Fluid Mechanics Flowrate Question
Fluid Mechanics Flowrate Question
(OP)
I am working on a fluid mechanics problem within a piping system and could use a little help:
There is a piping system that starts as a singular line and then branches out into two lines (equal pipe sizes). I have a known initial flowrate; and I realize that (assuming incompressible flow) the flowrate should remain constant. Additionally, I understand that (given the branch areas are equal), each branch experiences a constant flowrate that is 50% of the initial flow. However, in one of the branches there is a valve that is open only a very small amount, enough to still leak fluid through that valve. If I'm just using flowrates and not using Bernouilli's energy equation, does that branch still experience exactly 50% of the original flowrate? Perhaps there's something I'm overlooking because it doesn't seem as though that is possible. Thank you in advance for any input!
There is a piping system that starts as a singular line and then branches out into two lines (equal pipe sizes). I have a known initial flowrate; and I realize that (assuming incompressible flow) the flowrate should remain constant. Additionally, I understand that (given the branch areas are equal), each branch experiences a constant flowrate that is 50% of the initial flow. However, in one of the branches there is a valve that is open only a very small amount, enough to still leak fluid through that valve. If I'm just using flowrates and not using Bernouilli's energy equation, does that branch still experience exactly 50% of the original flowrate? Perhaps there's something I'm overlooking because it doesn't seem as though that is possible. Thank you in advance for any input!





RE: Fluid Mechanics Flowrate Question
At the point where the pipe branches, the pressure or head is equal. If the end point of the branches is the same height and pressure then the flow will depend on the friction losses in the branches. If one branch has a higher friction loss due to a "valve that is only open a small amount" than the other then it will have less flow than the other. All depends on the friction loss between the two branches. Internal size, length, material, fittings etc all have an impact on the friction losses.
Think electricity as an analaogy. You couple up two identical wires to the same juncton box, but in one you place a large resistor in line and then couple up the wires again. Which has the most current (flow)?? It cetainly isn't equal current lfow...
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: Fluid Mechanics Flowrate Question
Good luck,
Latexman
Need help writing a question or understanding a reply? forum1529: Translation Assistance for Engineers
RE: Fluid Mechanics Flowrate Question
TTFN

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Need help writing a question or understanding a reply? forum1529: Translation Assistance for Engineers
RE: Fluid Mechanics Flowrate Question
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: Fluid Mechanics Flowrate Question
in reality it is a function of reynolds number, fluid distribution, piping layout, etc., even if you are nursing a head cold, a great essay question for an exam
RE: Fluid Mechanics Flowrate Question
Gigawatts,
You make a statement that makes me nervous..
The continuity equation has no restrictions on incompressible flow, constant density, or any other flow parameter. That equation says that the mass flow rate must be constant within any line that does not add or remove fluid. So if you measured the mass flow rate in your system before the branch and after the lines come back together the mass flow rates MUST be identical. If they are not then you either have a leak, new fluid coming int, or a measurement error.
Also, Bernoulli only applies to pipe flow in cases where the distance traveled is VERY short pipe runs (inches, and not very many inches). It allows the AGA-3 equations to work because the distance between the ports is 1-inch which is too short for friction to be material. Any pressure drop due to friction at all invalidates the "inviscid flow" and the "no work is done on or by the gas" assumptions.
David Simpson, PE
MuleShoe Engineering
"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
The plural of anecdote is not "data"
RE: Fluid Mechanics Flowrate Question
Depending on the discharge coefficient, you end up with anything between shut off head and total head loss.
RE: Fluid Mechanics Flowrate Question
Is this a hydraulic 101 homework problem?
Q. " does that branch still experience exactly 50% of the original flowrate?"
A. No, how can it see 50% of the total the flow rate when in effect the line is closed off, the flow can only equal the leakage rate.
However, the pump will be trying to pump the total flow (50% + 50%) thru' the branch line still open, at this point neglecting any leakage thru' the closed valve on the other branch line, the result being - it will force the pump left on its operating curve and could result in the pump operating in an unstable point on its curve.
You need to calculate a system curve (head loss) for the normal operating condition, or at least know what flow and head was used for the pump selection -- a second system curve must be calculated for the system when operating with one leg closed - to see what this does to the pump operation in terms of where on its curve it is going to operate.
It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)
RE: Fluid Mechanics Flowrate Question
Good luck,
Latexman
Need help writing a question or understanding a reply? forum1529: Translation Assistance for Engineers
RE: Fluid Mechanics Flowrate Question
Good luck,
Latexman
Need help writing a question or understanding a reply? forum1529: Translation Assistance for Engineers
RE: Fluid Mechanics Flowrate Question
I realize that it wasn't logical to believe both branches received 50% of the initial flow rate, but I was being tripped up by thinking that by having a barely open valve in one branch (using Q=VA) this would just mean the velocity increases as the area shrinks to keep the flow rate constant. I see the error in that now since I was thinking of the fluid's initial entry to the branch line as opposed to the steady state where the many head losses would serve as a resistance to that flow.
Moving forward, I suppose the best way to approximate the volumetric flow rate through the branch with the failed valve is to use the electrical circuit analogy and approximate (unfortunately I don't have all the data available for this system) an equivalent line resistance for each of the branches and use the ratio of resistances to determine the percentage of the initial flow rate that each branch will experience. Does this seem sensible? Thanks again for your help.
Also, Zdas04, I realize the continuity equation has no restrictions on incompressible flow, but I am looking at a volumetric flow rate and not a mass flow rate. As such, I believe compressibility would be a factor since it can affect the volume of a fluid, but of course leaves the mass unchanged.
RE: Fluid Mechanics Flowrate Question
For branches with very different flows, you could probably estimate at the start that the one with very little flow has negligible friction losses in the pipe and all the pressure loss is concentrated across the valve where the upstream preassure is the same as that existing at the branch. As flow increases then this ceases to be valid.
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way