Abstract:

A better understanding of two-phase flows with evaporation allows leading to an optimal design of heat exchangers particularly evaporators. For that purpose, numerical simulations are very useful. In this paper, a numerical study using Fluent has been carried out in order to model and simulate the combination of a two-phase flow with evaporation in a vertical tube. For that purpose, the VOF multiphase flow model and a phase-change model for the mass transfer have been used. For an accurate modelling, the effect of axial conduction has been also taken into account using a conjugate heat transfer model. Our numerical simulation procedure has been validated by comparing our results with those obtained by other authors. Temperature and void fraction fields as well as the heat transfer rate have been calculated for various conditions. Indeed a parametric study has been carried out for various conditions (Reynolds number, imposed lateral heat flux, position along the tube, etc…). Since thermal oscillations are undesirable as they can lead to the failure of the tube, flow instabilities have also been analyzed, using FFT (Fast Fourier transforms), in order to comprehend their behaviour and influence. A control study of the flow instabilities in the tube is also presented. For that purpose tube inlet temperature has been varied using a gain control parameter.

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