Thermal and flow characteristics in a vertical spiral-type ground heat exchanger based on linear non-equilibrium thermodynamic principle

The thermal and flow performances in a single spiral-type ground heat exchanger in one-week operation were numerically simulated by considering the coupled thermal and moisture migration model for backfill and soil fields. The main factors involved Reynolds number (3000–10000), inlet temperature (30–40 °C), initial volumetric moisture content (6.95–20.8%), backfill materials (native sand, and sand/kaolin blend with 5% adding ratio of kaolin), spiral diameters (0.4–0.8 m), spiral pitches (0.1–0.8 m) and operation modes. The thermal performance of the heat exchanger without considering the coupled thermal and moisture migration model in unsaturated soil could be overestimated in long-term operation. The average temperatures in the blend enhanced by about 2% compared with the sand field, but the change degrees for the moisture content in the blend field were about 33% lower than that in the sand field, which imply that the kaolin additive could simultaneously enhance the thermal migration and water-holding capacity of the sand. The heat transfer rate of the heat exchanger in the blend enhanced by 24–36% with widening the spiral diameter from 0.4 to 0.8 m or narrowing the spiral pitch from 0.8 to 0.1 m, while the pressure drop rose by 50–92% and 4–5 times, respectively. 3-h-on/3-h-off mode among three intermittent operation modes (12-h-on/12-h-off, 6-h-on/6-h-off and 3-h-on/3-h-off) for the blend was more effective one to urge the soil temperature restoration. The intermittent operation mode should be also a feasible technique for prompting the thermal performance of the ground heat exchanger.