Fig. 8 Noding for numerical calculation of fin

Shown in Fig.9 is calculation result for the radial temperature distribution in the fin of 10mm fin pitch varying the temperature of fin tip. The temperature gradient is steep below the dew point indicating the high heat flux due to the condensation. Even when the base tube temperature decreases below the dew point, the tip temperature is approximately 90℃. When the base temperature further decreases to 0℃, the tip temperature is approximately 70℃ and much higher than the dew point. These supports the observation result that the most part of fin surface was dry when the condensation at the base tube initiated.

The fin efficiency can be obtained from the numerically calculated temperature distribution as

Fig. 9 Temperature distributions in fin

Fig. 10 Fin efficiency at pitch of 10mm

Shown in Fig. 10 is the relation of experimental fin efficiency and the average temperature of base tube comparing with those obtained from the numerical calculation and the first order approximation. In the first order approximation, the equivalent heat transfer is used to evaluate the fin efficciency in the empirical correlation of Eq.(16). Increasing the base tube temperature, the experimental fm efficiency sharply drops at the dew point. This behavior is considered to be due to the fact that a large amount of heat flows in the fin and the temperature difference between the fin tip and bottom becomes large when the condensation takes place. The experimental efficiency agrees well with that obtained with the numerical calculation assuming the symmetrical heat flow in the fin. The asymmetric temperature distribution at the lower fin observed in the above photographs does not have a significant effect on the total fin capability. However it is possible that the re-evaporation at the lower fin condenses the condensate and leads to the severe erosion.

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