Fluid Flow Measuring

Instruments Guided by: Mr. Kartik Ayer

Prepared by: Gaurav Dave(130990136011)

Contents Pitot tube

Introduction Working principle Construction and working Types of Pitot tube Applications

Current meter

Introduction Working principle Construction and working

Pitot Tube

• Pitot tube was invented by the French engineer Henri Pitot in the early 18th century.

• Pitot tube is a pressure measurement instrument used to measure fluid flow velocity.

• It is used for measuring the velocity of liquid flow at any point in a pipe or a channel.

Working principle of Pitot tube

• “If the velocity of flow at a point becomes zero, the pressure is increased there due to the conversion of the kinetic energy into pressure energy.”

• The point at which the velocity of flow becomes zero is called stagnant point.

• The pressure at stagnant point is called total pressure or head or stagnation pressure.

Construction and working

• Pitot tube consists of L-shaped glass tube, a tube bent at 90o and with the ends unsealed.

• The horizontal part is called ‘body’ and the vertical is called ‘stem’.

Construction and working

• p1 = pressure at point (1)

• V1 = velocity at point (1)

• P2 =pressure at point (2)

• V2 = velocity at point (20

• H = depth of tube in liquid• h = rise of liquid above the

free surface of liquid in tube

Velocity measurement and Coefficient of pitot tube

• Applying Bernoulli’s equation at point (1) and (2),

Datum head z1 = z2

V2 = 0Therefore, = H & = H + h

= hThus V1 = The above equation gives the theoretical velocity at point (1)Actual velocity,V1act = Cv

Continued…

The above equation gives the theoretical velocity at point (1)Actual velocity,V1act = Cv Where Cv is the Coefficient of pitot tube

Cv 0.96 to 0.99

Stagnation or Total HeadIt is the sum of static head and dynamic head. • Total head:

• Total pressure at point (1): = static + dynamic pressurep1 = p2 – ⍴• Total pressure at point (2): p2 = + p⍴ 1

Static pressure

“ It is defined as the pressure of liquid measured when the liquid

is static”.h1 = static pressure head

h2 = dynamic pressure head

Total pressure head = h1 + h2

Types of Pitot tube

Pitot tube with piezometer raised in flow through pipe

Pitot tube with differential manometer

Here, V1 = Cv

h = x[ - 1] X = Differential manometer reading in mm of mercury = Density of liquid flow

through pipe

Pitot static tube

h = x[ - 1] h in meter of liquid flow in channel or through pipe

Applications

• In industry, the flow velocities being measured are often those flowing in ducts and tubing where pitot tube is useful.

• In weather stations with high wind speeds.

Current Meter

• The Current meter is a device used to measure the velocity of water in rivers and in open channels.

• The current meter consists of hollow hemisphere or cups mounted on spokes so as to cause rotation about a shaft perpendicular to the direction of flow.

Principle

Mechanical• Mechanical current meters are

mostly based on counting the rotations of a propeller.

Acoustic• Doppler effect is used. It use a

ceramic transducer to emit a sound into the water.

Electromagnetic Induction• Charged particles (the ions in

seawater) are moving with the ocean currents in the magnetic

field of the Earth which is perpendicular to the movement.

• Using Faraday's law of induction , it is possible to evaluate the variability of the averaged horizontal flow by measuring the induced electric currents.

Tilt• Tilt current meters operate

under the drag-tilt principle. They consist of a sub-surface buoy that is anchored to the sea floor with a flexible line or tether.

Construction and working

• The entire setup is lowered into water from a bridge or a boat.

• The drag on a hollow hemisphere or cups is greater when its open side faces the liquid stream and so there is a net torque on assembly when flow comes from any direction in the plane of rotation.

• This rotation is converted into electrical signal by means of a circuit.

• A fixed number of revolutions of wheel produce a beat that can be heard clearly.

• The number of beats in a given period of time is a function of fluid velocity.

References

• Fluid Mechanics: Basic Concepts & Principles, Shiv Kumar

• http://www.sciencedaily.com/releases/2010/09/100914102112.htm

• http://www.aoml.noaa.gov/phod/floridacurrent/background.php