# Concentration boundary layer thickness

1. Oct 18, 2015

### Maylis

Hello,

I'm doing an experiment where I will be blowing warm air parallel to a stagnant water surface, and I will investigate the scaling of air velocity with mass transfer coefficient. I am trying to find some kind of scaling of the concentration boundary layer thickness with air velocity, and have been unable to find any literature that gives that scaling. Anyone know what that relationship is, or where I can find it? Thanks.

I should note I have found derivations of the momentum boundary layer thickness, but not the concentration. Perhaps there is some correlation between momentum and concentration boundary layer thickness?

Last edited: Oct 18, 2015
2. Oct 18, 2015

### Maylis

Nevermind, I found some powerpoint slides. It can be correlated with the Reynold's number and Schmidt number to various powers multiplied by some constant. As a follow up, I understand that the momentum, concentration, and thermal boundary layer thicknesses are not the same, but I wonder if these are real or are they just concepts used to explain the phenomena. How can one layer of air be thicker than another layer, like they are overlapping each other? Seems strange.

3. Oct 20, 2015

### bigfooted

Suppose you have air flow over a solid object but there is no temperature difference between the air and the object. The thermal boundary layer thickness will be zero, but the momentum thickness will have some finite value. The momentum boundary layer says something about how far away from the object your velocity reaches the free-stream velocity. The thermal boundary layer says something about how far away from the object your temperature reaches the free-stream temperature. These boundary layers are usually not the same.

Please, do not depend only on powerpoint slides for such a difficult topic. There are very good books on boundary layer flows and the questions you ask are all covered by most of these books. Finding out why the Sherwood number for mass transfer looks the way it does is an advanced topic, but for the quoted questions above, you can study the first chapter on laminar boundary layer flow in any book that covers boundary layer theory

4. Oct 21, 2015

### 256bits

While not directly applicable to your problem, this I found gives a brief synopsis of how some of the boundary layers work.

http://nature.berkeley.edu/biometlab/espm129/notes/Lecture%2024%20Leaf%20Boundary%20Layer%20Resistances%20and%20Mass%20and%20Momentum%20Exchange%20notes.pdf [Broken]

Last edited by a moderator: May 7, 2017