Experimental investigation of saturated flow boiling of water in vertical helically coiled tubes

Helically coiled tubes are used in the High Temperature Gas-cooled Reactor (HTGR) Once Through Steam Generator (OTSG). However, due to centrifugal forces and secondary flows, heat transfer characteristics inside these tubes may differ from that in straight tubes. A clear understanding of the heat transfer inside helically coiled tubes is essential for the design and operation of the OTSG. In this study, we experimentally investigated saturated flow boiling in helically coiled tubes with a large curvature ratio (δ = 0.109). The experimental parameters cover a broad range. The system pressure is from 3.5 to 7 MPa, mass flux is from 300 to 1100 kg/(m²·s) and heat flux is from 50 to 600 kW/m². Results show that the inner wall temperature distribution is uneven, with the highest temperature on the inner side and the lowest on the outer side. Increasing heat flux enhances the saturated flow boiling heat transfer coefficient. At low steam quality, the heat transfer coefficient is not significantly affected by mass flux variations. However, at higher steam quality, increasing mass flux improves heat transfer. An increase in system pressure enhances the heat transfer coefficient at lower steam qualities but reduces it at higher steam qualities. Six correlations for the saturated flow boiling were evaluated, with the Gungor-Winterton correlation originally developed for straight tubes showing accurate predictions for heat transfer coefficients in helically coiled tubes (MAPE is 12.72 %, RMS is15.36 %). This indicates that even in helically coiled tubes with a large curvature ratio, no significant difference is observed in the saturated flow boiling heat transfer coefficient compared to straight tubes.