Guarded hot plate apparatus for extreme conditions: temperatures up to 1000 °C and pressures from 10-8 to 10+2 bar

U. Gross1 and K. Raed1

1TU Bergakademie Freiberg, Institute of Thermal Engineering, Germany

Keywords: Knudsen number variations
property: effective thermal conductivity
material: porous media, insulating materials, variable atmosphere

Caused by increasingly rising energy costs, the development of thermal-protection systems, application of insulation materials and related thermal conductivity measurements are of increasing importance. The guarded hot plate method (GHP) is considered as a reference for thermal conductivity measurements of insulation materials. There are needs to develop GHP facilities for extended temperature and pressure ranges. In our lab, one GHP and two panel test facilities are already available for thermal conductivity measurements at temperatures from -10 C to 1650 C. However, all three of them are restricted to air atmosphere at ambient pressure. By design of a new apparatus the kind of atmosphere will be extended to arbitrary gases having pressures from 10-8 to 10+2 bar.

A GHP apparatus following ASTM C177 and ISO 8302 international test standards was designed and constructed at TU Bergakademie Freiberg. This facility allows measurements of the effective thermal conductivity of porous insulation materials ranging from 0.001 to 1 Wm-1K-1 at temperatures from 30 to 1000 C and pressures from 10-8 to 10+2 bar.

The GHP method is a stationary one, and it is characterized by a strictly one-dimensional heat flow through the square shaped specimen. To achieve this, the apparatus consists of an assembly of heating plates which are placed inside a pressure vessel (autoclave) being able to withstand the requested temperature and pressure conditions. There are various guard heating systems which are controlled to maintain the heat losses at an acceptable minimum by keeping zero temperature differences to the measuring plate. The autoclave is double walled containing a cooling water system to maintain the temperature of the outer shell close to ambient temperature for safety reasons.

The design facilities for the above mentioned temperature and pressure parameters in combination with the demands coming from the GHP methods is characterized by many technical challenges: selection of construction materials, design of the heating plates, integration of the various heaters, fixing the plates assembly to keep them in situ at different pressure and temperature conditions, optimization of the temperature distribution in the assembly, measuring the temperatures and many more. These problems and their optimized solutions will be described in detail in the paper.

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