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viscosity of fluid and impact on flow on a fixed supply line

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Old   December 27, 2021, 21:08
Default viscosity of fluid and impact on flow on a fixed supply line
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Hello, First off I do not study, or know much about fluid dynamics and hydrodynamics. However I am working on a project that I am hoping that someone on this message board can help with.

I have a system with a fixed diameter tube that runs into a needle valve and then a flow gauge so that the flow rate can be adjusted and measured "live", as the fluid flows through the pipe. the tube, needle valve and flow gauge are all the same diameter tube/fittings, so only the setting on the needle valve restricts flow. the system is 36in long from supply shutoff to discharge.

I am flowing water from a supply tank "A" to bleed the air out of the system, then shutting off tank "A", which stops flow, then opening tank "B" which is a more viscous, darker fluid to begin flowing toward the discharge.

I know as fluid B moves through the line that there is a mixing that takes place with the water ahead of it. That is not the issue, the issue is fluid B is so dark that I can no longer read the indicator on the gauge once fluid B reaches it.

So my question is... as fluid B enters the suction tube, can the flow rate be adjusted to achieve the target flow rate as the water is passing through the gauge ahead of fluid B, so that when fluid B reached the flow gauge, and it is no longer readable, the flow rate is already set.

I am not looking for a pin point flow rate, of say 90 ccm, but I am looking to have a flow rate between 80 - 100 ccm, so if I target 90 with the water in the line, then when fluid B reaches the gauge I will be within the target range.

In my mind, as fluid B enters the suction line, its viscosity is already impacting the flow of the fluid ahead of it, in this case water, so should the flow of water through the flow gauge read close to the correct amount needed for fluid B?

Any help, opinions, and explanations would be great.
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Old   December 27, 2021, 22:31
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For discussion purposes, if I can assume that all the pipe fittings and plumbing are the same for both the water runner and your mysterious fluid then...

You can use the metered flow rate with water and the head level in the tank (this gives you the driving pressure differential) to determine the head loss coefficient for your system for a particular needle valve position. The head loss is the so-called K-factor you see in fluids texts. The K-factor will be nominally the same for other fluids (although there will be a Reynolds number dependence). You can then calculate the flowrate for your dark liquid, using this K-factor. You can also do this for several needle valve positions to determine the K-factor for each position and generate a prediction for what the dark liquid flowrate will be at any position.

If you can adjust the flowrate using water (where you can read the gauge) for several flowrates, you can even build a K-factor vs Reynolds number curve and then apply this to your calculation to predict what the flowrate will look like.

Now let's say you have an extra 90 degree elbow on your dark liquid tank, you can take this into account by adding it to your K-factor accordingly.
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