Thank you for your message and for providing me with this helpful information.
The gas is a mixture of air and water vapor. Additionally, liquid water also enters through the inlet. I am curious to know if I need to consider species flow even if there is no liquid flow through the inlet. I am a bit confused about it.
I assume that the flow in the tank is laminar. Regarding the term headspace, I am not entirely sure what it means in this context. If you are referring to the condition outside the inlet, then it is simply atmospheric conditions.