Concentration Boundary layer thickness

[gfd43tg]

Hello,

I am simulating an experiment I did in the lab where we had air flow over a tray of water to determine the mass transfer coefficient scaling with velocity, as well as boundary layer thickness scaling with velocity.

Now I am using COMSOL to simulate the experiment, and here is the schematic set up

Where the dashed lines represents the control volume of the COMSOL simulation. I input my boundary conditions and here is what the concentration profile looks like

The blue area is the bulk air flow concentration, so it is clear that the teal area is the concentration boundary layer. Notice that the boundary layer gets thicker as you move more down the length of the tray. Why is this happening theoretically? I know it has something to do with the Blasius boundary layer.

Also, if anyone understands COMSOL or has any ideas how I could devise a way to figure out the boundary layer thickness as a function of position, that would be very helpful! I can plot the concentration gradient as a function of distance, so that might be a place to start to find the thickness.

Thanks

 

[gfd43tg]

I'm looking at some of the equations for mass transfer and I am a little confused

In this picture, it is saying the flux $N_{i}$ is equal to zero at the boundary layer highlighted in blue. This doesn't seem to make sense, so I am wondering if this is supposed to mean that there is no flux in the x-direction, and only flux in the y-direction, or if the negative sign just indicates that there is no flux back into the water surface, i.e. all water that leaves the water surface goes into the bulk and does not flow back in.