Heat transfer characteristics and heat conductivity prediction model of waste steel slag–clay backfill material

Abstract

The thermal performance of the backfill material used in ground source heat pump systems directly affects their heat transfer efficiency. To enhance the heat transfer efficiency, aiming at the low heat conductivity of existing backfill materials, this paper proposes a waste steel slag–calcareous clay mixture as the backfill material for buried pipes. First, the heat conductivities of 128 sets of waste steel slag–calcareous clay mixtures in different preparation states were tested by performing indoor thermal probe tests, and the influences of the steel slag mixture, moisture content, dry density, and particle gradation on the heat conductivity were investigated. Scanning electron microscopy and Charge coupled device imaging were employed to capture the microscopic morphology of the materials and to analyze their heat transfer mechanism. With the test results, a heat conductivity prediction model of the backfill material based on a back propagation neural network was developed. The incorporation of waste steel slag was found to significantly improve the heat conductivity of the mixed backfill materials, the heat conductivity of a mixture incorporated with 70% steel slag was the highest, with the value being 1.86 to 2.46 times higher than that of the clay material without steel slag. The established heat conductivity prediction model produced accurate results with an average relative error of 1.0007%. This study can provide reasonable thermal parameters for waste steel slag–clay mixtures as backfill materials in ground source pump systems, and the effective utilization of waste steel slag can improve its added value.