# Will air push the water or pass through as bubbles?

This doubt is about a conundrum which is common to multiple scenarios. One such scenario is this:

The valve is opened and the high pressure air is pushed towards the water in the channel with the hope that it will drive out all the water through the open end of the channel. Now, my doubt is whether air will drive out all the water or will it simply pass through the water as bubbles?

The scenario occurs in many other situations. In aquariums, the pump pushes in air which passes through as bubbles. In pressure-based cleaning devices (e.g. vacuum cleaners), air pressure drives out all other fluids.

So what happens in this case? And how do we even analyze such problems. In which cases does the air drive out the fluid and in which cases does it pass through as bubbles?

• This is a two phase flow problem. In two phase flow, there are several regimes, and many variables, such as tube diameter, liquid surface tension, air velocity, height of the water column that is being pushed up the tube, etc. Aug 31, 2019 at 2:38
• Thanks @DavidWhite "Two phase flow" : that is the key phrase! I will look into this. Aug 31, 2019 at 3:30

Treasure divers use a device not unlike your design. The horizontal section is short and the air is introduced where it joins the vertical. Most of the vertical section is underwater. The air bubbles reduce the density in the vertical section and it is lifted by the pressure of water flowing in at the bottom. The flow picks up sand (and other things) from the bottom and delivers them to a sieve at the top.

This is a known old invention called "Airlift pump", it was originally invented by Carl Emanuel Löscher in 1797. In many languages it's called "Mammut pump"

With this pump, it's even possible to dredge seabed or clean deep drill holes. Even the smallest airflow will cause this effect as it principally works with density differences, and thus the air is only needed to reduce density, while the fluid outside the pipe will provide the pressure and flow. So it's actually the buoyancy which lifts the water.

Your questions; In which cases does the air drive out the fluid and in which cases does it pass through as bubbles?

Answer; If the volume of the airflow is enough to replace the whole volume of the water channel all water will be blown out. If this volume is only half, then half of the water will be blown out.

Water is approximately 800 times heavier than air so it requires very large amount of push to do work on water. So yes if you can create very large velocity (F = phoareavelocity^2) in that same cross section than as the force is directly propotional to square of velocity than the water might rise but if you are forcing air from smaller cross section like a nozzle than water may displace side ways so air will definately apply force but it will pass through it as the case of aquarium. That's what i think.

• What happens if you compress the air to a pressure 800 times that of atmospheric? May 23, 2018 at 0:34
• As I said air will pass through because liquid has no resistance to applied force (fundamental properties of fluids so it will displace side ways and rise to some height) but if you can send the air stream exactly of the same dia of pipe with that much pressure surely it will displace water because now water has no where to go. And 800 times compressing air adiabatically means a huge temperature rise so it is I think impractical to design such compressor.
– LMB
May 23, 2018 at 17:53
• Have you ever heard of cooling the air after compressing it? Or using inter cooling between stages of a compressor? Who said anything about producing the compressed air by compressing it adiabatically? May 23, 2018 at 17:56
• @LMB Thank you for your answer. I think you are on to something, however could you be more elaborate and explain using equations and/or diagrams? May 23, 2018 at 22:41