An experiment is conducted to determine the average mass transfer convection coefficient of a small droplet using a heater controlled to operate at a constant temperature. The power history required to completely evaporate the droplet at a temperature of 37°C is shown in the sketch. It was observed that as the droplet dried, its wetted diameter on the heater surface remained nearly constant at a value of 4 mm.

(a) Calculate the average mass transfer convection coefficient based on the wetted area during the evaporation process when the droplet, heater, and the dry ambient air are at 37°C.

(b) How much energy will be required to evaporate the droplet if the dry ambient air temperature is 27°C while the droplet-heater temperature remains at 37°C?

Dry air T h Droplet Heater 50 100 I (min) P (mW)