Water flow venturi meter problem Heeeelp please

In summary: H2O to psi. Or find a conversion table online. From there, it should be easy to figure out the pressure change.In summary, Krisany was trying to figure out how to calculate the discharge coefficient for a venturi tube, but she wasn't sure what Cb was. She was also having trouble reading her results page. She found that M=A_{2} \sqrt{\frac{2 \Delta P \rho}{1-k^2}} and C_{d}=M_{actual}-M_{theoretical}. She was also not finding the pressures, and she measured them in her experiment. Lastly, she needed to find conversions from mmH2O to psi or...
  • #1
fcukniles
26
0
water flow venturi meter problem! Heeeelp please :)

Hi,
The objects of my experiment where
1) to find out the theoretical and experimental pressures in a venturi tube,
2) To determin the coefficient of discharge for the venturi

here is the data i collectec http://cniles0.tripod.com/work020.jpg

im having problems with my two objectives,
i have i have been playing arround with the following formula's but with no luck,
.
m = A2 [sqrt (2 x delta p x viscosity)/1-k^2

K=A1/A2

therefore K = 2.63998

.
V = 201.1 x sqrt [2 x delta p x 1 (waters viscosity coeffient)]/[1 - 2.64^2]

Here is the hand out i got, giving some formula's and it gives cross sectional values for the venturi tube but not sure what do do with these?

http://cniles0.tripod.com/venturi_water.doc

i have worked out the experimental rh but i don't know how to find out the therotical rh, and I am not sure what Cb (show on results table is).



thanks for any help ppl
Kris
 
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  • #2
any help would be really apricated thanks
 
  • #3
I'm not sure what you are measuring in your experiment...velocity I assume. Here's what I would think you would be doing.

You have a given Q (flow rate), and you are looking for coefficient of discharge. Well based on continutity, you know what the flow rate at any point along your tube must be the same. So given the area upstream of the venturi, you can find pressure and velocity using Bernoulli's Equation, and then Q=VA. Now using that last equation, you can see that if flow rate stays constant (which in your case it does), any change in the area will yield a proportional change in the velocity. With your new velocity, plug that back into Bernoulli's Equation to solve for a theoretical pressure. Now take your actual measured velocity and find your actual pressure. Divide these to get your coefficient.
 
  • #4
i have worked out the experimental rh but i don't know how to find out the therotical rh, and I am not sure what Cb (show on results table is).

I am not quite sure what your question refers to. What is "rh?" What is "Cb?" Do you mean Cd (which is the discharge coefficient)? Also, I am having a lot of troubles reading your results page that you scanned in.

So...let's talk in general.

The theoretical flow through the venturi is given by the equation you have already mentioned:

[tex]M = A_{2} \sqrt{\frac{2 \Delta P \rho}{1-k^2}}[/tex]

That is the theoretical flow (in a perfect world). With the measurements you have you should be able to calculate both the theoretical and what actually happened. There will be a difference. That difference is the discharge coefficient of the venturi:

[tex] M_{actual} = M_{theoretical} C_{d} [/tex]

I guess the first big question is do you understand what the data you took is for and where it fits into the venturi equation?
 
  • #5
yeh how do i work out the coefficient of discharge?
thanks
 
  • #6
I think I explained it decently well, but let me try again.

Your coefficent of discharge is a fraction that tells you how much flow you will actual get based on theoretical. Qactual = Qtheo*C

To get your theoretical flow, you will use a combination of Bernoulli's Equation, and the equation for flow, flow simply being Q=VA.

Since you have your areas and initial velocity, finding your theoretical flow through the venturi should be simple. It seems to me that you are measuring velocity in your experiment so...take your measured velocity and multiply by area to get your actual flow. Divide your theoretical by your actual to get the coefficient.
 
  • #7
im not quite sure how to find out the theoretical and experimental pressures in a venturi tube is it something to do with the thickness of the tube and the high difference?
 
  • #8
heres what am getting at the minute and it doesn't seam right:

A1=201.1 (area of crossection)
A2=530.9
K=0.37879
DeltaP= 195

Putting these numbers into the equation i get 17912.11 which is nothing like any of my other values any tell me what this means or where i have gone wrong?
thanks
 
  • #9
What are the units of your [tex]\Delta P[/tex]? That large of a number looks like possibly inHg or inH2O. You must use the proper units.

Also, you are not finding the pressures. You measured them in your experiment! You are calculating the theoretical flow using your measured pressures and comparing it to what you actually measured in the bucket.
 
  • #10
i measured the height difference in each of the tubes, these values are in mm how do i work out the pressure change? thanks for help :)
 
  • #11
millimeters of what? Water I am assuming. You need to use consistent units with your calculations. Look at the units you have for density and area.

Look for conversions from mmH2O to psi or Lbf/ft^2.
 
  • #12
He is measuring the mass flow rate. So the

Volume flow rate = 1/density * Mass flow rate

Isn't this the experimental flow rate?

So he is left with finding the theoretical flow rate.

fcukniles:
Do you have a diagram of your experimental set up? What are p1, p2 .p11? Difficult to read your data.
 
  • #13
Since there seems to be some confusion...here's what I see as confusing you:

The mass flow in the tube at any point along the line has to be the same (conservation of mass). Therefore, if you take any two points where you did the measurements, you should calculate the same theoretical mass flow rate. So, for example, take station #1 (the entrance to the venturi) and station #4 (the throat section, i.e. smallest area):

- You measured a mass flow rate of .424 kg/sec (trial #2)

- The [tex]\Delta P[/tex] is the absolute value of the difference between two pressure readings. In your case, P1=222 mmH2O and P2=20 mmH2O. You HAVE TO convert these readings to the proper units. Since this is in metric, that would be into Pascals or N/m^2 (the unit of Newtons can be further broken down into kg*m/s^2. That leaves you with an overall pressure unit of (kg*m)/(s^2*m^2))

- The area ratio, k (what is normally called the "beta" ratio) is the ratio of the smaller area to the larger area, or this case, 201.1/530.9. NOTE: Since this is a ratio, you don't have to worry about the units here. No matter what units you use, the answer will be the same.

- Since I didn't see it anywhere, I am assuming you used a value for the density of water of 1000 kg/m^3. Again, you have to use the proper units!

NOW you can calculate the THEORETICAL mass flow through the venturi. Once you get that value, compare that value to the measured mass flow rat to calculate the Cd.

I did run the numbers that you had for this trial and they came out pretty good. The Cd I calculated is along the lines of what I would expect for a venturi meter.
 
Last edited:
  • #14
how do i convert them? I've been playing around with the numbers but no luck, nothing that is close to 0.450
 
  • #15
ok I am lost...


me = wild sheep in new york
 
  • #16
baaaaaaaaaa
 
  • #17
The equivalence for pressure at the some height of water can be found from the relationship P = [itex]\rho[/itex]gh. Units have to be consistent.

However,

1 atm = 1.013 bar = 1.01325E+5 Pa = 760 mmHg = 1.01325E+4 mmH2O = 1.033 kg/cm2 = 14.696 psi

1 Pa = 1E-5 bar = 9.869E-6 atm = 0.0075 mmHg = 0.1 mmH2O = 1.02E-5 kg/cm2=0.000145 psi

Calculator for pressure - http://www.lenntech.com/unit-conversion-calculator/pressure.htm

Just put a number in one of the boxes and hit calculate.
 
  • #18
ok so if:
A1 = 30mm => 300 N/m^2
A2 = 195mm => 1950 N/m^2
k= 0.15385
so puttin these into the formula:

1950 x [sqrt (2x1650x1000)/1-0.15385^2]

3569835.251 ?

pretty sure this aint right!

any help?
 
  • #19
The "A" variables are the AREAS of the sections you are considering. How are you going from A in 'mm ' to A in N/m^2?

Short of actually doing it for you, I would suggest you post the work you have done. Every step. Then we can show you where you are taking the wrong steps.
 
  • #20
ok right here is what i think I am doing:
if A1= 221.7mm^2 A2= 530.9mm^2 need to convert these into N/m^2
so i divide these by 1000. Giving me A1 = 0.2217N/m^2 A2= 0.5309N/m^2.
so k = 0.4176. Putting the Change in height 165 mm into this http://www.lenntech.com/unit-conversion-calculator/pressure.htm I get Delta P to =1650.
So... putting these numbers into the formula I get...

0.5309N/m^2 x sqrt([2 x 1650 x 1000]/ 1-0.4176^2])

Ans 1061.4 ?
 
  • #21
The areas are there to calculate k (re read your handout, at the bottom of page 3 it says "The ratio of A2/A1 is often represented by the symbol 'k'" :

k = A4/A1
k = 221.7/530.9
k = .418

That gives you one piece you need to solve the flow equation.

The pressures are what you read from the manometers in mmH2O at the 11 points along the venturi, i.e. 222 mmH2O and 20 mmH20. These are the values that you need to convert into Pascals (N/m^2). Once you do the conversion, take the difference between the two values and that will be your [tex]\Delta P[/tex] which is another piece of the equation.

Keep trying. We'll get you there.
 
  • #22
ok right here is what i think I am doing:
if A1= 221.7mm^2 A2= 530.9mm^2 need to convert these into N/m^2
so i divide these by 1000. Giving me A1 = 0.2217N/m^2 A2= 0.5309N/m^2.


1000 mm in 1 meter
so how many mm in 1 meter squared?
 

1. How does a water flow venturi meter work?

A water flow venturi meter works by using the principle of fluid mechanics to measure the flow rate of water in a pipe. It consists of a constricted section (known as the venturi) in the pipe, which causes the water to speed up and decrease in pressure. This pressure difference is then measured, and the flow rate can be calculated using a specific formula.

2. What are the common problems with water flow venturi meters?

Some common problems with water flow venturi meters include clogging, inaccurate readings due to changes in temperature or pressure, and incorrect installation or calibration. It is important to regularly maintain and calibrate the meter to ensure accurate readings.

3. How can I troubleshoot a water flow venturi meter problem?

If you are experiencing issues with your water flow venturi meter, the first step is to check for any clogs or blockages in the venturi section. You should also ensure that the meter is installed correctly and calibrated properly. If the problem persists, it may be necessary to consult a professional for further troubleshooting.

4. What is the formula for calculating flow rate using a venturi meter?

The formula for calculating flow rate using a venturi meter is Q = A1V1 = A2V2, where Q is the flow rate, A1 and A2 are the cross-sectional areas of the pipe before and after the venturi, and V1 and V2 are the velocities of the water before and after the venturi.

5. Can a water flow venturi meter be used for all types of fluid?

No, a water flow venturi meter is specifically designed for measuring the flow rate of water. It may not be suitable for other types of fluids, as their viscosity and density can affect the accuracy of the measurements. It is important to use a meter that is specifically designed for the type of fluid being measured.

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