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Hot Wire Anemometry
By
Mudrika KhandelwalIIT Bombay
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Flow is all around
Blood inside us Air around us
Fuel and air flow in Rockets in spaceFlow of water around submarine
Flow of polymers during blendingFlow of fluids in reactors
Flow
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Why is flow measurement
important? Flow can be of solids, liquids or gases
Types: IncompressibleCompressible (Mac>0.3)
Measurements are important to monitorcontrol and analyse a process
Thus, a lot of considerations go into thedesigning the method of measurementand understanding the underlyingprinciple
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Measurement is a design exercise, doesnot matter how accurate you are, you canalways make mistakes in interpretation
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Flow Measurements rely onexperiments!!
The physics behind any process can beunderstood only through first hand
experience. Computers alone are lame : They simulate
situations only based on the inputparameters
Experimentation is a more lucid means ofdisseminating facts and knowledge. It helpsin proper visualisation and thus better 5
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Noninvasive
Invasive
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Techniques involving integral
property
a)Venturimeter
b)Rotameter
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Techniques involving local flow
properties
a)Hot Wire Anemometer b)Laser Doppler Anemometer
c)Particle Image Velocimetryd)Ultrasonic Technique
e)Magnetic Technique
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HOT WIRE ANEMOMETRY Invasive technique Local flow properties evaluation
Why HWA? Necessary resolution in terms of time and
space Correlations of the velocity fluctuations
requires local investigation
Such measurements can be made by meansof hot-wire or hot-film anemometry andLaser-Doppler anemometry
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Drawbacks: Inherent perturbations caused by the
introduced measuring sensors. Special designs of measuring sensors are
required to keep low measuring errors High turbulence intensity leads to
difficulties concerning the interpretation ofsignals Indirect measurement technique
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If velocity changes, convective heat transfer coefficientwill change, wire temperature will change and
eventually reach a new equilibrium.
Principles of Operation
Velocity U
Current I
Sensor (thin wire)
Sensor dimensions:length ~1 mmdiameter ~5 micrometer
Wire supports(St.St. needles)
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Governing Equation:
E = thermal energy stored in wire
E = CwTsCw = heat capacity of wire
W = power generated by Joule heatingW = I 2 Rwrecall Rw = Rw(Tw)
H = heat transferred to surroundings
W dt dE =
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Heat transferred to surroundings
( convection to fluid + conduction tosupports + radiation to surroundings)
Convection Qc = Nu A (T w -Ta)Nu = h d/k = f (Re, Pr, M, Gr, ),
Re = U/
Conduction f(Tw , lw , k w, T supports )
Radiation f(Tw4 - Tf 4)
H =
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ProbesProbes are the sensor which are the means
of measure of change of some property
Classification:On basis of number of sensor
SingleDual
Triple
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Classification Contd..
On basis of type of sensor
Miniature wiresGold-plated wires
Fibre-filmFilm-sensors
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Important ConsiderationsWire length should be as short aspossible
spatial resolutionprobe length
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Wire should resist oxidation. High Temperature toget good sensitivity, high signal to noise ratio
Temperature coefficient of resistance shouldbe high for high sensitivity, signal to noise ratio andfrequency response
Wires of less than 5 m diameter cannot be drawnwith reliable diameters
To overcome thermal inertia, reduce time constantby using thin wires, but are mechanical instability
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Temperature profile in Small Vs Long wire
Rw = Ro(1+ ())
Rw =wire hot resistanceRo =wire resistance at To =Temp.coeff. of resistanceTw =Wire temperatureTo =Reference temperature
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Properties disturbing the
measurement1. The boiling temperature of fluids is low.
2. Organic fluids can decompose.3. Fluids generally possess electric
conductivity.4. Fluids dissolve gases.
5. Fluids are usually moresoiled/contaminated than gases.6. In water and other fluids salts are
dissolved.19
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Types
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C C
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Constant Current
AnemometerPrinciple: Current through sensor is kept constant A fine current carrying wire is exposed to the flow
velocity. Wire attains equilibrium when heatgenerated equals heat loss. The equilibriumtemperature is a measure of velocity
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C T
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Constant Temperature
AnemometerPrinciple: Sensor resistance is kept constant by servoamplifier The current through wire is adjusted tokeep the wire temperature constant The currentrequired to do this is a measure of flow velocity
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Constant Temperature
Anemometry principle
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Heat Exchanges at Hot Wire
Involved heat transfers:
Radiation
ConductionSelf Convection
Forced Convection
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Radiation : can be neglected accounts foronly 10% of the black bodyradiation
Conduction : 2 pins therefore factor of 2
10-20 % of the total lossThis contribution becomes more prominent
as the length to diameter decreases25
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Self convection : buoyancy forces at the hotwire influences the flow field around thewire
According to collis and williamsFree convection neglected for v>0.1m/s
Forced convection: Major contribution tothe heat exchange
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Assumption: In general flow higher the 0.1m/s,
therefore no free convection If the velocity does not exceed a certain
limit, no compressibility effects
When ld, two dimensional heat flow
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Calibration
Uncertainties in the complicated process of
wire drawing (eg. about diameter)Uncertainties in the sensor length owing to
welding of pins to the wire tipOther influences like aging corrosion
homogeneity
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Calibration Procedure
The Probe is placed in a low turbulence air
stream of known and variable velocity, andthe anemometer output voltage E and theflow velocity U are determined in the rangeof planned measurement.
The calibration curve is obtained by plottingthe anemometer output voltage above theknown velocity.
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Calibration Procedure
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Determination of velocity:
Relating velocity and voltage output :
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Turbulence Measurement HWA is a good tool to measure turbulence
or fluctuations in velocity. A turbulent flow field varies in both space
and time, making it very difficult to
analyze, understand and control Fluctuations in velocity is captured as
change in voltage.
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Statistical Representation of
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Statistical Representation of
Turbulence Velocity takes random values as a function
of space coordinate and time according tosome probability laws. Laws determined by the experimental data
from the flow field, a lot of data is required. But the amount of data required is
drastically reduced by some statisticalprinciples.
Stationarity of fluctuations permits timeaveraging instead of ensemble 33
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At any instant bulk velocity U and
fluctuation component u can be known Mean velocity is calculated by time
averaging
As a result time mean fluctuation is zero
But time mean square of the fluctuationcomponent is not zero
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Analog measurement of
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Analog measurement of
turbulence
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In ol ed Eq tion
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Electrical Energy Generated Energy lost by Convection=Energy stored in the wire
Convective heat transfer coefficient varies as:
H=C0+C1 V
Energy lost by convection=HA(T w-Tf )Kdt
Temperature and resistance dependence:
Tw=Ktr (Rwo+r w)Change in heat transfer coefficient:
h= dV df
(V-V0)~
Kvv37
Involved Equation
For constant current :
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Where the time constant > 0.001 And thus limits frequency response to 160 cps
Turbulence studies require above 50000cps
Limitation is overcome by dynamic compensation38
I2(Rwo+r w)dt A(K tr (Rwo+r w)-Tf )K(C0+C1 V + Kvv)dt = MCdr w
I2(Rwo)dt A(K tr Rwo-Tf )K(C0+C1 V )dt=0
(I2r w A(Ktr r w)K(C0+C1 V -Kvv) Kc A(Ktr Rw0-Tf )Kvv)dt = MCK tr dr w
Ir w=e
1)(
+=
DK
Dve
where210
)( I V C C AK K
MC K
otr c
tr
=
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Improving Response
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Force a squarewave current
through hot wireOutput voltage
response to thiscurrent signal has
the same timeconstant as theresponse to the
flow velocity signal
Due to Change in current:
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Which is same as that for response to velocity
change40
Due to Change in current:
Energy change:(I0+i)
2(Rwo+r w)-Kc A(Ktr (Rw0+r w)-Tf )H=MCK tr dt
dr w
Kirchoffs law:
e=-Ri 2+(Rw0+r w)i+I0r w
1)(
+=
DK I
Die io
210 )( I V C C AK K
MC K
otr c
tr
=
For Constant Temperature:
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Systemic dynamic response can be obtained bysuperimposing previously obtained results for responseto velocity and response to current since in the
constant temperature mode both current and velocityare changing simultaneously.
Effect of velocity on r w:
1I/
)( m
+=
DK
Dvr w
Effect of current on r w :
1I/
)( m
+=
DK
Di
r ew
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The total effect on r is gi en b
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The total effect on r w is given by
superimposition
Finally the Time constant comes out to be farsmall than the normal. Therefore the responselimit for frequency increases and thus the bandwidth increases.
A typical instrument has frequency response of17000 cps when the average flow velocity is 30
fps, 30000 cps for 100 fps, 50000 cps for 300fps 42
1)(
0
+=
DK
Dve
ct
ct
WhereG
ct +
=1
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CCA Vs CTA
Current must be sufficient to increase the
temperature above fluid temperature But a sudden drop in fluid velocity cause
burn out of the wire No such drawback in constant
temperature as the feedback systemmaintains the safe temperature for everyvelocity
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Constant current does not provide proper
compensation while constant temperaturedoes owing to feed back But constant temperature set up suffers
larger noise level CC uses dc amplifier and hence usable to
low frequencies but CT uses ac amplifierwhich limits the use down to about 1cps.
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Determination of Direction
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Determination of Direction Generally a single wire responds to
velocity component perpendicular to the
wire if the angle is between 25 to 90therefore instead of velocity a componentis used.
To determine direction: Rotate the probe to find angle for max
current If the angle is roughly known, probe is
rotated in the opposite direction till we getsame current as before. Bisector is the 45
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Component and correlation
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p
measurement
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Wire position for sequence of hot-wire
measurements
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C t l ti
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Component correlation...
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at various angles like +45,-45 and 0
Film anemometr
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Film anemometry
Due to limited strength of thin wire , some
times films are used. Basically similar to constant temperature
anemometry Can be used at high temperature by
constructing internal water coolingchannels
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Why Laser Doppler
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Anemometry? Most flow measuring instruments measurephysical quantities which are functions of
the flow velocity. Measuring quantities by which flow
velocity is determined, often are functionsof the properties of state of the fluidmedium, which have to be known.
They have to be taken into account in thecalibration of the measuring method
These difficulties led to development of52
Laser Doppler Anemometry
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Laser Doppler Anemometry Two beams of collimated, monochromatic
laser light in the flow eg HeNe or Argon ion
Fluid is targeted, reflected radiation iscollected.
Change in wavelength is function oftargeted object's relative velocity (Dopplereffect).
Velocity is obtained by measuring thechange in wavelength by forming aninterference fringe pattern (i.e. superimpose
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Light source Generation of collimatedmonochromatic laser light
Change in wavelength
Interference fringe pattern
Velocity determination
Reference
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Reference
Ertunc(2001) Measurement of turbulence withHWA its application to Axisymmetrically strainedturbulence Doebelin Measurement Systems application and
design, Revised Edition, Mcgraw Hill LtdJorgensen (2002) How to measure turbulence withhot wire anemometers- a practical guideGoldstein RJ, editor (1983) Fluid MechanicsMeasurements, Hemisphere Publishing
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