Thermal performance of a ground U-shaped tube with twisted tapes in sand/graphite backfill materials

Thermal characteristics of water in a ground U-tube with twisted tape inserts were addressed. At inlet temperatures of 60°C to 75°C, and Reynolds numbers of 3000 to 9000, the surface heat transfer coefficients for water with twisted tapes ranged from 442 to 897 W m⁻² K⁻¹, which were higher than that in smooth tube, 384–810 W m⁻² K⁻¹. Average convective heat transfer coefficient for the smooth tube in sand/graphite blend ameliorated by 36% compared to that in sand. Average apparent thermal diffusivities of sand/graphite at hot end temperature of 70°C rose by about 17% compared to sand.