Heat transfer performance of a vertical buried internally helically groove tube in soil/polyacrylamide under hot climate

Heat transfer performance of ground heat exchanger with a vertical U-shaped internally helically groove tube for hot climatic condition was addressed in terms of the influences of inlet water temperatures (30-45 °C), Reynolds numbers (4000-10,000) and backfill materials (raw soil; soil/polyacrylamide blend). The temperature drops of the water in the tube buried in the blend increased with respect to the raw soil backfill materials. Thermal diffusivities and water retaining ability of the soil were enhanced with increasing the blending ratio of polyacrylamide from 1.5% to 3.0%. The surface heat transfer coefficients of the water in the tube buried in the blend were much higher than those in the raw soil. The dimensionless surface heat transfer correlations of the water in the internally helically groove tube heat exchanger buried in the two samples were obtained. The correlation can well predict heat transfer characteristics with high coefficient of determination (R2). Meanwhile, thermal and moisture diffusions in the two samples were evaluated. Thermal diffusion of soil was enhanced by adding the polyacrylamide, and water retaining ability of the additive was also verified. Adding the polyacrylamide into soil can be an effective manner for enhancing the heat transfer of the ground heat exchanger.