How Does an Air-Lift Work in Fluid Dynamics?

[rivermud]

Thanks for any replies in advance.

I am currently trying to discern the theory behind an air-lift. A very large group of people are currently debating exactly how an air-lift works.

Imagine a pond with a pipe in it. An airstone is place at the bottom end of the pipe and air is pumped into the pipe. the result is a lift causing the water to rise up the pipe and expel at a level higher than the surface of the pond. At question is why. A basic theory given is: air bubbles act as pneumatic pistons, pushing or drawing water up a pipe or stack as the rise and expand. A more advanced theory given, and the one I am more prone to subscribe to describes the pumping action as the result of an air-water mixture. The air-water mixture is less dense than (and therefore is displaced by) the surrounding water of higher density.

I propose that the air displaces the water, the surrounding pressure of the pond/reservoir is greater than that in the tube thus the water must move up the pipe to provide space for the air. However alternate arguments state that the air itself pushes the water and drags water behind it. I know both theories sound similar but one is based on pressure of the surrounding water where as the other seems to subscribe to aerodynamics; the bubble itself pushes the water rather than displaces it. Thus forcing the water to go upwards because the air itself is traveling that direction. The issue in my opinion with that argument is that the air itself is being farced that direction because it is lighter and less dense than the water and must escape so how could it push the water if the water is pushing it?

Could someone provide a definitive answer to the theory? Our group is grinding to a halt on this issue. Thanks.

 

[Kazza_765]

I don't know if your other theory is correct, but unless its a pretty short tube I think the air pushing the water up is incorrect. The buoyancy force acting on an air bubble is equal to the volume of water displaced. Therefore, in best case scenario, it could only push a volume of water equal to its own size out of the tube. Actually, I think I just realized what an airstone is, and if there is enough air going through it perhaps that is what happens (I was thinking along the lines of one big air bubble traveling up a tube). So nevermind, someone else will probably know this one.

 

[rivermud]

It's a tough theory to wrap your brain around. I can see both sides to the issue. In theory you could consider the air to push the water because it is being introduced with an airstone and is not present to begin with. Displacing is truly pushing, the argument then becomes why do the water always go up rather than simply exiting through the bottom of the tube. One argument say that sinc ethe air is moving up the water must then move up because it's being pushed. my theory is that since the pressue outside of the pipe is greater than inside the pipe do to use increasing the bouyancy by adding air the water must go towards the least resistance which would be "up" the pipe. I also contend that the size of the bubbles and the amount if air introduced to the system would greatly affect it's lifting capability and flow rate. Smaller bubbles stay in the tube longer therefore you can add a larger volume of air to the column faster and easier than simply cranking the pressure up with a bigger pump and massive bubbles. Of course then i would have to calculate the drag versus the pipe diamter and the back pressure generated for the efficiency rating. ... gah it just gets soo immense.. anyway, all of these things scream to me that it is pressure related not simply the push of an air bubble up on the water... Oh and maybe this should be moved to classical physics as it seems to fit in that forum a little better i apologize for not posting it in the correct forum in the first place. thanks again.

 

[rcgldr]

It's a combination of effects. If you look at air bubbles in a fish tank you can clearly see that some water is moved upwards at the surface. I think the two factors are the lower density in the pipe as mentioned, and also due to momentum (and viscosity) due to circulation of water around a rising bubble.

In the ocean, a very long veritcal pipe can create a similar reaction. At the bottom of the pipe, the water is cooler but less salty than the water on the surface. As the water rises, it's warms up becomes less dense than the surrounding salty water.