Experimental Investigation of Multi-Swirl Inserts for Enhancing High-Heat-Flux Subcooled Flow Boiling Under One-Sided Heating Conditions

Subcooled flow boiling experiments were conducted to investigate the effect of multi-swirl inserts on heat transfer, critical heat flux, and pressure drop under one-sided high-heat-flux heating conditions. Water was used as the working fluid, and four cooling-channel configurations were tested: a smooth circular tube, a single-swirl insert, a two-element multi-swirl insert, and a four-element multi-swirl insert. The experiments were performed under controlled pressure, inlet temperature, and flow velocity, while the heat flux was increased stepwise until the critical heat flux (CHF) occurred. The boiling curves showed that the multi-swirl inserts reduced the wall superheat compared with the smooth tube and the single-swirl insert. The maximum heat transfer coefficient increased from 61.2 kW/m² K for the smooth tube to 66.8, 115, and 154 kW/m² K for the single-swirl, two-element multi-swirl, and four-element multi-swirl inserts, corresponding to enhancements of 9.2%, 87.9%, and 151.6%, respectively. The CHF also increased from 3927 kW/m² for the smooth tube to 5086, 6086, and 7472 kW/m² for the corresponding insert configurations, corresponding to CHF enhancements of 29.4%, 55.0%, and 90.3%, respectively. The results show that multi-swirl inserts enhance subcooled flow boiling heat transfer and increase CHF under one-sided high-heat-flux heating conditions. A performance evaluation criterion (PEC) analysis further indicated that the hydraulic penalty should be considered when selecting the insert configuration for a target heat-flux range.