Dividing Flow Distribution Across a Perpendicular Junction

[kevinbrighton@bigpond.com]

Dear colleagues,

I'm looking for some help with an application. I have flow along an underground  longitudinal concrete lined cavern under construction, with vertical shafts of smaller cross sectional area. The latter are used for spoil removal and worker access etc. Looking at the simulation, most air escapes through the vertical shaft, which is driving me to add booster fans. However looking at the fan throw, my feeling is much more air should flow bypass  the shaft, based upon it's momentum and the fact that is the boundary layer rather than the peak velocity that is open to the shaft.

My research shows network models that account for the momentum, but what I'm really looking for is a sensible work around in Ventsim that I could use to tune the model.

Thoughts?

[Martin Griffith]

Hi Kevin,  probably the easiest way to finetune, would be to add a custom shock loss to the shaft, on the first tab of the Edit box. This custom shock loss would account for the change of direction of flow needed for air to flow through the exit and for the change in area. This will increase the resistance in the shaft, reducing the flow out of it. 

[kevinbrighton@bigpond.com]

Thanks Martin. Actually I've already tried a shock loss of 1, I'll try a higher one. Is there any sort of rationale I can use as a guide, so that I'm not just guessing?

[Martin Griffith]

You could try setting the shock as Auto High, this should calculate the shock as a function of the 90 degree turn and of the change of area, so should come out greater than 1, Otherwise, the flow will go out there if that is the path of least resistance; so it would pay to have a think about if there is any flow resistance in the shaft that you haven't account for in the model. 

[kevinbrighton@bigpond.com]

Okay thanks Martin I'll check that out.