How is the Energy Increasing in a Venturi Experiment?

[lefteris28]

Homework Statement

Hi
I have done an experiment applying the bernoulli equation on a venturi tube, according to bernoulli the energy must remain the same. However according to my calculation the energy increases when the tube contracts up to the point of the throat. Can someone explain how it is possible?

Homework Equations

p/ρg + v²/2g = total energy

The Attempt at a Solution

 

[JaredJames]

Can you show your calculations? It would make it a lot easier to see where the problem is.

 

[lefteris28]

Can you show your calculations? It would make it a lot easier to see where the problem is.

I can sent my excel file, the problem is that i have to explain how is that possible. Only if there was back pressure would cause an increase in head of fluid and conseqently the total energy.

 

[JaredJames]

Can I ask where you pressure is on that spreadsheet?

Is the experiment horizontal or vertical?

You have "head" in metres, but nothing to give you a pressure in your calculations.

 

[lefteris28]

Can I ask where you pressure is on that spreadsheet?

Is the experiment horizontal or vertical?

You have "head" in metres, but nothing to give you a pressure in your calculations.

head of fluif plus head of velosity = total energy . The experiment is horizontal.

 

[JaredJames]

Well your units are all to pot.

You have head in metres and velocity in m/s which, when added together give you another value in m/s which isn't energy.

 

[lefteris28]

Well your units are all to pot.

You have head in metres and velocity in m/s which, when added together give you another value in m/s which isn't energy.

The formula we use is that P/ρg + v²/2g +z=total energy
P/ρg= head of fluid v²/2g = head of velocity

 

[JaredJames]

The formula we use is that P/ρg + v²/2g +z=total energy
P/ρg= head of fluid v²/2g = head of velocity

Yes, but your units on the spreadsheet don't match this so it's difficult to see what goes where and does what.

 

[lefteris28]

Yes, but your units on the spreadsheet don't match this so it's difficult to see what goes where and does what.

This formula was given to calculate the total energy. The velosity head is also in meters. As i said before the height of fluid in piezometer tube is equal to p/ρg or pressure=hρg so we have head of fluid in meters plus head of velosity in meters.v²/2g gives meters. m²/s² χ s²/m = meters. My problem is that i can not justify what caused the energy gained in the contraction the only i could say is that there was back pressure that in turn causes an increase in height of piezometer.