Study on heat transfer characteristics of the deep-buried ground heat exchanger under different multi-pipe layouts

The pipe group layout and buried pipe spacing in large-scale applications are of great significance for guiding the design of pipe group systems and improving the utilization potential of regional geothermal resources. This work takes a deep-buried coaxial double-pipe heating project at a depth of 2,539 m in Xi'an as an example. It establishes three-dimensional full-scale numerical models that couple the internal and external heat transfer with a multi-pipe system based on the measured ground temperature distribution, ground thermophysical parameters, and an in-situ experiment. The multi-pipe models are divided into two categories based on the pipe layout's minimum basic unit: the straight-line and the equilateral triangle models. Both model categories contain three independent deep-buried coaxial double-pipes, and each model involves four spacings of buried pipes (5 m, 10 m, 15 m, and 20 m). Based on the established models, the heat transfer characteristics of buried pipes under different pipe layouts and buried pipe spacing are studied, and the influences of ground thermal conductivity on the heat transfer in multiple pipes are discussed. Consequently, this study provides a scientific reference for the design of pipe groups for large-scale utilization of geothermal energy in medium-deep layers.