IIT Madras Sreenath K Laser Doppler Anemometry

Laser Doppler Anemometry

Sreenath K

IIT Madras

OUTLINE

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Flow measuring techniques

LDA principle

Components of LDA

Applications

Single Phase, Multi Phase measurements

Phase Doppler Anemometry

 Quantification of fluid movement is pivotal in experimental and research studies

 Bulk Flow measuring devices

 Venturi, Rotameter, Flow nozzle, Orifice plate, Piston meter, Thermal mass flow meter

 Electromagnetic, Ultrasonic, Coriolis flow meters, Optical flow meters  Direct and Indirect measurement

 Local or Bulk, Time-Averaged or Instantaneous values

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Sreenath K

IIT Madras

 Convective heat transfer is correlated with velocity  Limitations

 Decomposition of fluid medium

 Perturbations caused by measuring probes especially in regions of Thin boundary layer Recirculation regions, Flow separation

 Deposition of solids

 Particulate content

Fig 1: Hot wire anemometer, Reference (1)

 Electrical conductivity 4


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IIT Madras

 Indirect measurement

 Measured quantities are Thermodynamic state dependent

 Proper knowledge of Fluid-property influence for good calibration

 State parameter fluctuation e.g. Two phase flows, chemical reactions  Blockage to flow in ducts with small dimensions  Hostile environment. Flames etc  FLOW DISTURBANCE

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IIT Madras

 No flow disturbance

 Almost direct measurement

 Needs particles which can faithfully follow the flow  Optical access

 Takes advantage of particulate content to an extent  High Spatial and temporal resolution

 Additional instrumentation for size, concentration measurements

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Sreenath K

IIT Madras

OPTICAL TECHNIQUES Image formation

Tracer Technique

Position Tracking

Photoelectric image tracing 7

Interference pattern formation

Photoelectric Interferometry

Photoelectric position tracking


Sreenath K

IIT Madras

LASER DOPPLER ANEMOMETRY (LDA) Characteristics

 Invented by Yeh and Cummins in 1964  No mechanical probe

 No flow disturbance  No pressure loss

 Velocity, velocity changes, Instantaneous velocity and its correlations  Rapid response to velocity changes  High accuracy without calibration

 Measures local, instantaneous velocity of tracer particles suspended 8

in the flow


Sreenath K

IIT Madras

LDA Principle     

Doppler shift

Laser light is sent through the flow region Scattered by seeded particles Scattered light is detected

Frequency of scattered light is Doppler shifted by an

Fig 2: Doppler Shift, Reference (1)

amount related to velocity of tracer particle

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IIT Madras

LDA Principle

Fig 3: LDA principle, Reference (2)

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IIT Madras

LDA Principle

 Doppler shift (~ 105 )is only a small fraction of incident frequency (~1015)

 Estimating a small value as a difference of two big values - High uncertainty

 Use two beams, find beat frequency.

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IIT Madras

LDA Principle

Fig 3: LDA principle, Reference (3)

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IIT Madras

Optical System a)

Reference beam

c)

Dual Scattered Beam

b)

Dual beam

Fig 4: Reference (4)


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IIT Madras

Dual beam LDA

DOPPLER MODEL

Fig 7: Doppler model to explain dual beam LDA, Reference (3)

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IIT Madras

Dual beam LDA

FRINGE MODEL

Fig 8: Fringe model to explain dual beam LDA, Reference (2)

 

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Gaussian Intensity distribution

Particle moving little away from centre Laser Doppler Anemometry

Sreenath K

IIT Madras

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Sreenath K

IIT Madras

Particle size

 

2𝜋𝜋𝑟𝑟𝑝𝑝 ∆𝑥𝑥 2𝜋𝜋𝑟𝑟𝑐𝑐 𝐼𝐼𝑆𝑆 = 𝐼𝐼𝑆𝑆,𝐴𝐴 �1 + � � sin � � cos � �� 2𝜋𝜋𝑟𝑟𝑝𝑝 ∆𝑥𝑥 ∆𝑥𝑥

Very small particles – modulation depth is fully reflected (A) As particle diameter increases – signal shape similar to (B)



Particle of diameter EQUAL to one fringe spacing (C)



diameter larger than several fringe spacing





Good “visibility” have been obtained with particles of Incompleteness of fringe model Mie’s theory (scattering)

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Fig 9: Effect of Particle Size on signal quality, Reference (5)

Sreenath K

IIT Madras

Particle concentration   

Constructive or destructive superposition of signals can occur

Modulation Depth is likely to be small at large particle concentrations Poly-dispersed particle size distribution

Fig 10: Effect of Particle concentration on signal quality, Reference (5)

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Sreenath K

IIT Madras

Directional ambiguity   

Same velocity in opposite directions result in same frequency shift Shift frequency of one incident beam using bragg’s cell

Rotating diffraction grating

Fig 11: Direction determination by frequency shifting, Reference (2)

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IIT Madras



Particle generator



Transmitting Optics



Backward scattering

Fig 12: Components of a typical LDA, Reference (6)

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IIT Madras

Optics set-up    

Ensure velocity information in signals obtained

Matching particle size to angle between two beams Effective measuring volume as seen by detector

Signal quality at the outer region of ellipsoid measuring volume is poor.

Fig 13: Influence of geometrical parameters, Reference (5)

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IIT Madras

Multidimensional measurement  

One pair of beams per dimension

Different frequency for each pair for meaningful detection by filtering


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IIT Madras

Signal Processing 



Signal Characteristics 

Frequency spectrum of signal from PMT



Modulation depth varies with particle size and concentration



Signal is not present at all times. Noise is always present

Selection on the basis of 

Property to be measured



Precision



SNR



Particle concentration



Turbulence intensity etc.

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IIT Madras

Fig 15: Various aspects of signal processing, Reference (5)

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Sreenath K

IIT Madras

Fig 16: Various aspects of signal processing, Reference (5)

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Sreenath K

IIT Madras

Phase Doppler Anemometry • Extension of LDA principle

• Simultaneously measures velocity and size as well as concentration, mass flux

• First introduced by Durst and Zare (1975)

• Commercial instrument in 1984 • No calibration required

• Very high accuracy and spatial resolution

• Preconditions: Optical access, Sphericity, homogeneity, known refractive indices, size and concentration

• Principle : Phase shift between scattered signals at different locations is proportional to diameter

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Sreenath K

IIT Madras

X

Set up of PDA

Detector 1

Flow

• Optical parameters

Z

θ

• Beam intersection angle θ

ψ

• Scattering angle Φ

ϕ

• Elevation angle ψ

• Optical configuration • Collection lens • Mask

ψ

Y

Scattering plane

Detector 2 Fig 17: PDA optical setup, Reference (6)

• Vertical slit

Fig 18: PDA optical setup, Reference (7)

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IIT Madras

Scattering • Scattering of plane waves by spherical particles – Mie theory

• Scattering modes

• Phase shift between detectors

• Depends on θ, Φ, ψ, diameter, scattering mode, wavelength of laser

Fig 18,19: Scattering by a spherical particle, Reference (6)

• Phase – diameter correlation • Linear for each mode

• Intensity of scattered light • Scattering angle • Polarisation 28


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IIT Madras

2π ambiguity • Phase difference is a modulo 2π function • Three detector setup increases measurable size range

Fig 20: Illustration of 2π ambiguity in PDA, Reference (6)

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IIT Madras

Dual PDA • Trajectory and slit errors eliminated • Optimised for spray with transparent particles • Reject non-spherical particles • Improved concentration and mass flux estimation

Fig 21: Dual PDA optical setup, Reference (6)

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Sreenath K

IIT Madras

Dual PDA • Combination of conventional and planar PDA

Fig 22: Conventional PDA, Reference (6)

Fig 23: Planar PDA, Reference (6)

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Fig 24: Dual PDA, Reference (6)

Sreenath K

IIT Madras

Tracer particles

• Scattering properties different from that of fluid • Small scattering particles penetrating the fringe system • Suitable density

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Set up

• Particle concentration • Turbulence • Properties to be measured • Appropriate Signal Processing

Sreenath K

IIT Madras

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Characteristics

Two Phase

• Much more complex • Variety of flow patterns • Flow properties of each phase should be measured

• Bubbles/Suspended particles for secondary phase flow properties • Additional tracer elements to extract main flow characteristics • The scattering properties should be distinct.


Sreenath K

IIT Madras

Fig 25: Two Phase flow patterns, Reference (6)

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Sreenath K

IIT Madras

Fig 26: Setup for two phase flow measurement, Reference (7)

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Sreenath K

IIT Madras

Fig 27: Setup for three phase flow measurement, Reference (8)

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IIT Madras

Pictures from Reference (6) 37


Sreenath K

IIT Madras

1

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•Aircraft Fuel Injection


Sreenath K

IIT Madras

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• Water flow measurement in a pump model


Sreenath K

IIT Madras

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• Helicopter Rotor model testing


Sreenath K

IIT Madras

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• Velocity profile measurement in a pipes


Sreenath K

IIT Madras

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• Measurement of flow around a ship propeller in a cavitation tank


Sreenath K

IIT Madras

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• Measurement of flow field around a 1:5 scale car model in a wind tunnel


Sreenath K

IIT Madras

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• Measurement of air flow field around a ship model in a wind tunnel


Sreenath K

IIT Madras

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•Nozzle Design


Sreenath K

IIT Madras

T Y P I C A L A P P L I C AT I O N S  Sprays and liquid atomization processes  Water sprays  Fuel injection  Bubble dynamics  Cavitation  Multiphase mass transfer  Aerodynamics  Hydrodynamics  Turbomachinery  Combustion process  Verification of turbulence models and CFD predictions 46


Sreenath K

IIT Madras

 Importance of undisturbed flow measurements in “fragile” flows  Doppler effect applied in LDA

 Dual beam LDA can be explained by Doppler model and fringe model  Complete LDA system was analysed  Principle of PDA was discussed

 Physical conscience behind Multiphase Flow measurements were presented

 We looked at some applications of this technology

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Sreenath K

IIT Madras

1. 2. 3. 4. 5. 6. 7. 8.

http//www.efunda.com http://web.mit.edu/fluids-modules/www/exper_techniques/LDA.text.pdf Fluid Mechanics – An Introduction to the theory of fluid flows, F. Durst. Laser-doppler anemometry and Its application to flow investigations Related to the environment of vegetation, F. DURST and M. Zari~ “Principles and Practice of Laser-Doppler Anemometry” by F. Durst, A. Melling and J. H. Whitelaw http://www.dantecdynamics.com Measurement of velocities in gas-liquid two-phase flow using laser Doppler velocimetry ,P. F. Vassallo, T. A. Trabold, W. E. Moore, G. J. Kirouac Extended Phase-Doppler Anemometry for Measurements in Three-Phase Flows* By Heiko Braeske, Günter Brenn, Joachim Domnick, Franz Durst, Adrian Melling, and Maris Ziema

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Sreenath K

IIT Madras

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Sreenath K

IIT Madras