Jordanian German Winter Academy

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• Jordanian German Winter Academy
• Amman, 4-11/ Feb. 2006
• Hot-Wire Anemometry
• HWA

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Discussed Topics

• Definition.
• Features.
• Applications.
• Operation and Measurement principle.
• About probes.
• Operation Modes.
• Governing Equations.
• Calibration.

• Deficiencies and Limitations.
• Measurements in 2 and 3 dimensions.
• Data acquisition.
• Steps of a Good HWA.
• Ending and Discussions.

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• Hot-wire anemometry is the most common method
  used to measure instantaneous fluid velocity. The
  technique ( found in the early 70s by King and
  others) depends on the convective heat loss to
  the surrounding fluid from an electrically heated
  sensing element or probe. If only the fluid
  velocity varies, then the heat loss can be
  interpreted as a measure of that variable, (
  relate heat loss to flow ).

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Features

• Measures velocities from few cm/s to
  supersonic.
• High temporal resolution fluctuations up to
  several hundred kHz.
• High spatial resolution eddies down to 1 mm
  or less.
• Measures all three velocity components
  simultaneously, and Provides instantaneous
  velocity information.

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Applications

• Aerospace Automotives Bio-medical
  bio-technology Combustion diagnostics Earth
  science environmen Fundamental fluid
  dynamics Hydraulics hydrodynamics Mixing
  processes Processes chemical engineering
  Wind engineering Sprays (atomization of
  liquids)

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Principles of operation

• Consider a thin wire mounted to supports and
  exposed to a velocity U.
• When a current is passed through wire, heat is
  generated ( I 2 Rw ). In equilibrium, this must
  be balanced by heat loss (basically convection)
  to the surroundings.
• If the velocity changes, convective heat transfer
  coefficient will change, so the wires
  temperature will change and eventually reach a
  new equilibrium.

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Principle of operation
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Measurement Principles

• The control circuit for hot-wire anemometry is
  in the form of a Wheatstone bridge consisting of
  four electrical resistances, one of which is the
  sensor. When the required amount of current is
  passed through the sensor, the sensor is heated
  to the operating temperature, at which point the
  bridge is balanced. If the flow is increased, the
  heat transfer rate from the sensor to the ambient
  fluid will increase, and the sensor will thereby
  tend to cool. the accompanying drop in the
  sensor's electrical resistance will upset the
  balance of the bridge. This unbalance is sensed
  by the high gain DC amplifier, which will in turn
  produce a higher voltage and increase the current
  through the sensor, thereby restoring the sensor
  to its previously balanced condition. The DC
  amplifier provides the necessary negative
  feedback for the control of the constant
  temperature anemometer. The bridge or amplifier
  output voltage is, then an indication of flow
  velocity.

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Probes
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Probe Types

• Hot film , which is used in regions where a hot
  wire probe would quickly break such as in water
  flow measurements.
• 2. Hot wire , This is the type of hot wire that
  has been used for such measurements as turbulence
  levels in wind tunnels, flow patterns around
  models and blade wakes in radial compressors.

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Hot wire sensor
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Hot film sensor
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Probe selection

• For optimal frequency response, the probe should
  have as small a thermal inertia as possible.
• Wire length should be as short as possible
  (spatial resolution want probe length size)
• Aspect ratio ( L/d ) should be high (to
  minimize effects of end losses)
• Wire should resist oxidation until high
  temperatures (want to operate wire at high T to
  get good sensitivity, high signal to noise
  ratio)
• Temperature coefficient of resistance should be
  high (for high sensitivity, signal to noise
  ratio and frequency response)
• Wires of less than 5 µm diameter cannot be
  drawn with reliable diameters

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Modes of operation

• Constant Current anemometry (CCA)
• Constant Temperature anemometry (CTA)

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Constant current anemometer CCA
Principle Current through sensor is kept
Constant Advantages - High frequency
response Disadvantages - Difficult to use -
Output decreases with velocity - Risk of probe
burnout
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Constant Temperature Anemometer CTA
Principle Sensor resistance is kept constant by
Servo amplifier Advantages -Easy to use -High
frequency response -Low noise -Accepted
standard Disadvantages -More complex circuit
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Governing equations I
Governing Equation E thermal energy stored
in wire E CwTs Cw heat capacity
of wire W power generated by heating
W I² Rw recall Rw Rw(Tw) H
heat transferred to surroundings
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Governing equations II

• Heat transferred to surroundings

( convection to fluid H sum off
conduction to supports
radiation to surroundings)
Convection Qc Nu A (Tw -Ta)
Nu h d/kf f (Re,
Pr, M, Gr,a), Re ?U/µ
Conduction f (Tw , lw , kw,
Tsupports) Radiation f (Tw?- Tf ?)
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Simplified static analysis I

• For equilibrium conditions the heat storage is
  zero
• and the Joule heating W equals the convective
  heat transfer H
• Assumptions
• Radiation losses small
• Conduction to wire supports small
• Tw uniform over length of sensor
• - Velocity impinges normally on wire, and is
  uniform over its entire length, and also small
  compared to sonic speed.
• Fluid temperature and density constant

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Simplified static analysis II

• Static heat transfer
• W H I ² Rw hA(Tw -Ta)
  I²Rw Nu kf/dA( Tw -Ta)
• h film coefficient of heat transfer
• A heat transfer area
• d wire diameter
• kf heat conductivity of fluid
• Nu dimensionless heat transfer coefficient
• Forced convection regime, i.e. Re Gr(1/3 )
  (0.02 in air) and Re
• Nu A1 B1 Re n A2 B2 U n
• I ² Rw ² E² (Tw -Ta)(A B U n) Kings
  law
• Then the voltage drop is used as a measure of
  velocity.

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Heat transfer from Probe

• Convective heat transfer Q from a wire is a
  function of the velocity U, the wire
  over-temperature Tw T0 and the physical
  properties of the fluid. The basic relation
  between Q and U for a wire placed normal to the
  flow was suggested by L.V. King (1914). In its
  simplest form it suggests
• where Aw is the wire surface area and h the
  heat transfer coefficient, which are merged into
  the calibration constants A and B.

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Hot-wire static transfer function
Velocity sensitivity (Kings law coeff. A 1.51,
B 0.811, n 0.43)
Output voltage as fct. of velocity
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HOT-WIRE CALIBRATION

• The hot-wire responds according to Kings Law
• where E is the voltage across the wire, u is the
  velocity of the flow normal to the wire.
• A, B, and n are constants. You may assume n
  0.5, this is common for hot-wire probes. A can be
  found by measuring the voltage on the hot wire
  with no flow, i.e. for u 0, so A E2 as we
  can see.
• Make sure there is no flow, any small draft is
  significant. The HWLAB software operating in
  calibration mode will give you a voltage.
• Once you know A, you can measure the wire
  voltage for a known
• flow velocity and then determine B from Kings
  law, were B (E 2 A)/ U n )

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Calibration curve
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Problem sourcescontamination I

• Most common sources
• - dust particles
• - dirt
• - oil vapors
• - chemicals
• Effects
• - Probe Change flow sensitivity of the sensor
  (DC drift of calibration curve)
• - Reduce frequency response
• What to do
• - Clean the sensor
• - Recalibrate

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Problem SourcesProbe contamination II

• Drift due to particle contamination in air
• 5 ?m Wire, 70 ?m Fiber and 1.2 mm Steel Clad
  Probes

(From Jorgensen, 1977)
- Wire and fiber exposed to unfiltered air at 40
m/s for 40 hours - Steel Clad probe exposed to
outdoor conditions 3 months during winter
conditions
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Problem SourcesProbe contamination III

• Drift due to particle contamination in water
• Output voltage decreases with increasing dirt
  deposits

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(From Morrow and Kline 1971)
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Problem SourcesProbe contamination IV

• - slight effect of dirt on heat transfer were
  heat transfer may increase !
• effect
• low velocity indication, for increased surface
  vs. insulating effect
• High Velocity,
• - more contact with particles especially in
  laminar flow, were turbulent flow has a cleaning
  effect
• Influence of dirt INCREASES as wire diameter
  DECREASES
• Deposition of chemicals INCREASES as wire
  temperature INCREASES
• FILTER THE FLOW, CLEAN SENSOR AND RECALIBRATE

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Further Problem SourcesBubbles in Liquids I

• Drift due to bubbles in water
• In liquids, dissolved gases form bubbles on
  sensor, resulting in
• - reduced heat transfer
• - downward calibration drift

(From C.G.Rasmussen 1967)
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Bubbles in Liquids II

• Effect of bubbling on
• portion of typical
• calibration curve ( noised signal )
• Bubble size depends on
• - surface tension
• - overheating ratio
• - velocity
• Precautions
• - Use low overheat
• - Let liquid stand before use
  
• - Dont allow liquid to mix with air
  
• - Clean sensor

(From C.G.Rasmussen 1967)
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Stability in Liquid Measurements

• Fiber probe operated stable in water
• - De-ionized water (reduces algae growth)
• - Filtration ( should be better than 2 ?m)
• - Keeping water temperature constant (within
  0.1oC)

(From Bruun 1996)
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Eddy shedding I

• Eddy shedding from cylindrical sensors
• Occurs at Re 50
• Select small sensor diameters/ Low-pass filter
  for signal

(From Eckelmann 1975)
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Eddy shedding II

• Vibrations from prongs and probe supports
• - Probe prongs may vibrate due to there own
  shedding or due to induced vibrations from the
  surroundings via the probe support ( effects of
  resonance and vortices ).
• - Prongs have natural frequencies from 8 to 20
  kHz
• Always use stiff and rigid probe mounts.

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Temperature Variations I

• Fluctuating fluid temperature
• Heat transfer from the probe is proportional to
  the temperature difference between fluid and
  sensor.
• E2 (Tw-Ta)(A BUn)
• As (Ta ) varies
• - heat transfer changes
• - fluid properties change
• Air measurements
• - limited effect at high overheating ratio
• - changes in fluid properties are small
• Liquid measurements effected more, because of
• - lower overheats
• - stronger effects of T change on fluid
  properties

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Temperature Variations II

• Anemometer output depends on both velocity and
  temperature
• When ambient temperature increases the velocity
  is found to be low if not corrected for.

(From Joergensen and Morot1998)
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Temperature Variations III
Film probe calibrated at different temperatures
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Temperature Variations IV

• To deal with temperature variations
• Keep the wire temperature fixed (no overheat
  adjustment), measure the temperature along and
  correct anemometer voltage prior to conversion
• Keep the overheat constant either manually, or
  automatically using a second compensating sensor.
• Calibrate over the range of expected temperature
  and monitor simultaneously velocity and
  temperature fluctuations.

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Measurements in 2D Flows I

• X-ARRAY PROBES (measures within 45o with respect
  to probe axis)
• Velocity decomposition into the (U,V) probe
  coordinate system
• where U1 and U2 in wire coordinate system are
  found by solving

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Measurements in 2D Flows II

• Directional calibration provides the
  coefficients k1 and k2

(Obtained with Dantec Dynamics 55P51 X-probe and
55H01/H02 Calibrator)
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Measurements in 3D Flows I
TRIAXIAL PROBES (measures within a 70o cone
around axis)
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Measurements in 3D Flows II

• Velocity decomposition into the (U,V,W) probe
  coordinate system
• where U1 , U2 and U3 in wire coordinate system
  are found by solving
• left hand sides are effective cooling
  velocities. Yaw and pitch coefficients are
  determined by directional calibration.

Measurements taken for previous situation
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Measurements in 3D Flows III

• U, V and W measured by a Triaxial probe, when
  rotated around its axis. Inclination between flow
  and probe axis is 20o.

(Obtained with Dantec Dynamics Tri-axial probe
55P91 and 55H01/02 Calibrator)
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Measurement at Varying TemperatureTemperature
Correction I

• Recommended temperature correction
• Keep sensor temperature constant, measure
  temperature and correct voltages or calibration
  constants.
• I) Output Voltage is corrected before conversion
  into velocity

-This gives under-compensation of approximately
0.4/ C in velocity.
Improved correction
Selecting proper m (m 0.2 typically for wire
probe at a 0.8) improves compensation to better
than 0.05/C.
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Measurement at Varying Temperature Temperature
Correction II

• Temperature correction in liquids may require
  correction of power constants A and B
• In this case the voltage is not corrected

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Data acquisition I

• Data acquisition, conversion and reduction
• Requires digital processing based on
• Selection of proper A/D board
• Signal conditioning
• Proper sampling rate and a number of samples

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Data acquisition II
A/D boards convert analogue signals into digital
information (numbers), They have the following
main characteristics

• Resolution
• - Minimum 12 bits (1-2 mV depending on range)
• Sampling rate
• - Minimum 100 kHz (allows 3D probes to be
  sampled with approximately 30 kHz per sensor)
• Simultaneous sampling
• - Recommended (if not sampled simultaneously
  there will be phase lag between sensors of 2
  and 3D probes)
• External triggering
• Recommended (allows sampling to be started by
  external event)

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Data acquisition III

• Sample rates and number of samples
• Time domain statistics (spectra) require sampling
  2 times the highest frequency in the flow
• Amplitude domain statistics (moments) require
  uncorrelated samples. Sampling interval minimum 2
  times integral time scale.
• Number of samples should be sufficient to provide
  stable statistics (often several thousand samples
  are required)
• Proper choice requires some knowledge about
  flows nature
• It is recommended to try to make autocorrelation
  and power spectra first, as basis for the choice

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CTA AnemometrySteps needed to get good
measurements

• Get an idea of the flow (velocity range,
  dimensions, frequency)
• Select right probe and anemometer configuration
• Select proper A/D board
• Perform set-up (hardware set-up, velocity
  calibration, directional calibration)
• Make a first rough verification of the
  assumptions about the flow
• Define experiment (traverse, sampling frequency
  and number of samples)
• Perform the experiment
• Reduce the data (moments, spectra, correlations)
• Evaluate results
• Recalibrate to make sure that the
  anemometer/probe has not drifted

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• Thank you for listening

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