Study on heat transfer performance of variable-diameter energy pipe piles under intermittent operation

This study investigates the heat transfer performance of Precast High-strength Concrete (PHC) energy piles by combining field tests on small-diameter specimens (300mm) with numerical simulations of variable large-diameter piles (600-1000mm) under intermittent operation. Quantitative results demonstrate that the IR-4 mode ("4h operation/8h shutdown") is optimal for small-diameter piles, increasing unit heat exchange by 13.1% and 21.7% compared to IR-5 and IR-6 modes, respectively. For large-diameter piles, convective heat transfer intensity remains stable for 600-800mm diameters with Nu values around 7.5, while the Nu for 1000mm piles decreases by 30.0% as thermal resistance becomes the dominant factor. Furthermore, the IR-4 mode exhibits superior thermal stability and the lowest heat transfer attenuation rate of 39.4%, whereas IR-6 reaches 44.7%. Analysis of thermal short-circuiting reveals that an increase in the Bi number by 0.01 reduces short-circuiting intensity by 13.0%, while the THR growth rate stabilizes at 0.1 m/h. These findings facilitate precise matching between pile diameters and operation cycles to maximize geothermal efficiency.