Enhancing heat transfer in ground collectors using internal fins
We evaluate, using fully resolved Direct Numerical Simulations (DNS), the thermohydraulic performance of geothermal collector pipes with internal surface modifications (fins). The purpose of the fins is to enhance the convective heat transfer of the brine without significantly increasing the pressure drop. Due to fluctuating demands, geothermal heat pumps operate at varying heat loads, often regulated using the brine circulation pump. This regulation gives brine flow rates and typical Reynolds numbers in the range 1300 < 𝑅e < 3300, where the transition to turbulence for smooth pipes is at approximately 𝑅e = 2300. Consequently, the brine is operated in both laminar and unsteady flow conditions. During laminar flow, the brine convective heat transfer resistance is a major contribution (˜50%) of the borehole thermal resistance. At turbulent conditions, the convective resistance is, however, very small compared to the borehole resistance. A suitable fin design in the collector pipes should thus trigger an unsteady flow at 𝑅e < 2300, without significantly increasing the pressure drop at 𝑅e > 2300 (that decreases system efficiency). Here, we use DNS to find a fin design that achieves both objectives. We start by evaluating three fin design strategies; straight axial fins; helical fins continuously rotating around the pipe axis; and fins rotating in alternating directions around the pipe axis. We also assess the influence of the fin size, the number of fins and their twist rate on the heat transfer rate and pressure drop. We find a promising design of alternating helical fins that triggers an unsteady flow down to about 𝑅e = 1700. This design enhances the heat transfer rate by 300-600% in the range of 1700 < 𝑅e < 2300 compared to a smooth pipe with only minor increase in the pressure drop (max about 2%) outside of this range.