hot wire anemometer & laser doppler anemometer

8/10/2019 Hot Wire Anemometer & Laser Doppler Anemometer

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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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hot wire anemometer & laser doppler anemometer

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