An investigation on shell-side thermal-hydraulic performance of helically coiled tube heat exchanger for deep underground space

Helically coiled tube heat exchanger (HCHE) was extensively used to enhance the thermal environment of deep underground space. The inefficiency of heat transfer in the mine cooling system results in increased operational energy consumption. To enhance heat utilization, this study examines the impact of air temperature, air velocity, coil diameter, pitch, and elliptical tube aspect ratio on the shell-side thermal performance of HCHE through experimental and numerical analysis. The performance evaluation criterion (PEC) and field synergy are introduced to provide a comprehensive assessment of performance. Results suggest that the air velocity can more significantly affect the thermal performance of HCHE than air temperature. Augmenting the coil diameter from 30 - 40 mm and coil pitch from 7.5 - 9.0 mm led to an increase in the Nu number by 16.5 % and 6.7 %, as well as an increase in friction factor f by 575.2 % and 147.1 %, while a reduction in PEC by 38.4 % and 21.1 % when the air velocity is 5 m/s, respectively. For the helically coiled elliptical-tube heat exchanger (HCHE-E), the thermal performance is significantly dependent on the long axis's direction and its aspect ratio. When the air velocity is 3 m/s, the increase in aspect ratio from 1.0 to 1.8 resulted in a decrease and increase in Nu by 14.7 % and 14.6 %, an increase and decrease in PEC by 24.5 % and 28.4 %, as well as a reduction and increase in f by 31.5 % and 48.1 % for the long axle in the horizontal and vertical directions, respectively. The PEC for HCHE is not significantly dependent on the air velocity. The increase in air velocity can result in the augment in PEC of HCHE-E regardless of long axis's direction. The variation of synergy angle is consistent with the change of Nu. These findings offer valuable insights for optimizing the design parameters of HCHE in deep underground spaces.