Calculating change in Relative Humidity
 

I have inlet air and water temperature and initial Relative Humidity of the air.
I have a CFD model that can calculate the change in Temperature of water due to evaporation in a Control Volume.
However, change in temperature or evaporation is accompanied by an increase in Relative Humidity. How can I calculate the increase in Relative Humidity for my Control Volume? I can calculate increase in vapor mass (that's how I'm calculating evaporation energy)

What's your psychometric model? How do you define humidity?

It might also be helpful to describe any details of your model beyond "I have a CFD model"