Bibliographic analysis of predicting heat transfer coefficients in boiling for applications in designing liquid food evaporators
2008
Ali Adib, Tarif | Vasseur, Jean
In this article 16 different correlations to predict the boiling heat transfer coefficient, h L, 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 h L are heat flux Φ (Wm⁻²) and liquid viscosity η L (Pas). However, several other parameters, such as the boiling regime (nucleate or non-nucleate) and the flow in falling film which lead to different h L 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 h L 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 Re F) 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, B O, 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 h L value on the basis of the existing formulas found in the literature.
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