THERMAL SCIENCE

International Scientific Journal

Songying Zhao

,

Yasheng Wang

,

Yusong Xiong

,

Jialong Qian

,

Hong Chang

,

Junyan Ma

STUDY ON HEAT TRANSFER PERFORMANCE OF VARIABLE-DIAMETER ENERGY PIPE PILES UNDER INTERMITTENT OPERATION

ABSTRACT
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.
KEYWORDS
PHC energy pile, intermittent operation, large diameter, thermal influence zone, thermal short-circuiting effect
PAPER SUBMITTED: 2026-02-13
PAPER REVISED: 2026-04-11
PAPER ACCEPTED: 2026-04-12
PUBLISHED ONLINE: 2026-05-17
DOI REFERENCE: https://doi.org/10.2298/TSCI260213063Z
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Study on heat transfer performance of variable-diameter energy pipe piles under intermittent operation

© 2026 Society of Thermal Engineers of Serbia. Published by the Vinča Institute of Nuclear Sciences, National Institute of the Republic of Serbia, Belgrade, Serbia. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International licence