A review on the two-phase heat transfer characteristics in helically coiled tube heat exchangers

•Detailed review on the two-phase heat transfer characteristics in helical tubes.•Curvature effects on the boiling heat transfer coefficient are not significant.•There is significant conflict in literature on the applications of nanofluids.•There is a significant gap in literature in the field of non-boiling two-phase flow.

Helically coiled tube heat exchangers are the most widely used from the family of coiled heat exchangers. This is due to their compact design, ease of manufacture and enhanced heat transfer efficiency. The purpose of this review is to summarise and critically review the published studies on the heat transfer characteristics of two-phase flow in helically coiled tubes. The first section presents the experimental and theoretical results for the boiling heat transfer characteristics reported by several authors whilst the second section focuses on the results for the heat transfer characteristics with nanofluids. Therefore, this review provides researchers in academia and industry with a practical summary of the relevant correlations for the calculation of the two-phase heat transfer coefficient. A significant scope for further research was also identified in the field of two-phase flow at non-boiling conditions and in the application nanofluids.