Heating test of a silty sand in a short cylinder with a vertical hydronic grouted heating pipe

Abstract

Ground heat exchangers (GHEs) consist of HDPE pipes embedded in the ground to exchange thermal energy with the surrounding soils. Vertical GHEs are backfilled with grout materials to seal the soil boreholes, which are critical to the heat transfer at the pipe and soil interface. In this study, a short vertical soil cylinder was developed to measure the heat transfer process at the soil and pipe interface. The soil test cylinder consists of a 30 cm tall acrylic cylinder with a vertical 2.5 cm diameter (ID) HDPE pipe at its center. A grout of 85% bentonite and 15% graphite was compacted at 36% moisture content around the HDPE with a 1.4 cm radial thickness. A silty sand was chosen as the testing soil and compacted into the cylinder at 9% moisture and a dry density of 1.26 g/cm3. Moisture, temperature, and heat flux sensors were installed along the radial direction to measure the heat transfer process. The HDPE pipe was connected to a temperature-controlled water bath for a heating source at the specified constant temperature. The heating test of the silty sand with and without grout was compared. The thermal contact resistance at the pipe-grout interface was determined. The results demonstrated the impact of thermal grout on the heat transfer of GHEs, and the findings will help improve its design.