Two nozzles of different output diameters (0.20 mm and 0.25 mm) are being used to take mass measurements for an analysis over a time period for the same input pressure.
It has been observed that the standard deviation in the measurements increases in with an increase in diameter of the nozzle.
Is this true? Does an increase in diameter causes greater fluctuations in the flow for the same input pressure?
Please look at the "de Lavel nozzle mechanism" as a realistic application of the "fluid flux conservation" to get a detailed answer for your flow dynamics.
You can consider two parameters:
1) the mass flow rate, if it is taken constant an increase in the diameter causes a diminishing averaged velocity.
2) the Reynolds number, if the averaged velocity is taken constant the Re number increases. From 0.20 to 0.25 you are increasing the 25% and the Re number can be such that transition to turbulence can occur,
well it depends whether the flow in incompressible or compressible. for incompressible flow if pressure remain constant then massflow rate icreases...
In-compressible, Constant pressure, Diameter increases :- result,
decrease in velocity, constant mass, the range of pressure desire the focal point of delivery and micro level losses are more when compared with the existing diameter.
A lot goes into this answer, especially with some parameters missing, such as whether the fluid is compressible or not.
Mr. Karmakar's referencing of the deLaval nozzle is a good place to start. One issue I would want to address is how the standard deviation of flow rate was determined with the differing nozzle diameters. Given the small diameters of the nozzles in question, could the apparent flow fluctuation be a result of measurement uncertainty, that is, the flow rate didn't change appreciably, just the output from the flow rate determination. This instrumentation artifact could present as an unstable flow rate when the root cause is some instability in the flow rate determination.
Just a thought...
What about the measured roughness of the two nozzles? Are the valuies equal in absolute/relative sense? It may happen that the Reynolds number plays a secondary rule, and the pressure drop (therefore the fluid flow) depends on the relative roughness.
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