"Venturi Meter" this term is very famous in the Mechanical Engineering field, but do we actually know how it is work? Today after reading this article you will get an idea about Venturi Meter, it's working principle, parts and applications. So let's get started with the definition.
What is a Venturi Meter?
Venturi Meter is a device in which pressure energy is converted into kinetic energy and it is used for measuring the rate of flow of liquid through pipes. It is invented by an American Engineer Clemans Herchel and named by the Italian physicist Giovanni Venturi. It works on the basic principle of Bernoulli's Equation.
Parts of Venturi Meter:
A Venturi Meter is consisted of:
Converging cone or Diameter (the area is decreasing).
Throat Diameter (the area is constant).
Diverging cone (the area is increasing).
let's consider a pipe in which there is a venturi meter is fixed. In the pipe, fluid is flowing so first it enters into a converging cone then Thorat and then Diverging Cone.
When water flowing through this cone the area is decreasing, therefore, the speed of flowing water increases and pressure decreases.
When water flowing through this cone the area remains constant therefore the speed of flowing water and pressure remains constant.
When water flowing through this cone the area is increasing, therefore, the speed of flowing water decreases and pressure decreases.
Working Principle of Venturi Meter:
As I already told that Venturi Meter works on Bernoulli's Principle, so let's find out how it depends on Bernoulli's Principle.
Suppose the quantity of liquid v1 enter to the pipe, as per continuity equation volume flow rate at the inlet (Q1), is equal to discharge at the outlet (Q2), so if v1 amount of water enters to the inlet of the venturi meter the same amount of water should be discharged at the outlet, that means at unit second v1/t1= v2/t2.
As the area of section 1 (according to the above diagram) is more than the area of section 2, that means due to the decrease area the pressure at throttling section is decreased and velocity will be increased to maintain the flow (Q1=Q2).
In the throat position, the velocity of flow is maximum and pressure is minimum.
After throttling there again a diverging cone (diffuser) which restores the pressure as nearly possible to the actual value.
By this, we can easily determine the volume flow rate with the help of the U-Tube Manometer which is shown in the above diagram, by finding the pressure difference between section 1 (converging section) and section 2 which is throat.
Derivation of Discharge:
The several notations use in this derivation:
A1= Inlet area in m2.
D1= Diameter of Inlet.
D2= Diameter of the throat.
A2= Throat area in m2.
P1= Pressure at the inlet in N/m2.
P2= Pressure at the throat in N/m2.
v1= Velocity at inlet in m/sec
v2= Velocity at throat in m/sec.
h= Pressure heads.
Cd= Coefficient of Discharge. This is unitless.
Qact= Actual discharge in m3/sec.
Qthe= Theoretical discharge in m3/sec.
Applying Bernoulli’s equations at sections 1 and 2, we get:
As pipe is horizontal Z1= Z2,
Where [h= (p1-p2)/ρg], difference of pressure heads at sections 1 and 2.
From the continuity equation at sections 1 and 2, we get,
This expression is the Theoretical Discharge of Venturi Meter. In general actual discharge is always less than Theoretical Discharge. So if we multiple Cd (Coefficient of discharge to the above equation, then we get an actual discharge, and here is the expression of actual discharge,
The other way to find h (Pressure heads) by using differential U–Tube Manometer:
The liquid in the manometer is heavier than the flowing fluid in the pipe.
Sh= Specific gravity of the heavier liquid.
x = Difference of the heavier liquid column in U-tube
S0= The Specific gravity of flowing fluid.
Sl= Specific gravity of the lighter liquid.
h = x [ (Sh / S0) – 1]
The liquid in the manometer is lighter than the flowing fluid in the pipe.
h = x [1- (Sl / S0)]
Applications of Venturi Meter:
Venturi Meter is used in various field like:
Calculating the flow rate of fluid that is discharged through the pipe.
In the industrial sector, it is used to determine the pressure as well of the quantity of gas and liquid inside a pipe.
The flow of chemicals in pipelines.
This is widely used in the waste treatment process where large diameter pipes are used.
Also used in the medical sector for the measure the flow rate of blood in arteries.
This also used where high-pressure recovery is required.
Advantages of Venturi Meter:
The advantages of Venturi Meter are:
Power loss is very less.
This can be used where a small head is available.
High reproducibility (the extent to which consistent results are obtained when an experiment is repeated).
Accuracy is high over wide flow ranges.
This can also be used for a compressible and incompressible fluid.
This device is easy to operate.
The coefficient of discharge (Cd) for the venturi meter is high.
This is widely used for a high flow rate (Discharge).
Disadvantages of Venturi Meter:
Although there are few disadvantages of Venturi Meter, and those are:
The installation cost of a venturi meter is high.
There are little difficulties while maintenance.
This device can not be used where the pipe has a small diameter of 76.2 mm.
This system occupies more space as compared to the orifice meter.
It has a limitation of the lower Reynolds number of 150,000.
It is expensive and a little bulky.
So this is all about Venturi Meter, I hope you understand the concept of venturi meter and also got an idea about how it works. If you like this stuff then do not forget to share this article on your favorite social media platform. I will see you in any other article, till then Stay healthy and keep learning Mechanical Engineering with us.
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