Study on reasonable selection of insulation depth of the outlet section of vertical deep-buried U-bend tube heat exchanger

Around the scientific prediction of insulation depth of the outlet section of vertical deep-buried U-bend tube heat exchanger (THE) in geothermal heating, in this study, a full-scale numerical calculation model combining the heat transfer processes inside and outside the tube was established. This model was further validated through in-situ experiment for a vertical deep-buried U-bend THE system with a depth of 2505 m in Xi'an. Four constant heat conditions with different magnitudes were selected and the highest average outlet temperature of the circulating water within the tube in the entire heating season corresponded to the optimum insulation depth. Furthermore, the effect of insulation depth at the outlet section of the buried tube on the heat transfer performance was analyzed by numerical calculation. The results showed that the critical exothermic depth of the outlet tube without insulation in a heating season was exactly the optimum insulation depth. Moreover, the linear fitting relationship between the initial temperature of the soil at the optimum insulation depth and the average water temperature at the outlet in the entire heating season of the insulated vertical deep-buried U-bend tube was obtained by numerical method under the determinant conditions of rock structure, temperature distribution, and geometrical parameters of the buried tube system, which can provide the optimum insulation depth of deep-buried THE projects under the geological conditions in the construction plan phase.