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Old Nov 8th 2014, 07:44 AM   #1
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Question Coefficient of velocity

Hi there!
I have done an experiment to calculate the coefficient of velocity of a jet flow through an orifice, and I got results with an interesting pattern. So I have done the experiment with two different heads and orifice sizes and got the following results for the coefficient of velocity:
For high head (0.390m):
3mm orifice-0.971
6mm orifice-0.930
For low head (0.300m)
3mm orifice-0.922
6mm orifice-0.940
This made me wonder why high head and small orifice is more efficient, and low head large orifice is more efficient? Is there a relationship between them? or there is just an error in the experiment?
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Old Nov 10th 2014, 04:27 AM   #2
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I would expect that the velocity of flow through the larger orifice would be greater than flow through the smaller orifice, based on it having less drag from the edge of the orifice. And it makes no sense for the flow through the 6mm orifice to be slower under the high head condition than the low head - are you sure of that reading?

It's impossible to say whether there is an error in the experiment without knowing more about how it was set up and data taken. How did you maintain the pressure head? How did you measure velocity? How many readings did you take? Is your flow meter properly calibrated?

Last edited by ChipB; Nov 10th 2014 at 08:31 AM.
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Old Nov 11th 2014, 05:44 AM   #3
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I took readings of y (vertical distance of the trajectory) and x (horizontal distance of the trajectory). The head with a overflow tube to keep the head constant. the velocity is calculated by Bernoulli equation v=sqrt(2gh) and the coefficient of velocity is calculated by the gradient of sqrt(yh) vs x graph.
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Old Nov 11th 2014, 10:11 AM   #4
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Originally Posted by CosecX View Post
I took readings of y (vertical distance of the trajectory) and x (horizontal distance of the trajectory). The head with a overflow tube to keep the head constant. the velocity is calculated by Bernoulli equation v=sqrt(2gh) and the coefficient of velocity is calculated by the gradient of sqrt(yh) vs x graph.
Please define coefficient of velocity - is it a ratio of actual velocity to Bertnoulli's sqrt(2gh)? And please define the calculation for coefficient of velocity, as I don't understand what you mean by "gradient of sqrt(yh) vs x." Using the apparatus shown I would think you would determine v from

v= x sqrt(g/(2 delta_y))
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Old Nov 11th 2014, 05:12 PM   #5
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Just a guess.

Perhaps if you expand the experiment with a range of heads for each orifice it might produce a fuller picture of what is happening.
Plot a curve of head against velocity with five or six different heads and see what you get.
I am guessing, from the limited results you have posted, that the relationship between head and velocity for a given orifice size might be non-linear.
There is possibly also a non-linear relationship between velocity and orifice size for a given head.
If this is the case, then you might find there is an optimum head for any given orifice size (and visa versa an optimum orifice size for any head).
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