TP2:The evaporating droplet
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| Main subject | |
|---|---|
| Important Concepts | |
| Next Module | |
| Mixers and agitators | |
Introduction
Evaporation is a process that occurs on the surface of a liquid as it changes into the gas phase. The surrounding gas must not be saturated with the evaporating substance.
The process is modeled with both a mass balance (the evaporation rate must exactly balance the rate of mass transfer into the gas phase) and an energy balance. The latent heat that is absorbed at the liquid/gas interface due to evaporation must exactly balance the rate of heat transfer to the interface. The latent heat is proportional to the evaporation rate, and therefor both balances must be solved simultaneously. To simplify the problem, we will only consider isothermal evaporation
Objective
To learn how to model evaporation from an idealized falling droplet under isothermal conditions.
Learning Outcomes
- To correctly set-up the "unsteady-state" mass transfer equation using a quasi-steady state assumption.
- To determine the terminal velocity and rate of mass transfer from a falling droplet.
- To use a Runge-Kutta numerical integration method to predict the droplet radius as a function of time.
- To explain why the isothermal approximation is most likely incorrect and how to determine the true temperature at the evaporating surface.
Practice Exercise
1. You have a water droplet (1 cm diameter) falling at its terminal velocity at room temperature. Estimate the droplet lifetime. You will need to find the appropriate correlation in Perry's handbook. Assume the process is occurring under steady-state conditions. Also assume there is no water vapor in the air (no humidity) and that the process of evaporation is isothermal.
2. How would you approach the problem if you could not make the isothermal approximation?