Consider a hemispherical droplet of liquid water residing on a flat surface, as shown in the figure on page 526. Still air surrounds the droplet. At an infinitely long distance from the gas film, the concentration of water vapor in the air is effectively zero (dry air). At a constant temperature of 30°C and 1.0 atm total pressure, the evaporation rate of the droplet is controlled by the rate of molecular diffusion through the still air. The vapor pressure of water at 30°C is 0.042 atm, and the molecular diffusion coefficient of H₂O vapor in air at 1.0 atm and 30°C is 0.266 cm²/s. 

a. What is the total evaporation transfer rate (W_A) of water from a water droplet of radius 5.0mm in units of mmole H₂O per hour? 

b. Determine the time it will take for the water droplet to completely evaporate at 30C and 1.0 atm total system pressure if the initial droplet radius is 5.0 mm.

Transcribed Image Text:

Still air WA. A = H20 B = air Pure liquid H20