In order to get a deep insight of the heat transfer mechanism of falling film evaporation outside a horizontal tube, the characterization of the microscopic mechanics was numerically investigated. A numerical model was developed to obtain the dynamic and heat transfer performance including film thickness, film velocity, film temperature and local heat transfer coefficient. The simulated results were found to be in a good agreement with the reported data. The results show that the convection plays an important role in the thin film evaporation even at low Reynolds number through the analysis on the profiles of the local film velocity and local heat transfer coefficient in the falling film. The distribution of the thermal developing region and thermal developed region along the tube circumference for a single tube and a tube bundle was predicted. The local film temperature increases and local heat transfer coefficient stabilizes at the top thermally developing region while the former stabilizes and the latter decreases at the fully thermal developed region which is at the bottom part of the tube.
In order to get a better understanding of distribution characteristics of the dynamic and heat transfer performance in a horizontal-tube evaporator, a numerical calculation was developed to simulate the internal film condensation and external falling film evaporation in a tube bundle. The temperature field, film thickness and local heat transfer coefficient were predicted along the tube length and in-between tubes. In view of a good agreement of the simulated predictions with the data of the practical desalination plant, the theoretical model was proved to be valid and accurate. The results show that the flow field and heat transfer rate are improved by means of optimizing the flow density distribution of liquid film outside a tube bundle on basis of the variation of the internal condensation process. The internal vapour condensation temperature reduces sharply at the outlet of the second pass. The local overall heat transfer coefficients tend to decrease from the inlet of the tubes to the outlet and approach the maximum at the bottom of the bundle.
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