Thermo-hydraulic performance analysis of a horizontal slinky-type geothermal heat exchanger under turbulent flow vonditions

Horizontal Slinky-type geothermal heat exchangers (GHEs) are widely used in ground-source heat pump systems due to their compact layout and low installation cost, yet their performance is often constrained by shallow burial depth and weak internal flow mixing. This study presents a three-dimensional CFD analysis of a horizontal Slinky GHE enhanced with Twisted-tape inserts. The effects of Twisted-tape pitch (100, 250, and 350 mm), burial depth (0.5-1.5 m), and water mass flow rate (0.30-1.0 kg/s) are investigated under turbulent flow conditions using ANSYS Fluent with the realizable k-ε model. The novelty of this work lies in the combined optimization of Twisted-tape geometry and burial depth, evaluated through both first-law (Nusselt number, pressure drop) and second-law (entropy generation, exergy destruction) performance metrics. Twisted-tape inserts significantly enhance convective heat transfer: compared with the plain tube, the average Nusselt number increases by up to 28-35% for the 100 mm pitch and 22-30% for the 250 mm pitch. Although the 100 mm pitch provides the highest thermal enhancement, it also results in a 40-52% pressure-drop increase. The 250 mm pitch yields the maximum thermo-hydraulic performance criterion (PEC), improving PEC by 12-18%, indicating the optimal balance between heat-transfer augmentation and hydraulic penalty. Increasing burial depth improves thermal stability and heat extraction, enhancing the heat-transfer coefficient by approximately 15-21%. The optimal configuration is identified as a 250 mm pitch at a burial depth of 1.0-1.5 m. These findings provide practical design guidance for improving the efficiency of horizontal Slinky geothermal heat exchangers. The findings offer practical design guidance for horizontal Slinky geothermal systems, indicating that careful optimization of coil pitch, burial depth, and operating mass flow rate can substantially improve heat extraction performance while keeping hydraulic losses within acceptable limits under realistic ground and ambient conditions.