Numerical study on the heat transfer performance of coaxial shallow borehole heat exchanger

Abstract

l CFD simulations on the coaxial shallow borehole heat exchanger without grouting. l Analyzing the effect of design parameters on the heat transfer performance. l Investigating the effect of thermal short-circuiting. l Providing some guidelines for the coaxial shallow borehole exchanger design. In the present work, a numerical investigation on the coaxial shallow borehole heat exchanger based on Computational Fluid Dynamics (CFD) technique in Hefei city of China has been performed. The effects of design parameters, including inlet flow rate, inlet fluid temperature, inner pipe material and outer pipe diameter, on the heat transfer performance were systematically studied. Besides, the thermal behavior along the pipe has been carefully examined with focus on the thermal short-circuiting phenomena. When the fluid inlet velocity is less than the critical value, the turbulence intensity increases and the Nusselt number increases with the inlet flow rate increasing. However, there is sufficient time for heat transfer between the fluid in inner pipe and outer pipe because of low flow rates, leading to large heat loss, i.e., thermal short-circuiting phenomenon. It is found that with the inlet flow rate increasing, the heat transfer increases first and then decreases, and the rate of reduction slows down gradually. When the inlet flow rate increases, the pumping power undergoes exponential growth. As the inlet temperature increasing, the heat transfer decreases almost linearly. Moreover, when the soil temperature at the top of the casing is lower than that of the fluid in the casing, heat is transferred from the fluid in the casing to the soil, and the heat loss increases with the increase of the inlet fluid temperature. The material of inner pipe with high heat conductivity would result in large heat loss under the influence of thermal short-circuiting. The heat load increases while the pumping power required decreases with the increasing of outer pipe diameter. This study is very beneficial for the coaxial shallow borehole exchanger designs and energy conservation of buildings.