Effect of gradation on the thermal conductivities of backfill materials of ground source heat pump based on loess and iron tailings

Abstract

The addition of iron tailings to backfill soil of ground source heat pump (GSHP) systems can efficiently improve backfill thermal conductivity. The engineering properties of iron tailings and soil are significantly affected by particle gradation. Therefore, the effect of gradation on the thermal conductivity of backfill material composed of iron tailing and loess was investigated. The thermal conductivities of natural (bad) gradation and manmade (good) gradation materials with different iron tailings content, water content, and dry density were tested and analyzed. Results showed that the thermal conductivity of manmade gradation materials was usually higher than that of natural gradation materials. For the same water content and dry density, the influence of gradation on thermal conductivity decreased with increasing water content. For saturated material, the maximum difference in thermal conductivity caused by gradation ranged from 39.1% to 8.6% compared to dry material. The maximum increases in thermal conductivity caused by gradation were approximately 0.12–0.24 W/(m·K). The maximum increase percentage of thermal conductivity increases with decreasing water content, and the value range is 28.4–60.5%. The empirical formula for thermal conductivity was established based on the water saturation. The influence of gradation on formula parameters is analyzed. Finally, suggestions for gradation selection in practical applications of GSHP backfill materials are given.