Apparatus Control of Edge Heat Loss
Daniel R. Flynn, William M. Healy, Robert R. Zarr
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 to that of the meter plate so as to promote one-directional heat flow through the test specimen(s) that are located on one or both sides of the hot plate. 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. However, for high-temperature apparatus, environmental chambers are normally not used and some form of edge guarding is used to control 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 the current 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.
, Healy, W.
and Zarr, R.
High-Temperature Guarded Hot Plate Apparatus Control of Edge Heat Loss, Proceedings | 28th |International thermal Expansion Symposium, CA, [online], https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=861062
(Accessed June 8, 2023)