Air entrainment / Vortice formation

[packdad]

Imagine you have a pipe tee with the branch oriented vertically downward. The tee (which is connected to a piping network) is partially full of water, so that there is a water/gas interface directly above the branch connection.

Now, imagine that you are drawing water down through the branch connection. Given values for pipe size, flow rate, and water level in the tee, you want to be able to predict when gas will start to become entrained into the branch flow. This is important because it flows to the suction of a pump.

How do you predict this? I remember reading an article somewhere, at some time, that said that, as long as the Froude number is less than 1.0, you will not have entrainment due to the dominance of buoyancy forces. Is this always correct? Are there any good references available for a scenario like the one I've described?

 

[Mech85]

Packdad, Refer to the following website:

http:/

http://www.freecalc.com/selffram.htm

This site calculates a self-venting pipe size

The calculation is based on the technical paper: "Designing Piping for Gravity Flow"; Hill; Chemical Engineering; Sept 05, 1983.

As a broad rule of thumb, the Froude Number for vertical downflow service should be less than 0.3 to avoid air entrainment- ie vapour bubbles will rise and the pipe will be self venting. For fully flooded vertical downflow, the Froude number should be greater than 0.6.

 

[BRIS]

I am wondering how you calculate the Froude number in a vertical gravity flow pipe.

fr = V/(g * a/t)^0.5

??

and how it can be less than 1 in a vertical free flow pipe ??

I suggest that the criteria may relate to free surface flow in sloping pipes rather than vertical pipes.

Brian

 

[Mech85]

Dear Bris,

The Froude number for vertical downflow pipe is calculated as follows :

Fr = V/SQRT(gu*D

Where
V = Velocity (m/s)
gu = g(Sl - Sg)/Sl
Sl = density of liquid (kg/m3)
Sg = density of gas or vapour (kg/m3)
g = 9.81 m/s2
D = internal pipe diameter (m)

Refer to the technical paper: "Designing Piping for Gravity Flow"; Hill; Chemical Engineering; Sept 05, 1983. It uses the term J for dimensionless volumetric flux which equates to Froude number

 

[BRIS]

Dear Perth

Thanks for the info – One of the benefits of the forum is the free exchange of ideas between disciplines, particularly between discipline that you would not normally work with. My discipline is civil hydraulic engineering.

The use of Froude number as a relationship for the design of vertical pipes is new to me although I have designed a number of vertical shaft spillway but typically comprising 6.0m diameter pipe.

I would be interested in seeing the technical paper by Hill – do you know how I can access a copy?

(Where is V measured - is it the real free fall velocity or an equivelant velocity V/a ?)

I apologise for straying from the original question.

Regards Brian

 

[packdad]

Thanks for the input. I'll try to get a copy of that paper, as well.

In the meantime, I did manage to find the report that I originally had in mind. It's actually a Westinghouse nuclear publication - WCAP-11916. It concerns taking a suction from the bottom of a partially-filled reactor coolant loop. It finds critical submergence ratios as they are related to pipe orientation and Froude number.

It does not appear to be a proprietary document, but I also doubt it is widely available.

 

[BRIS]

I meant equivalent velocity q/a (flow divided by full flow area)

not equivelant and not v/a - a bad tryping day