A disk of 20-mm diameter is covered with a water film. Under steady-state conditions, a heater power of 200 m W is required to maintain the disk-water film a 305 K in dry air at 295 K and the observed evaporation rate is 2.55 x 10-4 kg/h.

(a) Calculate the average mass transfer convection coefficient, hm for the evaporation process.

(b) Calculate the average heat transfer convection coefficient, h.

(c) Do the values of hm, and h satisfy the heat-mass analogy?

(d) If the relative humidity of the ambient air at 295 K were increased from 0 (dry) to 0.50, but the power supplied to the heater was maintained at 200 m/W, would the evaporation rate increase or decrease? Would the disk temperature increase or decrease?

Transcribed Image Text:

Air T = 295 K Water film, T, = 305 K wwww Disk, D = 20 mm Heater, 200 mW