6. Effect of water vapour mass fraction w0

decreases from the inlet to the tube exit, resulting in the reduction of the heat transfer across the condensate film. Also, the slopes of the temperature of the mixture are decreasing along the tube owing to the absorbed energy transferred from the gas flow to the liquid film. The distributions of the mass fraction of water vapour in the gas region are illustrated in Figure 7. It is interesting to observe that the vapour mass fraction w0 decreases from the entrance to the tube exit, which implies that the condensation rate is decreasing along the tube. Consequently,

In the desalination units, the tube geometry of which the water vapour condenses (whether it is the length or the radius) contributes positively to the improvement of the condensation

To reveal their impacts, we first examined the impact of the length of the tube on the liquid film thickness and the condensing mass flux at the interface along the tube. Figure 8 shows the influence of the tube length on the thickness of the film and condensing mass flux at the interface. It is noted that δ<sup>x</sup> increases with increasing the tube length L. The results also indicate that, for a high tube length, the mechanism of the condensation is important when the distance is less than (X = 0.8), especially for L = 4.5 m and 6.0 m, because for a fixed Reynolds gas number (the inlet velocity is fixed too), the condensed vapour decreases with the increase of tube length. This means that near to the tube exit, the condensation process becomes almost unimportant. It is

Figure 8. Effect of the tube length on the variation of the liquid film thickness and the condensing mass flux at the

w0 is reduced from the Centre line (η = 1) to the liquid-vapour interface (η = 0).

5. Effect of the tube geometry (length L and radius R)

process if they are well dimensioned.

68 Desalination and Water Treatment

interface along the tube.

In the majority of thermal desalination units, the water vapour that does not condense at the first effect, with all the non-condensable gases content, is transferred to the second effect, and this produces gas accumulation up to inadmissible concentrations. These gases cause a reduction in the performance of the system.

Figure 10. Effect of the inlet vapour mass fraction on the variation of the liquid film thickness and the condensing mass flux at the interface.

Figure 11. Effect of TW on the evolution of the liquid film thickness and the condensing mass flux at the interface.

Computational Study of Liquid Film Condensation with the Presence of Non-Condensable Gas in a Vertical Tube

http://dx.doi.org/10.5772/intechopen.76753

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Figure 12. Effect of TW on the accumulated condensation along the tube.

This result is confirmed in Figure 10, which shows that the thickness of the film increases considerably from the inlet to the exit of the tube. It is also observed that the increase in the mass fraction of water vapour w0 considerably improves the condensation mechanism along the tube. Indeed, for a constant T0, an increase of w0 affects the thermo-physical properties of the gas mixture at the inlet, which leads to an augmentation of the vapour partial pressure and the temperature at the vapour-liquid interface. Consequently, the condensing mass flux at the interface J" increases significantly with w0 leading to an increase in the rate of condensation, which improves the thickness of the liquid film. On the other hand, a small amount of w0 (inversely proportional to the mass fraction of the non-condensable gas) causes a remarkable reduction of the condensed mass flux rate and the axial variation of the thickness of the film along the tube. This is due to the presence of air, which plays the role of thermal and mass transfer resistance at the vapour-liquid interface.
