Experimental studies

(ii) For the wet conditions, the average heat transfer coefficient h is given by

0.69

 U∞Dh 

The Biot number for the wet conditions is 0.83.

Reading Figure 14c for θ at ξ = 1 (fin tip) on the curve for Tfb = 0C, we have

Tft = Ta - 0.72 (Ta - Tfb ) = 7 oC .

or

Bi = (0.83)1/2 = 0.91, so that Figure 15a for Ta = 25C gives

qwt = 1.45 qd = 159.8 W.

Reading the curve for Ta = 25C in Figure 15c, the ratio qws/qwt for

The latent heat transfer is 59.1 W.

Experimental studies

Experimental studies of finned coiled heat exchangers have been carried out by several

workers including Bryan (1962), Bettanini (1970), Yoshi et al. (1971) and Guillory and

McQuiston (1973). These studies have confirmed that the performance of finned coils is

significantly reduced when dehumidification occurs. This reduction is the consequence of

lower fin efficiency for wet conditions.

Kazeminejad (1987) and Coney et al. (1989b) conducted an experimental study to inves-

tigate the performance of a vertical rectangular fin (Fig. 13) when moist air in turbulent

flow dehumidifies on the surface of the fin. The study revealed that the wet fin surface

temperature increases with increase in free stream velocity, relative humidity and dry

bulb temperature. The increase in wet fin surface temperature also occurs when the fin

base temperature is decreased. These observations confirm the theoretical predictions

shown in Figure 14. The study also noted that smooth and clean surface copper fins

promote dropwise condensation rather than filmwise condensation. Although the effect

of mass transfer on the heat transfer coefficient was small, the fin efficiency was markedly

reduced under wet conditions. Another interesting conclusion was that the shape of the

leading edge of the fin affected the heat and fluid flow. The heat transfer to the fin was

higher when the leading edge of the fin was blunt (with attendant flow separation and

reattachment) than when the leading edge was elliptical.

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