The coupled effect of coil turns and pitch on the hydrothermal characteristics of a compact spiral heat exchanger

This study presents a numerical investigation of the effect of geometrical parameters on the thermal and hydraulic performance of a compact spiral coil heat exchanger (SCHE) using the commercial CFD code. Compact SCHEs are gaining importance due to their high surface-area-to-volume ratio and their ability to enhance fluid mixing through secondary flows, making them highly suitable for power generation, refrigeration, chemical processing, and renewable energy systems. Two key parameters, the number of coil turns and the coil pitch, were systematically analyzed under various Reynolds numbers. The evaluation was performed regarding heat transfer coefficient, Nusselt number, pressure drop, and the overall thermal performance factor, reflecting the trade-off between thermal enhancement and hydraulic losses. The results showed that the three-turn configuration consistently outperformed the four-turn configuration, especially at lower Reynolds numbers, due to reduced flow resistance and lower pumping requirements. Similarly, increasing the coil pitch from 40 mm to 55 mm improved overall thermal performance across all flow conditions by promoting better fluid circulation and reducing pressure drop, while maintaining strong heat transfer. The findings confirm that a three-turn coil with a pitch of 55 mm represents the most effective design among the cases studied, providing the optimal balance between heat transfer efficiency and hydraulic performance. The results demonstrated that the three-turn configuration (Case A) consistently outperformed the four-turn design (Case B). AtRe = 10,000, Case A achieved a maximum η of around 1.96 compared to around 1.33 for Case B, representing an improvement of nearly 47 %, while atRe = 16,000, Case A maintained a around 14 % advantage. Similarly, increasing the coil pitch from 40 mm to 55 mm enhanced performance across all Reynolds numbers. AtRe = 10,000, the wider pitch provided η ≈ 1.61, 29 % higher than the 40 mm pitch case. Even atRe = 16,000, the 55 mm pitch retained a 7 % improvement.