3D CFD investigation on thermal performance of a U-tube borehole heat exchanger

Journal of Mechanical Research and Application

دوره 11، شماره 3، بهمن 2021، صفحه 99-76   

نویسندگان

Ali Abjadi^{* 1}؛ Mohammadreza Asadbeigi²؛ Shayan Farjyar³؛ Farzad Ghafoorian⁴

¹Department of Mechanical Engineering, South Tehran Branch, Islamic Azad University, Tehran, Iran

²School of Mechanical Engineering, Iran University of Science and Technology, Tehran, Iran.

³Department of Civil Engineering, Architecture and Art, Islamic Azad University, Science and Research branch,Tehran,Iran

⁴Turbomachinery Research Laboratory, Department of Energy Conversion, School of Mechanical Engineering, Iran University of Science and Technology, Tehran, Iran.

چکیده

In the present study, a special model of geothermal heat exchangers called coaxial borehole heat exchanger (CBHE) was numerically analyzed. For this numerical solution, computational fluid dynamics (CFD) method was used. As the name of this system indicates, this system consists of two coaxial pipes. The parameters are studied in the research are the inlet velocity of the operating fluid into the inlet pipe, the groundwater seepage velocity, the soil porosity of the area and the use of nanofluids instead of pure water. Studies on each of the above conditions have shown that they can have a significant effect on increasing the temperature of the operating fluid flowing inside the outer pipe. The results show that when the operating fluid passes through the outer pipe at high speed, it does not have enough time to heat up and its temperature rises less. Also, the presence of groundwater seepage which is a natural factor, will reduce the temperature of the working fluid. The decrease in temperature is related to the velocity of groundwater flow. Clearly considering the porous medium and the amount of soil porosity and the empty space between particles affect on the thermal performance of CBHE. Obviously, high porosity increases thermal resistance and decreases thermal conductivity. The utilization of nanofluids as operating fluids instead of pure water was studied. With increasing thermal conductivity in nanofluids, the rate of temperature increase along the outer pipe increases. Therefore, the use of suitable nanofluids with high thermal conductivity is recommended as the operating fluid.