Venturi meter/ coefficient of discharge experiment

[headshrinker]

1. The problem statement, all variables and given/known data
[LIST]

It was a standard venturi meter experiment where we calculated the theoretical and actual mass flow rates for different pressures and velocity's. the follow up questions were:
-Compare the results of the coefficient of discharge obtained with typical values given in BS.
-Comment on the results for the percentage pressure loss
-Comment on the probable sources of any errors in your results

2. Relevant equations

my problem is not with the maths or equations but the ones used were the standard for mass flow rate and the coefficient of discharge. also:

pressure loss (%) = 100(P1-P11)/P1

3. The attempt at a solution
I have completed all the equations and found that both the coefficient of discharge and the percentage pressure loss decrease as the mass flow rate decreases and i was just wondering what the scientific reason for this was?

can anyone help?

[Jonny6001]

The exact geometry and velocities have a big effect but as the velocity through the orifice changes, the reynolds number of the flow will also change. In general this will mean that the thickness of the boundary layer or rather, the growth of the boundary layer along the pipe will be reduced for higher flow speeds. This will serve to increase the effective orifice diameter from a simplified view.

This should in turn reduce the pressure losses but without knowing more about the specific problem it's not possible to say. There are other factors to consider such as the development of turbulence along the length of the pipe or the method and location of measurements. You could be measuring pressure in a nice laminar attached region at one air speed which separates at a higher speed which means your measurements would be in a separated region of the flow.

[headshrinker]

thanks for your reply, but i still dont understand. why do the percentage pressure losses decrease as the mass flow rate decreases? what do u mean by boundary layers?

thanks