Numerical investigation on air-side heat transfer enhancement of wavy finned tube air cooler for underground tunnels

Air coolers used in underground tunnels are traditionally bare tube units with limited thermal performance, negatively impacting refrigeration systems' energy efficiency. To enhance heat transfer efficiency, this study explores the application of finned-tube air coolers in underground tunnels or shafts with low dust concentrations during construction. The potential of wavy finned-tube air coolers to improve thermal performance under high wind speed conditions is investigated using numerical simulations. The RNG k-epsilon model was used to numerically investigate the effects of fin and finned tube design parameters, delta-wing vortex generator (VG-D) and slit configurations on the thermal-hydraulic performance of wavy finned tube air cooler. Key performance indicators, including the Nusselt number (Nu), friction factor (f), fin efficiency and composite performance factor (JF) are utilized to assess the enhancements. The results demonstrate that augmenting the height and thickness of wavy fins and the longitudinal tube pitch leads to improvements in both Nu and f. Reducing the transverse tube pitch enhances the Nu, fin efficiency and JF. Moreover, the integration of VG-D boosts Nu by 10.1% and JF by 7.3% compared to the base design. The addition of top-bottom slit perforations further augments the Nu and fin efficiency by 10.3%, 4.9%, and reduces the JF by 2.9%. Empirical correlations for the air-side performance of wavy finned-tube air coolers are also proposed, with deviation error less than 15%. These results can offer valuable insights for the design of high-efficiency wavy finned air coolers specifically suited for underground tunnel applications.