How internal fins enhance the thermohydraulic performance of geothermal pipes: A direct numerical simulation study

Journal article, 2026

Geothermal heat exchanger systems provide stable, efficient, and sustainable heating and cooling solutions. Their performance can be enhanced by achieving more effective convective heat transfer in the collector pipes without causing excessive pressure drop. This study investigates the use of internal fins in pipes as a means to improve the thermohydraulic performance under operating conditions typical of geothermal applications (Reynolds numbers 1300≤Re≤3300 and Prandtl numbers 20≤Pr≤75). Direct numerical simulations are employed to examine three fin configurations: straight axial fins, continuous helical fins, and fins with alternating helical orientations along the pipe axis. The alternating helical fin design is shown to trigger an unsteady flow at significantly lower Re than the other designs. Using the alternating design, we analyse the influence of the fin height, number of fins and fin twist rate on the thermohydraulic performance. The results demonstrate that the proposed alternating helical configuration triggers unsteady flow down to 1700<Re<1800 while maintaining nearly the same friction factor as a smooth pipe at both Re≤1700 and Re≥2300. The augmented flow gives 3–6 times higher heat transfer compared to a smooth pipe in the interval 1700<Re<2300. The findings highlight a promising fin design strategy for geothermal systems where minimizing pressure losses is essential, and they facilitate general guidelines for developing efficient fin-enhanced heat exchanger pipes in related applications operating under similar conditions.