9. Conclusion

liquid film becomes thicker. From this figure, it is also observed that the condensing mass flux is important for a large value of the inlet-to-wall temperature difference and then decreases along the tube. The results indicate that J" decreases as the gas flow progresses along the tube because the condensation is accompanied by a diminution in the temperature of the vapour phase and the heat transferred by the latent mode during the condensation to the liquid film. The effect of wall temperature on the accumulated condensation mcd is illustrated in Figure 12. It can be seen that mcd becomes important for low temperatures. When the temperature difference (T0 - TW) increases, the heat transfer increases, and consequently the density of the condensed flux increases. This explains why the condensation process is favoured for a higher-

In thermal desalination units, when the water vapour condenses, the presence of a noncondensable gas hinders this phenomenon. The accumulation of non-condensable components at the vapour-liquid interface plays the role of an obstacle for heat and mass transfer. This causes a reduction in the efficiency of the system and therefore an increase in costs in most

In this section, we analyse the influence of the non-condensable gas type during water vapour condensation. We considered mixed mixtures of water-oxygen, water-air and water- nitrogen. The molar mass of oxygen, air and nitrogen are equal to 31.99, 28.95 and 28.01 g/mol, respectively. Since the liquid film thickness δ and the condensing mass flux at the interface J" determine the condensation efficiency, Figure 13 illustrates the variation of δ and J" along the

Figure 13. Effect of the type of non-condensable gas on the evolution of the liquid film thickness and the condensing mass

temperature difference.

72 Desalination and Water Treatment

8. Effect of the non-condensable gas type

desalination units using phase change.

flux at the interface along the tube.

A numerical analysis has been carried out to investigate the liquid film condensation of the water vapour with the presence of non-condensable gas inside a vertical tube. The main conclusions drawn from this study are as follows:

