Date of Award
Mechanical, Automotive, and Materials Engineering
Coaxial, Curvature ratio, Dean number, Total heat transfer, Total pressure loss coefficient, U-bend
David S.-K. Ting
Creative Commons License
This work is licensed under a Creative Commons Attribution-NonCommercial-No Derivative Works 4.0 International License.
The numerical simulation of flow and heat transfer in U-bend and coaxial borehole heat exchangers is carried out using the OpenFOAM CFD solver and post-processed with ParaView. The purpose of this study was to find the minimum pressure loss coefficient with a high total heat transfer. Detailed flow structures and heat transfer characteristics were investigated in three U-bends at Reynolds number of 600 (Dean numbers of 190, 300, 425) and 60000 (Dean number of 19000, 30000, 42500) representing low and high Reynolds numbers. For the flow at Re = 600, the increase of Dean number due to the increase of curvature ratio results in a consistent decrease of total pressure loss coefficient up to 9%. At Re = 60000, Dean number of 30000 has the minimum total pressure loss coefficient up to 6.9% of drop. The flow at a low Reynolds number has nearly 16 times better total heat transfer due to a higher residence time compared with a high Reynolds number at the exact same Dean number. However, this betterment of heat transfer happens at the cost of 5.8 times higher total pressure loss coefficient. In the coaxial model, the effects of Reynolds numbers of 2000, 10000 and 20000 show that the increment of Reynolds number reduces the total pressure loss of the system and increases the total heat transfer. At Re = 20000, a short (X/Dh = 0.6) and a long (X/Dh = 1.6) bucket space, a space between the inner pipe and the bottom of the borehole, require a large pumping power due to the increase of the total pressure loss coefficient. Also, no significant improvement in the total heat transfer is achieved as a result of changing the length of the bucket space. Hence, the length of X/Dh = 1 provides the most efficient pumping power. This length brings about a minimum total pressure loss up to 15%, meaning that installing the center pipe in a position that causes the least total pressure loss needs to be taken into account. At the Reynolds number of 20000, also, it was found that the center-in flow provides 5.7% better heat transfer performance with a significance of lower total pressure loss coefficient than the case of annulus-in flow.
Pakoohinejad, Hani, "Modeling the Flow and Heat Transfer in U-bend and Coaxial Borehole Heat Exchangers" (2020). Electronic Theses and Dissertations. 8385.