Air Flow through cooling tower- fluid mechanics problem

[Ritz_physics]

Hey everyone!

I am working on improving natural draft counter flow cooling tower efficiency as my final year B.E. project. I came across a research paper that said that at transverse cross section of the tower, above drift eliminators, the air velocity decreases from the periphery to the middle of the tower, while the temperature increases. The statement is also supported by CFD simulation of the tower.

If viscosity effect is considered, the velocity at the periphery of the cross section of the tower should be less than that at the center. I am assuming that since the flow of air takes place from the edges of the tower at the bottom, the velocity must be progressively reducing as air approaches towards the center of the tower, than at the periphery. Is this assumption acceptable?

Also, could the hyperbolic shape of the tower have got anything to do with the influence on air velocity profile across tower section?

 

[jlefevre76]

My first thought is, unless you expect compressible flow (which I would assume is not an issue on a cooling tower), the volume flow rate at any horizontal cross section of the tower would be constant. That is, the volume flow rate at the bottom, the volume flow rate at the narrowest point, and the volume flow rate at the top, will all be approximately equal. This might not be 100% true due to the change in humidity of the air as it evaporates the fluid that's being cooled, but the changes in air density due to the increase in humidity should be negligible. So, depending on what the inside structure of your cooling tower looks like (I've never seen one except on the internet, but there are different designs I think), the maximum velocity would always be at the center of the cooling tower, furthest away from the walls.

The exception to that is where the air intake is, it might be slower at the center of the tower than where the air intake is located. I hope that helps.

 

[jlefevre76]

Oh wait, I think I understand now what you mean. In that case, the periphery where the intake is, the velocities would probably be...well, they would still be lower than the choke point at the center of the tower. So, a lot of slower moving air coming in from all around at the bottom of the cooling tower will equal faster average moving air at the center choke point of the cooling tower, where the cross section is more narrow.

I think, if I understand what you're saying, that's how I'd look at it. Though, admittedly, I don't really know what the inside of a cooling tower looks like or where the air flows exactly. A diagram might be helpful here if you have something you can post.