A 2-mm-thick layer of water on an electrically heated plate is maintained at a temperature of Tw = 340 K, as dry air at T∞ = 300 K flows over the surface of the water (case A). The arrangement is in large surroundings that are also at 300 K.

(a) If the evaporative flux from the surface of the water to the air is n"A = 0.030 kg/s ∙ m2, what is the corresponding value of the convection mass transfer coefficient? How long will it take for the water to completely evaporate?

(b) What is the corresponding value of the convection heat transfer coefficient and the rate at which electrical power must be supplied per unit area of the plate to maintain the prescribed temperature of the water? Property values of the air are p = 1.08 kg/m3 ∙ c p = 1008 J/kg ∙ K. and k = 0.028 W/m ∙ K. The binary diffusion coefficient for air and water vapor is DAB = 0.29 X I 0-4 m/s, and the emissivity of water is ε = 0.95.

(c) If the electrical power determined in part (b) is maintained after complete evaporation of the water (case B), what is the resulting temperature of the plate, whose emissivity is εp = 0.60?

Transcribed Image Text:

Case B Case A Air Air Water (T Heated plate, PecTp Heated plate, P