How is Schrage's kinetic limit equation useful?

[Yinxiao Li]

The kinetic limited mass flux due to evaporation at the liquid–vapor interface surrounded by an air–
vapor mixture is given as:
mass flux = 2σ/(2-σ) *(M/2piRT)^0.5 * (Pv,eq -Pv)
where Pv,eq is the saturated vapor pressure of liquid and Pv is the partial pressure of vapor in the gas phase very close to the liquid vapor interface.

The question is, in real evaporation process, there is a mass diffusion boundary layer very near the interface. In other words, how do we know Pv? Pv could be very small far away from the interface, and becomes bigger and bigger as it approaches the interface.
If this Pv cannot be determined accurately, how is this equation useful?

 

[Illmatic1]

The equation given is an approximation based on the assumption that there is a linear variation of vapor pressure with distance from the liquid–vapor interface. This is usually a valid assumption in most evaporation processes. The equation can still be used to calculate approximate values for the mass flux if the vapor pressure at the interface (Pv,eq) is known. In more complex cases where the vapor pressure cannot be assumed to be linear, numerical simulation tools such as CFD can be used to determine the exact vapor pressure profile near the interface. This allows for more accurate estimation of the mass flux.