Non-Steady State Venturi Flow
Non-Steady State Venturi Flow
(OP)
Most of my fluid dynamics classes dealt with nice and neat steady state flow problems, but it turns out that now I need to know about how a system will start up.
Dilemma: I have a system where I have an inlet and then two flow paths (the fluid is water, say at room temperature). The goal is to have the water in branch A reach the 'same' endpoint/reservoir before branch B. However, I need branch B to have a much larger flow rate than branch A when they finally meet at the reservoir. I have limited space so just lengthening branch B could be an option, but it would be very difficult. Furthermore, the process being used to create the geometry is difficult to control (+/- 0.05").
One thought I had is to have a series of venturi orifices in branch B order to slow down the flow initially, but when it reaches steady state, the losses will be minimal. Is it worth it to perform a test?
Just to give you an idea of the dimensions: branch B has a diameter of ~0.75" and a length of ~18"; branch A has 2 sharp-edged orifices of diameter ~0.25" then goes into a chamber and then into 20, ~0.2" diameter sharp orifices (imagine a hose with holes in it).
There exists a proper 'ratio' of the two flow rates, but that is during steady state.
Alternate ideas on how to solve this problem would be wonderful BUT I'm mostly interested in an evaluation of my series venturi's idea.
-Con
Dilemma: I have a system where I have an inlet and then two flow paths (the fluid is water, say at room temperature). The goal is to have the water in branch A reach the 'same' endpoint/reservoir before branch B. However, I need branch B to have a much larger flow rate than branch A when they finally meet at the reservoir. I have limited space so just lengthening branch B could be an option, but it would be very difficult. Furthermore, the process being used to create the geometry is difficult to control (+/- 0.05").
One thought I had is to have a series of venturi orifices in branch B order to slow down the flow initially, but when it reaches steady state, the losses will be minimal. Is it worth it to perform a test?
Just to give you an idea of the dimensions: branch B has a diameter of ~0.75" and a length of ~18"; branch A has 2 sharp-edged orifices of diameter ~0.25" then goes into a chamber and then into 20, ~0.2" diameter sharp orifices (imagine a hose with holes in it).
There exists a proper 'ratio' of the two flow rates, but that is during steady state.
Alternate ideas on how to solve this problem would be wonderful BUT I'm mostly interested in an evaluation of my series venturi's idea.
-Con





RE: Non-Steady State Venturi Flow
RE: Non-Steady State Venturi Flow
Thanks in advance,
-Con
RE: Non-Steady State Venturi Flow
http://virtualpipeline.spaces.msn.com
"What gets us into trouble is not what we don't know, its what we know for sure" - Mark Twain
RE: Non-Steady State Venturi Flow
On startup "A" would be full and fluid would exit from the discharge as soon as new fluid entered the pipe. You might need some sort of weir with a drain hole in it at the end of "B" but it should be possible to use the whole sloping section as a reservoir that has to be filled on start up.
Or just open the valve to "A" before you open the valve to "B"
Harvey
Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com
RE: Non-Steady State Venturi Flow
I understand what you are saying about the pressure losses vs. resulting velocities. I was trying to minimize pressure loss with the venturi's (perhaps I don't really show that in the sketch, sorry) but keep the steady state flowrates the same. My goal is really to just delay the flow in the beginning and not introduce any real restrictions for the flow.
Another correction about the sketch: the resulting flows from branch A and B end up going out to atmosphere are there really isn't any backpressure at the exit of B other than a few inches of water.
In terms of laminar vs. turbulent, I don't think that's possible since the inlet flowrate is >10gpm and the ratio of flow through branch A to B is between something like 1:4 to 2:3. That ratio is important.
Thanks!
-Con
RE: Non-Steady State Venturi Flow
http://virtualpipeline.spaces.msn.com
"What gets us into trouble is not what we don't know, its what we know for sure" - Mark Twain
RE: Non-Steady State Venturi Flow
Yes, having the water in the reservoir on shutdown would help. That is a good idea. We have generally believed that having standing water, however, would invite bacteria, mold, or a fungal bloom. BUT, we already have standing water at the output of the two branches and do not have issues/complaints. The difference is that the branch A reservoir does not receive the proper coating that the reservoir of the two branches receives and cannot in the process.
This is worth looking into. It's nice when people can look at a problem objectively!
Now, tell me about my friend Venturi. :)
-Con
RE: Non-Steady State Venturi Flow
Having little accumulators was the idea, it's just due to the lack of space I drew them the way I did. You are correct.
-Con