Determination of the diameter ratios for minimum entropy generation in a borehole geothermal heat exchangers using numerical simulation of the flow and heat transfer

In this paper, diameter ratio of a geothermal borehole heat exchanger is determined in a manner that the entropy generation is minimized. The SST k-ω model is used to model the turbulent flow. Pressure loss calculation for different diameter ratios of the heat exchanger shows that for diameter ratio of 0.7, the total pressure loss is minimum. Results of the bulk flow temperature showes that the outlet temperature increases at higher heat resistance of the internal wall. The total entropy generation for different diameter ratios at different heat resistance of internal wall are also presented. Results show that when the heat resistance of internal wall increases, the minimum entropy generation occurs at higher diameter ratios. On the other hand, reduction in heat transfer coefficient of the outer wall and increment in heat transfer coefficient of internal wall are not favorable. Increment in heat resistance of the internal wall at higher diameter ratios of the heat exchanger, reduces both of these unfavorable effects.