Shown in Fig.4 is a schematic image of heat flux distribution on a fin. The solid line depicts the heat flux distribution when the condensation takes place at the lower part of fin. As the dew point locates at the middle of fin, the heat flux sharply increases toward the base of fin from the dew point. The dash line is the calculated result with the equivalent heat transfer coefficient of Eq.(21). The heat flux at the fin base is the same as the actual heat flux shown by solid line and the heat flux gradually decreases toward the fin tip without the effect of dew point. Generally the fin efficiency evaluated with Eq.(21) yields slightly the higher value than the actual one. However, at the white fuming condition when the saturation temperature merges with the gas temperature, the dew point never locate on the fin and the error due to the use of Eq.(21 ) becomes smaller.
For an analogous mass transfer, the simple analogy between heat and mass transfer gives,
Shf = jRef Scf 0.33 (22)
3.3 EXPERIMENTAL RESULTS AND DISCUSSIONS
3.3.1 Fin tube of pitch 10mm
Shown in Fig.5 is a photograph when the temperature of base tube became approximately the dew point (Δ Tsub = 0 K). The condensation behavior like fish-scales could be seen on the base tube between fins at around 45℃ from the tube top. However no condensation on the fin surface could be recognized.
Further decreasing the base tube temperature as the subcooling was Δ Tsub = 8.5 K, the base tube was covered with a thin condensate film as shown in Fig.6. In this subcooling, no condensation on the fin surface could be recognized.
Shown in Fig.7 is a photograph when the subcooling became Δ Tsub = 13.3 K. The base tube was almost covered with condensate but the most part of fin surface was still dry. The condensate flowed down and droplet of condensate was hanging on the fin of the lower part. So the temperature distribution of fin is not considered to be symmetric. It is possible that the re-evaporation takes place at the lower fin.
Fig. 5 Incipience of condensation (Δ Tsub= 0 K)
Fig. 6 Condensation behavior at Δ Tsub = 8.5 K
Fig. 7 Droplet hanging on fin
