2022-12 - De-aeration in Vortex Drop Shaft

De-aeration chamber to remove air entrained at bottom of vortex drop shaft is simulated by OpenFOAM. This vortex drop shaft and de-aeration chamber system is part of the water supply system in Hong Kong.
Hong Kong has indirect catchment to intercept surface runoff at hilly terrain for water supply. Flow intercepted is conveyed via vortex drop shaft to deep water supply tunnel up to maximum 100m below ground. The water supply tunnel will be surcharged under extreme rainfall. The challenge here is to prevent air entrained at bottom of vortex drop shaft from entering water supply tunnel, which may reduce tunnel capacity or result in gushing at drop shaft.
This was an extremely challenging problem for engineers 60 years ago in 1960s. They could only rely on physical model for the design. However, the estimated air volume in prototype passing into the feeder tunnel increases with increase in scale from 1:30, 1:20 to 1:10. That is, actual volume of air being conveyed to tunnel would be under-estimated by physical model.
There are a number of reasons for the under-estimation:
(i) air bubble size may be of similar size irrespective of scale, so the ratio of the upwards to forward velocities would decrease with the increase in scale;
(ii) increase in model size also resulted in more air bubbles being held in suspension and prevented from coalescing;
(iii) stronger turbulence in larger scale model may result in greater proportion of fine bubbles present
In view of the uncertainty of air entrainment in prototype, the engineer designed the de-aeration chamber based on physical model and then increased length of chamber by judgement finally.
Numerous designs of de-aeration chamber were tested by physical model. One of the intermediate designs tested in 1:20 scale is simulated by CFD to visualize interaction of air and water flow.
The de-aeration chamber in prototype is 18.3m long and 5.8m high. Length of drop shaft is about 70m long. There are ribs of semi-circular shape at roof of de-aeration chamber with 0.6m height to enhance collection of air. There are tiny holes at top of the ribs to allow movement of air at roof of the chamber also.
This is a computationally highly demanding model. As HPC is more assessable nowadays, it is interesting to carry out detailed validation for this highly complex and strong mixture of air-water flow using the detailed physical model results available in literature.

[@abhishekray9476]

It would be great for beginners like me if you would post the tutorial as well . Looking forward to it .

[@EA-uj3ur]

thanks for the upload. This is great. I'm an openFoam enthusiast. I believe the solver is interFoam?

[@gkatie248]

Could you explain plz