Thermal propagation around heat supply pipes - Determining thermal conductivity of soil specimens

摘要

The Institute for Soil Mechanics and Foundation Engineering at the Graz University of Technology is investigating thermal soil properties within a temperature range concerning e.g. the efficiency of remote heat supply pipes. Until now heat loss from the pipe is minimized by enlarging the polyurethane (PU) - insulation thickness around the pipe. An innovative method to minimize it could be the usage of thermally insulating bedding material. Conventional bedding sands cover all necessary soil mechanical properties, but have a high thermal conductivity of λ =1,5 - 1,7 W/(m K). A newly found bedding material ,,Thermosand" shows excellent thermal insulating properties of only λ=0,18 W/(m K) (dry) up to 0,88 W/(m K) (wet). To investigate the influence of varying moisture content on thermal conductivity a large scale model test was established, which provides a basis for developing numerical models regarding unsaturated soil mechanics with heat propagation, including the drying out of the soil during heat input. To interpret all measurements correctly, the thermal conductivity λ of the bedding materials is determined at different dry densities and water contents. The paper introduces a very sophisticated procedure for measuring the thermal conductivity λ with a new half space linear probe method. These tests were carried out in cooperation with the Chair of Geophysics, University of Leoben, Austria. With this system one can easily carry out repeated measurements under identical conditions and enlarge the accurateness and reliability of the results.