Bibliographic analysis of predicting heat transfer coefficients in boiling for applications in designing liquid food evaporators Review Article

In this article 16 different correlations to predict the boiling heat transfer coefficient, hL, on the product side are analyzed and compared to evaluate their use in evaporator design, particularly in situations where viscosity increases with concentration. The main factors to determine hL are heat flux Φ (W m−2) and liquid viscosity ηL (Pa s). However, several other parameters, such as the boiling regime (nucleate or non-nucleate) and the flow in falling film which lead to different hL values are also involved. First, we compare six nucleate-pool boiling correlations by observing the parameters used and their exponents. When compared with the same Φ assumption and the properties of the same product; the calculated hL values vary in a ratio of 1–5 among formulas in the case of pure water and in a ratio of 1–40 for the viscous liquid. Ten formulas for the situation of falling film (vertical tubes), considering the two boiling regimes (non-nucleate and nucleate) and the two flow regimes (laminar or turbulent according to the film Reynolds numbers ReF) are then compared. Although the transition from the non-nucleate to the nucleate boiling regime remains difficult to predict, the use of the boiling number, BO, is described for the nucleate boiling regime. The cases of high viscosity and/or of non-Newtonian liquids are also analyzed. From this critical bibliographic review, it appears that it is difficult to predict the hL value on the basis of the existing formulas found in the literature.