2020-10 - Airlock and Waterfall in Pipe

Airlock and waterfall in pipe is simulated by OpenFOAM to demonstrate how air may be trapped at local high point of pipeline, and how releasing the trapped air may help to ease the situation.

In pipework, air may be trapped at local high point to form air pocket. The source of air can be due to filling up of pipe, cavitation, air bubble in wet well being pumped into the system etc. Formation of air pocket will lead to reduction in flow area and increase in head loss.

Recently, I watched a youtube video explaining airlock in pipe and waterfall flow due to presence of air in pipeline. I am surprised by formation of waterfall flow which results in large head loss in the system equal to height of fall.
   • Waterfall Flow & Airlocks in Pipes Ex...  

Three hypothetical CFD cases are created to pump water with entrained air to head reservoir tank.
(i) Baseline case without local high point
(ii) Local high point is created
(iii) Local high point with air release

For pipeline without local high point or fall, air bubble can be pushed out of pipeline by water. However, if there is local high point, air pocket will be formed. Size and location of air pocket appears to relate to water flow velocity and supply of air. Releasing trapped air makes the system to return to full bore flow.

It appears that large waterfall flow in pipe will only appear under special circumstances (not in my case). In my simulation, a small fall of water is observed at local high point. It adds to my knowledge that formation of waterfall in pipeline is another reason for head loss, in additional to reduction in flow area, when air is present.