Convection and evaporation potential Essay

Figs. 10 and 11 show the variation in the convection and evaporation potential, respectively, as the fill height changes from 0 to 1.2 m, for different inlet wet-bulb temperatures (from 286 to 294 K). In Fig. 11, it is noted that evaporation potential decreases as the convection potential increases (Fig. 10) for the increasing inlet wet-bulb temperature. As it can be seen in Fig. 10 convection potential decreases with decreasing of wet-bulb temperature in the bottom part of the fill and this situation changes after crossing point . This is possible because of the negative convection heat transfer takes place at this part of the fill. It is noted that, with decreases of wet-bulb temperature value air is drier and the evaporative potential of air increased. The results of exergy analysis in cooling tower is shown in Figs. 12 to 15 . Fig. 12 shows water exergy, , and water temperature distributions. Exergy of water describes the available energy of water can be given to air. This parameter because of decreasing in water temperature from top to bottom of the fill, decreases continuously. This situation changes at negative convection region.
Fig. 8. Variation of water temperature versus height of fill for different wet bulb temperatures. Fig. 9. Variation of air temperature versus height of fill for different
wet bulb temperatures.
Fig. 10. Variation of convection potential versus height of fill for different wet bulb temperatures. Fig. 11. Variation of evaporation potential versus height of fill for different wet bulb temperatures.
Fig. 12. Water exergy and temperature distributions. Fig. 13. Air temperature and convective exergy distributions.
Fig. 14. Air humidity and evaporative exergy distributions. Fig. 15. Evaporative, convective and total air exergy.
Also, it is necessary to evaluate the exergy of the air against its "ultimate" dead state, achieved when the thermal, mechanical and chemical equilibrium is reached with the environment. Dry-bulb temperature and convective exergy of air distributions versus height of the fill depicted in Fig. 13. It can be seen from this figure that, from top to bottom of the fill, and air temperature decrease between to and their variations tend to increase for because of existence of negative convection. This point specifies conformity with the results discussed earlier in Fig. 4. The intersection point of and (Fig. 4) indicates no temperature difference. Therefore, there is not any convective heat transfer between air and water. This also indicates the minimum value of . Exergy of air via evaporative heat transfer and its humidity ratio are shown in Fig. 14 . The value of the humidity ratio increases continuously but evaporative exergy of air for height of , decreases and its variation start to increases for . In Fig. 15 convective , evaporative and total exergy of air depicted versus of fill height. When comparing with exergy of water in Fig. 14, it can be seen that the values of are more than those of . The comparison of results shows the good agreement with obtained results by reference [13].