Flow boiling heat transfer correlations in horizontal circular tubes: A review

Two-phase flow boiling is fundamental in the design of heat exchangers across diverse applications, from refrigeration to power generation systems. With the continuous growth of advanced power generation technologies, such as organic Rankine cycles (ORCs), this review systematically evaluates established correlation model approaches, assessing their physical foundations and predictive capabilities against experimental data representative of these applications. By analyzing each correlation type and examining its ability to capture the influence of key operating parameters (heat flux, mass velocity, and saturation temperature), this work demonstrates how the underlying assumptions of these models are rooted in the heat transfer mechanisms typical of refrigeration, characterized by low heat fluxes and low saturation temperatures, contrasting with the demanding operating conditions of power generation applications. Furthermore, this review compiles experimental results of the flow boiling heat transfer coefficient for low-GWP organic fluids in conventional tubes where existing correlations achieve acceptable accuracy ( MRAD < 20%) and presents them in an unconventional column-chart format that explicitly displays the operating and dimensionless parameter intervals characterizing each dataset. This visualization enables designers to identify which correlations are suitable for their specific operating conditions. The analysis reveals a critical finding: validated experimental datasets for all studied fluids cluster exclusively within the refrigeration-scale parameter space, with minimal representation of the high-heat flux, elevated-saturation temperature conditions typical of ORC systems. Consequently, for R1234yf , R1234ze(E) , R600a , and other low-GWP power-generation fluids, no ORC-relevant conditions for which there are validated correlations currently exist.