2. 2
• Classification of flow meters based on
1.Weight / quantity (or) volume
2.Rate of flow
3. 3
Quantity Meters
• A quantity meter is defined as one in which fluid
passing through the primary Element is accurately
quantified in terms of weight or volume of the fluid.
It measures volume in liters.
•
Eg:- Positive displacement meter
Reciprocating piston
Nutating discs etc
4. 4
Rate of Flow Meter
• A flow meter can be defined as one the fluid passing
through the primary element in a continuous stream.
• Rate of flow means quantity of flow per unit time.
Eg:- Orifice Plate
Turbine meter
Electromagnetic flow meter
5. 5
1. Head type flow meters based on differential
pressure measurements
a) Orifice plate
b) Venturi tube
c) Flow nozzle
d) Pitot tube
2. Electromagnetic flow meters
3. Rotameters (variable area meters)
Classification of Flow Meters
6. 6
4. Mechanical flow meters
a) Positive displacement
b) Turbine flow meter
5. Anemometer
a) Cup type anemometer
b) Hot wire anemometer
6. Ultrasonic flow meter
7. Vortex flow meter
8. 8
Principle of Head Type Flow Meter
• In this ,a restriction is placed in fluid path.
• Restriction creates pressure difference
• The pressure difference indicates flow rate.
• The relationship based on Bernoulli's theorem
9. 9
Head Type Flow Meter
• The Head type flow meters have a common feature in
that they produce a pressure difference when fluid flow
is maintained through them .
• There is a certain linear relationship between the
pressure difference and flow rate of the fluid
• Head type flow meters follows Bernoulli's theorem
10. 10
Bernoulli’s Theorem
•
•
•
•
•
It states that in a fluid stream, the sum of
Pressure head,
Velocity head
Elevation head
At a point is equal to their sum at any other point
removed in the direction of flow from the first point plus
loses due to the friction between the two points.
12. 12
• Consider a flow tube of varying cross sectional area and
having a difference in level as shown in fig.
An incompressible fluid density ‘ ρ’ is assumed to
steadily flowing through the pipe
The flow tube axis inclined above datum line ‘XY’ line
Applying the Bernoulli’s theorem , the relationship for the
fluid flow under equilibrium conditions can be expressed
as
•
•
•
Description
14. 14
ρ = Fluid density
g = Acceleration due to gravity
h1= Height of centre of gravity of volume BCED above
datum line
h2= Height of centre of gravity of volume FGIH above
datum line
16. • The orifice plate is basically a thin metal
plate with circular opening
Definition
9AEI306.59-60 16
17. Classification of Orifice Plate
• Concentric
• Eccentric
• Segmental
• The concentric type is by far the most widely used.
17
18. 18
• The materials used for construction of
orifice plates are
• Mild steel
• Stain less steel
• Phosphor bronze
Orifice Plate
19. 9AEI306.59-60 19
• The orifice meter is most common type of head
type flow measuring device for medium and large
pipe sizes.
• The office plate inserted in a pipe line causes an
increase in the flow velocity and a corresponding
decrease in pressure.
Orifice Plate
21. 21
• The flow pattern shows an effective decrease in the
cross-section of the flow beyond the orifice plate with
maximum velocity and minimum pressure
• The particular position where the velocity is maximum
and static pressure is minimum is called vena
contracta
Working
22. 22
Functioning of orifice plate
• An orifice plate installed in a pipeline creates a
pressure differential as the fluid flows through it
• This differential pressure is proportional to the
rate of flow
23. 23
•
•
They offer low cost over other types of flow meters
Especially in a large line sizes and have proved to
be rugged ,effective and reliable over many years
• It has low installation cost and a turn down of not
more than 4 : 1
Merits of orifice plate
25. 25
Working of Orifice Plate
• The orifice plate is inserted in pipe line between two
flanges.
• The fluid flow through orifice causes increase in flow
velocity and decrease in the pressure .
26. 26
• At a particular position beyond the orifice plate the
velocity is maximum and pressure is minimum
• This position is called vena contracta.
• Before the vena contracta the fluid velocity decreases
and pressure increases.
• It reaches to a position where the velocity and
pressure equal as upstream side.
27. • The volume of flow can be
determined by the equation –
Qv CEA2
27
2gPd
e
28. 28
Where
Qv = Volume flow rate; m3 / sec
C = Discharge Coefficient
A = Area of the orifice plate; m2
29. 29
Pd = Differential pressure; pascals,
g = Accelaration due to gravity; m/sec2,
ρ = Density of a fluid; kg/m3,
30. 30
E = Velocity approach factor
• By knowing the values of Cd, E, A, g, ρ and Pd
the volume flow rate can be determined.
34. 9AEI306.61 34
• It is head type flow meter
• It follows Bernoulli's theorem
• It works on the principle that by reducing the
cross sectional area of the flow passage a
differential pressure is created
• This differential pressure is proportional to the
discharge through the pipe
Venture Flow Meter
39. 9AEI306.61 39
• Converging Conical section : converging cone
converges from diameter D at its upstream side to
diameter d at this down side stream.
• As the flows takes place in the convergent cone the
velocity increases and pressure decreases.
•
41. 9AEI306.61 41
Throat
• It is a small portion of circular pipe in which
diameter is kept constant .
• In this section the flow velocity neither
increases nor decreases i.e. in steady state
43. 43
Diverging Section
• The downstream side of the throat examples from
throat d to D is known as divergent cone .
• The angle of divergent cone is 5 to 150
• It results in pressure recovery
44. Operation
• The pressure at different locations are measured
• By knowing the pressure differences, we can
calculate the flow rate using the equation
2
9AEI306.61 44
1
Q EA
2g(P1P2)
45. 9AEI306.61 45
Where
Q = Flow rate
ø= Expansion factor
E = Velocity approach factor
A2 = Area of cross factor
e1 = Density at pressure
Pd = P1 – P2 = pressure differences
g = Acceleration due to gravity
48. 48
• The pressure tapings can be placed at the
upstream entrance to the convergent cone and at
the throat.
• The flow rate or the discharge rate can be
determined by the following equation .
49. • A1= Cross sectional area at the inlet
• A2= Cross sectional area at the throat
H = Difference of pressure head in a U-tube
•
• G = Acceleration due to gravity
1
9AEI306.61 49
2
A2
Q
A1 A2 2 g h
A2
54. 54
• It was invented by henry pitot in 1732 to measure the
fluid velocity
It is used in wide range of flow measurement and
applications such as
Air speed in racing car
Air force in fighter jets
•
•
•
55. 55
• It consists of a cylindrical probe is inserted in to the
fluid stream
• In this device the velocity head is converted in to an
impact pressure
• The difference between the impact pressure and
static the pressure is a measure of flow rate
Principle
56. A Blunt Object Is Placed In A Fluid Stream Sa
Obstruction To The Flow As Shown In Fig B
56
57. • As the fluid approaches the object, the
velocity will decrease until it reaches zero at
the point where it impinges on it.
• This results in increase in the pressure on
downstream side.
57
58. Mathematical Expression
•
• At the point of impact v2is zero .in other words
• The kinetic energy has been converted in to potential
energy ,
the result is reflected in the value of p2at the impact
point.
P P
2g 2g
V 2
V 2
1
1
2
2
1 2
58
59. 59
• where
V1= velocity of the fluid on the upstream
V2= velocity of the fluid on the down stream
ρ1= density of the fluid on the upstream
ρ2= density of the fluid on the downstream
60. • This new pressure , known as the total pressure,
comprises the normal static pressure and pressure
produced as a result of energy conversion when v2= 0
For incompressible fluids ρ1=ρ2=ρ
The equation (1) becomes
•
•
2
1 p
2 g
v 2
p
1
60
61. • v1= 2g(p2-p1)
ρ
•
•
G and ρ are constant for particular fluid
The pressure difference is proportional to the
velocity of fluid
61
65. 9AEI306.6622 65
• When the blunt object is replaced with a tube having a
small opening ,
• Facing the direction of the fluid flow, connected to a
differential pressure gauge as show in the fig B.
• As there is no flow through the tube and since the flow is
brought to rest ,
• The new pressure developed and sensed is impact
pressure p2
Operation of Pitot Tube
66. 66
• A static pressure reading p1is taken upstream a little
away from the tube .
• By measuring the differential pressure ,the velocity can
be computed by knowing the density of the fluid
• It is very convenient to measure the static pressure in
the close neighborhood of the tube
Operation of Pitot Tube
67. 67
Advantages
•
•
•
It is a simple and low cost device
It does not produce appreciable pressure loss
It can be easily inserted through a small hole in to the
pipe
It is very useful for checking the mean velocities of the
flows in venturi, nozzle ,orifice plate
•
68. 9AEI306.6622 68
Disadvantages
• It is not suitable for measuring low velocities, bellow 5
m/s
• Is sensitive to misalignment of probe with respect to free
stream velocity
• It is not suitable for the measurement of highly
fluctuating velocities i.e. highly turbulent flows
69. 69
Industrial Applications
• It is used to measure air flow in pipes ducts and stacks
• It is also used to measure velocity of liquid flow in pipes
and open channels
71. 71
Rotameter
• It consists of a vertical tube with a tapered cone in which
float assumes a vertical position corresponding to each
flow rate through the tube.
• It is also called as constant pressure drop, variable area
meter.
72. Definition
• A Rotameter is a device that measures the flow rate of
liquid or gas in a closed tube.
72
73. The Fundamental Equation For An Incompressible
Flow Through A Tube
Qv CEA2
73
2gpd
------------------- (A)
C = Discharge coefficient
E = Approach factor
A2= Orifice area
g = Acceleration due to gravity
Pd= Pressure difference
= Density
74. 74
•
•
•
•
•
Earlier we have discussed orifice , venturi tube ,pitot
tube.
• If C, E, A2,g, are constant for particular fluid
• Then the flow rate is proportional to the pressure
difference.
In the case of Rota meter A2= Area between the vertical
tube and float
If C, E, Pd ,g, are constant for particular fluid
Then the flow rate is proportional to the A2.
That is why it is also called constant pressure drop with
variable area type flow meter.
75. 75
Operation
• It is a vertical tube of conical shape, the area gradually
expanding from bottom to top.
The fluid allowed to flow in an upward direction in the
tube.
If a disc is placed which is free to move in the fluid path,
it acts as a float in the fluid.
An orifice is setup between the perimeter of the disc and
inside surface of the tube with a corresponding pressure
drop.
•
•
•
76. 76
• Initially when there is no fluid flows through the
Rotameter then float is at equilibrium in a vertical tube.
• When fluid flows through the Rotameter it, will effect the
pressure drop, altering the relation between the inlet and
outlet pressure.
• Thus upsetting the equilibrium for force acting on the
disc (float).
77. 9AEI306.63TO64 77
• The disc (float) will then move up or down the tube there
by creating variable area of the orifice until the pressure
drop is at original value when the forces are again at
equilibrium.
• The position of the float in the tube is then measure of
the rate of flow.
78. 78
Analysis of Rotameter
• Consider the forces acting on the float in the vertical
column of liquid as shown in fig. 2
• The effective weight ‘W’ acting on the float
W = Vf (2 - 1)
Where Vf = Volume of the float
2= Material density of the float
1= density of the liquid
•
81. 81
• Then the float is free, it will moved in the direction of flow
• As it moves upward it increases the orifice area due to the
expanding sectional area of the tube and pressure
differential falls proportionally
82. 82
• This operation continues until (p1-p2) reaches its original
value then the equation are equilibrium again
• The new float position is the measure of the new flow rate
83. • The equation can be written as
• Where
1
v
2
Q KcxE 2g
Vf
A
f 1
x
83
K
A2
= Proportionality constant
84. 84
Advantages of Rota Meter
• It gives direct visual indication on a linear scale
• Low cost
• It has high accuracy
85. 85
Advantages
•
•
A rotameter requires no external power or fuel,
It uses only the inherent properties of the fluid, along
with gravity, to measure flow rate.
•
•
A rotameter is also a relatively simple device
It can be mass manufactured out of cheap materials,
allowing for its widespread use.
86. 86
Limitations of Rota Meter
• Due to its use of gravity, a rotameter must always be
vertically oriented and right way up, with the fluid flowing
upward
87. 87
Disadvantages
• Graduations on a given rotameter will only be accurate
for a given substance at a given temperature.
• Rotameters normally require the use of glass (or other
transparent material), otherwise the user cannot see the
float. This limits their use in many industries to benign
fluids, such as Water.
Rotameters are not easily adapted for reading by
machine
•
88. 88
Applications of Rota Meter
•
•
•
Laboratory
Testing and production lines
It can be easily integrated for instrumentation with
Alarms
Indicators
Controllers
Recorders
1.
2.
3.
4.
90. 90
Description
• It is non- friction displacement type of mechanical
flow meter
• It consists of two parts
• The rotor with multiple blades
• Variable reluctance tachometer
92. 92
• The rotor consists of turbine blades
• It consists of an axially mounted freely rotating turbine
wheel / (rotor). It is placed in the path of a fluid steam.
• When the flowing fluid impinging on the turbine blades
imparts a force on the blade surfaces
• Due to this force the rotor in motion with an angular
velocity “v”.
Operation
93. 93
• This angular velocity is proportional to the fluid of
whose velocity to be measured.
The turbine flow meter with an electrical output suitable
for measuring the flow in the tubes as shown in the
figure 2.
The turbine flow meter consists of a rotor with multiple
blades. The rotor is supported by ball bearing and is
located centrally in the pipe .
•
•
97. 97
• When the flowing fluid impinging on the turbine blades
imparts a force on the blades surfaces
• The angular velocity of the can be sensed by the means
of a proximity type of pick of reluctance type.
• A permanent magnet is encased in the rotor body and
each time the rotating magnet pass the pole of the pickup
coil,
Working Principle of Turbine Flow Meter
98. 98
• The change in permeability of the magnetic circuit
produces a voltage pulse at the output terminals
• These voltage pulses are counted by the means of
electronic digital counter
99. 99
• The relationship between the volume flow rate and
the angular velocity of the rotor is
Q = kn
Q= The volume flow rate
n = The rotor angular velocity in rad/s
k= Constant for any given meter
100. 100
• Alternatively the frequency is converted in to voltage
and is fed to analog/digital voltmeter
• The output voltage of analog/digital voltmeter is
proportional to the volume flow rate of the fluid.
104. 124
Definition of Anemometer
• Velocity-measuring devices for obtaining velocity of a
fluid stream
•
•
Such as air flow in a ventilating duct
Wind tunnel
Water flow in a closed channel
Wind speed as in meteorology
•
•
109. 109
• Vertical spindle rotating freely about the vertical axis
mounted on bearings
•
• Spindle is coupled to three equally-spaced horizontal arms
• Hemi spherically-shaped cup is mounted at the end of each
arm with the meridian plane vertical
When placed in an air stream ,a difference of pressure is set
up between the concave and convex sides of the cups
110. 110
• Resulting in a rotational torque at the vertical spindle
• The spindle is coupled to a mechanical or electrical
counter calibrated in the units of velocity i.e m/s
• The readings on the counter integrated over a specified
period gives an indication of the wind speed.
111. 111
• Three cup anemometers are currently used as the
industry standard for wind resource assessment studies
• They can measure velocities up to 3000 m/s
• Due to frictional losses, the device is not very accurate
and needs calibration periodically
115. 115
Principle of Hot-Wire Anemometer
• When a fluid flows over a heated surface
• Heat transferred from surface causes temperature
reduces
• The rate of reduction of temperature indicates velocity
of the fluid stream.
118. 118
Operation Of Hot-wire ANEMOMETER
• Fluid flows over the platinum wire, its temperature
reduces
• Resistance of wire changes ,bridge unbalanced
• The bridge is balanced by adjusting the current
through wire
• Temperature remains constant
119. 119
• Current measured due to voltage drop across resistance
• Heat generated=I2R
• Under equilibrium condition
• Heat generated=Heat loss
120. 120
• I2R= a (vρ +b)1/2
• V=[I4R2/a2–b] / ρ
• Temperature and resistance of a wire kept constant
• Velocity measured by measuring current (i), through
the heated wire
121. 121
Hot-Film Anemometer
• It is commonly used to measure the mean and
fluctuating velocity in fluid flow
• The flow sensing element is a platinum tungsten wire
• It is welded between two prongs of the probe
• It is placed in one arm of the Wheat stone's bridge
• It is heated electrically
122. 122
Hot-Film Anemometer
• The probe is introduced in the fluid stream
• Then it tends to get cooled by the instantaneous velocity
• Consequently its resistance decreases
123. 123
Hot-Film Anemometer
• The rate of cooling depends on
1. Shape, size and physical properties of the wire
2. Temperature difference between the heated hot
wire and the fluid stream
3. Physical properties of flowing fluid
4. Velocity of fluid stream
124. 124
Hot-Film Anemometer
• The first three conditions are generally constant
• So the instrument response is direct measurement of
the velocity
• There are two ways to measure the velocity using the
H. W. Anemometer
1. Constant current mode
2. Constant temperature mode
125. 125
Hot-Film Anemometer
• In both modes the bridge is initially balanced
• When there is a fluid flow the hot wire/film resistance
changes
• This unbalances the bridge and some output voltage is
generated
• That voltage is proportional to the velocity of fluid flow
127. 127
Hot-Film Anemometer-Range
Hot-Film probes are used for measurements in liquids
for flow-rates up to 25m/s.
Frequency response extending up to about 150kHz
130. 130
Operation of Hot-Film Anemometer
• Hot-Wire Anemometer is another version of Hot-Film
transducer.
• Sensor is the thin film of platinum deposited in a
glass or quartz substrate.
• The film replaces the Hot –wire , remaining circuit is
same as Hot-wire
• The film transducers gives mechanical strength .
131. 131
• It can also be used at very high temperatures , using
cooling arrangements
• The directional sensitivity of the probe, maximum
right angles to the flow
• In the angle 450 <θ 1350 effective velocity , u rms =u
sinθ.
132. 132
• This property directly utilized in flow- direction
measurements.
• In steady-flow conditions by rotating probe, until
sharply-defined null is obtained.
137. 137
• The basic principle of operation of Electromagnetic
flow meter is faradays laws of electromagnetic
induction
Principle of Electromagnetic Flow Meter
138. 138
Faradays Laws of Electromagnetic
Induction ?
• First law states that whenever a conductor cuts lines of
magnetic field ,an induced emf is generated.
• Second law states that the magnitude of this emf is
proportional to the rate of which these lines are cut.
• The emf is perpendicular to the plane of conductor and
the magnetic field.
141. 141
Construction
• A permanent magnet or electromagnetic it may either
ac or dc around a non conducting pipe
Two electrodes are inserted in tube, their surfaces
being flush with the inner surface of the tube and in
contact with liquids
As the conductive liquid flows through the insulated
tube with an average velocity v,
It may be considered as a series of flat conductor
discs passing through the magnetic field
•
•
•
143. 143
According faradays law induced emf generated by
E Bdv108
(1)
•
•
•
E = induced voltage in volts
B= magnetic flux density in tesla
D=the distance between the electrodes in m
• V= the average velocity of liquid in m/s
Mathematical Expressions
145. •
•
•
The volume flow rate Q= Av
A= cross sectional area of the pipe
V= Average velocity of the fluid
Substituting the value of from equation(1) in equation(2)
Q e
A
108
0
Bd
145
Mathematical Expressions
146. 146
• As A,B and d are constants for particular
electromagnetic flow meter,
• the induced voltage is proportional to the volume
flow rate
150. 150
Applications
• It is particularly suitable for flow velocity or volume
measurement of
Slurries
Corrosive acids
Sewage
•
•
•
• Detergents ,greasy and sticky fluids
152. 172
Ultrasonic Flowmeters works in two different
principles :
• Doppler Effect Ultrasonic Flowmeter
• Transit time/Time of flight Ultrasonic Flowmeter
155. 155
Principle of operation
• Ultrasonic Signals are passed through the fluid,
• the particles suspended in the fluid shows a frequency
shift
• It is proportional to the velocity of the fluid
156. 156
Working Principle :
• It is used for reflected electronic sound to measure the
fluid velocity
• Measuring frequency shift between frequency source ,
receiver , fluid carrier , relative motion is measured
• Resulting frequency shift is called doppler effect
159. 159
Expression
Where :
•
•
• fr= receivedfrequency
• ft= transmission frequency
• v = fluid flow velocity
• Ø = relative angle between the transmitted ultrasonic
beam and the fluid flow
c = velocity of sound in the fluid
This method requires there is some reflecting particles in
the fluid
160. 160
Advantages
• Obstructs less flow
• Can be installed outside the pipes
• The pressure drop is equal to the equivalent length of a
straight pipe
• Low flow cutoff
• Relative low power consumption
161. 161
Limitations
• Doppler flow meters performance highly dependent on
physical properties of fluid Such as :
• Sonic conductivity
• Particle density
• Flow profile
163. 163
Transit Time Ultrasonic Flowmeter-Principle
• The Time for the sound to travel between the
transmitter and a receiver is measured
• This method is not dependable on the particles in the
fluid
165. Transit Time Ultrasonic Flow meter
Receiver ‘B’
Receiver ‘B’
Transmitter ‘A”
Transmitter ‘B’
Flow’ v’
165
166. 166
Principle
An Ultrasonic flowmeter is mounted at an angle or
parallel to the pipe wall
Short duration Ultrasonic waves are transmitted across
the fluid
The velocity of the ultrasonic waves is increased or
decreased by the fluid velocity depending upon the
direction of fluid flow
167. 167
Construction
The figure shows the schematic arrangement of
ultrasonic flowmeter of transit time type
Two transmitters of piezo electric device A&B are at
the down side of the flow tube with an angle
Two piezo electric receivers A&B are connected to the
pipe at top side with an angle
168. 168
Operation
The fluid in the pipe flows at a velocity
The transmitter transmits short duration ultrasonic
signals through the fluid at a velocity ‘l’
The signal received by the receiver A is increased to
C+ cos θ because it is in the direction of fluid flow
The reception frequency of the receiver pulse fAwill be fA
= (C+ cos θ)/(l)
169. 169
Operation
Where θ= angle between the path of sound and pipe
wall
l = distance between the transmitter and
receiver
The velocity of the ultrasonic signal transmitted by A is
received by the receiver B will reduced by the fluid
velocity
It creates a retardation of C+ cos θ
170. 170
Operation
If the reception frequency is given by fB= (C- cos θ)/(l)
The difference in frequencies is given by
Δf = fA- fB= (2 cos θ)/l
Time duration = ΔT= (l)/ (2 cos θ) (since ΔT=1/Δf )
171. 171
Operation
By measuring the difference in repetition frequency Δf
and by knowing the value of θ and l the velocity of fluid
can be measured
Or
The flow velocity can be computed by measuring the
time difference between the two pulses in either
directions
172. 172
Advantages
Bidirectional measuring capability
Good accuracy
Fast response
Wide frequency range
Used for any size of pipes
Measurement is independent of the velocity of sound ‘c’
175. 175
Limitations
It requires reliability high frequency sound transmitted
across the pipe
Liquid slurries with excess solids or entrained gases
may block the ultrasonic pulses
These are not recommended for primary sludge, mixed
liquor ,septic sludge and activated carbon sludge
Liquids with entrained gases cannot measured reliably
179. 179
Laser Doppler Anemometer
• It is most recent advancement of flow meter
• It is also known as optical type velocity meter
• It measures the instantaneous velocities of gasses or
liquids flowing in a transparent (glass) channel
181. 181
Principle
• It is based on the Doppler shift in frequency of the light
scattered by an object moving relative to the radiating
source
• The technique basically consists of focusing laser beams
at the point in the fluid where velocity is to be measured.
• At this focal point the laser light scattered from the fluid
or fluid particles contained in the fluid
182. 182
Principle
• Signal processing of the photo-detector output gives the
magnitude of Doppler frequency shift.
• Which is directly proportional to instantaneously velocity
of the flow
183. 183
Features of LASER
• It provides much higher quality of monochromatic (single
wavelength) light source
• It is coherent i.e. it stays in phase with it self over long
distances
Its frequency is very stable .this enables to accurately
detect the Doppler shift frequency
Its wave length is less effected by changes in ambient
pressure ,temperature or humidity.
•
•
184. 184
Materials suitable for production of laser beams
• Ruby (aluminium oxide crystal doped with a small
amount of chromium)
• Nd-YAG ( type of garnet stone doped with a small
amount of neodymium)
• Carbon dioxide gas
• Neon gas
186. 186
Working
• The laser source (helium-neon laser) produce laser
beam .
This laser beam is split in to two equal parts by means of
a beam splitter .
The beam splitter is either a rotating optical grating or an
optical prism as shown in the figure 3 .
The focussing lens is put in the front of the beam splitter
It focuses the two beams at a point where the velocity of
the fluid is to be measured
•
•
•
•
190. 9AEI306.73-74 190
• At the focal point the two split beams cross each other.
• Thus forms an interference fringe pattern.
• It consists of alternate regions of low and high intensity,
as shown in the figure.
• If the small traces particles (dust or dirt particles present
in tap water or air flows) pass through the region of high
intensity ,they would scatter light and cause a Doppler
shift in the frequency of the scattered light.
191. 191
• This scattered light received by the photo detectorwill
show a varying electrical signal.
• The frequency of this electric signal is proportional to the
rate at which the particles cross the interference fringes.
192. • The spacing between the fringes
expression
is given by the
• Where = The angle between two converging beams
• = The wave length of the laser beam
2
x
sin (1)
2
x
192
193. • The tracer particles( assumed to have a velocity equal to
that of the fluid flow) pass across the fringes with a
velocity ‘v’ in the direction perpendicular to the fringes.
• The signal experiences a Doppler shift in frequency given
by
= The wave length of the laser beam in the fluid.
2
9AEI306.73-74 193
f
2v
sin (2)
194. • The equation (2) can also be written as
• Where n = The index of refraction of the fluid
• 0= The wave length of the laser beam
in the vacuum.
0
194
2
f
2nv
sin (3)
195. 195
• If n, 0are constant Doppler shift in frequency is
proportional to the velocity of the fluid at particular point
196. 196
Advantages of Laser Doppler Anemometer
• There is no transfer function involvement i.e. the output
voltage of the instrument is proportional to the
instantaneous velocity of the fluid.
• Non –contact type of measurements i.e. no physical
object is inserted in the flow field.
• Flow rate is undisturbed by measurement.
197. 197
Advantages of Laser Doppler Anemometer
• It has very high frequency response, in MHz range
• It has very high accuracy
• Suitable for measurement in both gas and liquid flows
198. 198
Disadvantages of Laser Doppler Anemometer
• It involves the need for a Transparent channel
• The measurement technique is not suitable for clean
flows
• It is highly expensive and requires a high degree of
experience and skill in operation .
199. 199
Applications of Laser Doppler Anemometer
• Remote sensing of wind velocities
• Blood flow measurements.
• Measurement of flow between blades of turbines and jet
propulsion system
• Used for both laminar and turbulent flow measurement
