# Homework Help: Flow rate measurement: Venturi vs Sharp edged circular orifice. Help!

1. Oct 20, 2011

### davekardle

Flow rate measurement: Venturi vs Sharp edged circular orifice. Help! :)

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

Hello guys, I am having trouble answering these questions.
I know the the discharge coefficient for a venturi is always higher
than that of the sharp edge orifice. Can you guys help me out with the following questions ?

1. Think about the head profiles and the loss of head from inlet to outlet of the venturi tube in P1. Are any experimental values of Hn greater than the theoretical values (Hn)t? If so, can you suggest a reason? (Look at the pressure tappings in this region).

2. What is the head lost in the venturi and is this significant compared with the total theoretical head? How does this loss of head arise?

3. (a) Why is Cd quite close to 1 for the venturi, but significantly less than 1 for the sharp-edged orifice? Compare with values of Cd in the text books.

(b) Is Cd constant within the range of each experiment?

(c) Would the meter need recalibration if it were used with a fluid of different viscosity or density? (Does Cd depend on the fluid properties)?

4. Why is the jet narrower than the orifice?

5. What are the advantages and disadvantages in practice of venturis and orifice plates as flow meters? In what circumstances is each most often used?

2. Relevant equations

3. The attempt at a solution

2. The loss is caused by friction.

3a: In venturi the Cd equals to the actual value/ theoretical value. It is typical 0.98. WHEREAS for the Sharped edge orifice, the Cd = Cc x Cv . Cc is the coefficient of contraction ( typical 0.60).

3b. Yes

I am really clueless about these questions guys, help me out ;(

2. Nov 4, 2014