Achieving optimal thermohydraulic efficiency: Comparative study of Semi-Cylindrical and Semi-Conical Coil geometries for heat exchanger design

The growing demand for high-performance thermal management in compact cooling systems and advanced heat transfer applications has necessitated the exploration of innovative geometries and hybrid nanofluids. Despite recent progress, the synergistic effect of dynamically varying curvature in semi-conical geometries and the impact of the Aspect Diameter Ratio (ADR), representing the ratio of internal to external tube diameter, remain insufficiently explored. This study numerically investigates the thermohydraulic performance of Semi-Cylindrical (SCYC) and Semi-Conical (SCOC) Coil heat exchangers using a SiC-CNT/Water hybrid nanofluid. The research was conducted in two sequential stages: first, a comparative performance analysis was performed between the two configurations, and second, the superior model was further enhanced by investigating the influence of the ADR parameter. Results indicate that the SCOC geometry is superior, achieving a Performance Evaluation Criteria (PEC) above 1.30, driven by the generation of intensified Dean Vortices. Further investigation into the tube thickness revealed that an ADR of 0.625 significantly improves performance by accelerating fluid velocity and strengthening internal secondary flows. The final enhanced design (Semi- Conical Coil- SiC-CNT/Water (SCOC-SCW) with ADR=0.625) achieved a peak Nusselt number of 13.1 and a maximum PEC of 1.35 at Re=1500, marking a 35% net gain over the baseline model. These findings provide an efficient design blueprint for next-generation thermal systems requiring high power density and compact footprints.