Venturi meter/ coefficient of discharge experiment

In summary: In general, the thicker the boundary layer, the more drag the fluid exerts on the walls of the pipe, and the higher the pressure losses will be. Additionally, as the velocity through the orifice changes, the Reynold's number of the flow will also change, and this will affect the thickness of the boundary layer.
  • #1
headshrinker
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Homework Statement




  • 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

    Homework 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

    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?
 
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  • #2
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.
 
  • #3
thanks for your reply, but i still don't understand. why do the percentage pressure losses decrease as the mass flow rate decreases? what do u mean by boundary layers?

thanks
 

1. What is a Venturi meter and what is its purpose?

A Venturi meter is a device used to measure the flow rate of a fluid in a pipe. It consists of a constriction in the pipe, which causes a pressure difference between the inlet and outlet. This pressure difference can be used to calculate the flow rate of the fluid.

2. How does a Venturi meter work?

A Venturi meter works based on the principle of Bernoulli's equation, which states that as the velocity of a fluid increases, the pressure decreases. In a Venturi meter, the fluid is forced to speed up as it passes through the constriction, causing a decrease in pressure. This pressure difference can be measured and used to calculate the flow rate.

3. What is the coefficient of discharge and why is it important?

The coefficient of discharge is a dimensionless constant that represents the relationship between the actual flow rate and the theoretical maximum flow rate in a Venturi meter. It is important because it accounts for any losses in the system, such as friction or turbulence, and allows for more accurate flow rate calculations.

4. How is the coefficient of discharge experiment conducted?

The coefficient of discharge experiment involves setting up a Venturi meter in a pipe with known dimensions and a steady flow of fluid. The pressure difference between the inlet and outlet of the meter is measured using pressure gauges. The flow rate is also measured using a flow meter. By using these measurements, the coefficient of discharge can be calculated.

5. What are the sources of error in a Venturi meter experiment?

Some potential sources of error in a Venturi meter experiment include incorrect measurements of the pipe dimensions, variations in fluid density and viscosity, and improper placement of the pressure gauges. Additionally, any leaks or obstructions in the system can affect the accuracy of the measurements and result in errors in the calculation of the coefficient of discharge.

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