laser doppler velocimeter
Post on 28-Apr-2015
73 Views
Preview:
TRANSCRIPT
Manish Shandilya 2010me20778
LASER DOPPLER VELOCIMETER
Deepak Vijay
2010me20914
By:-
The most recent advancement in the area of flow measurement
It is also known as laser Doppler anemometer
Measures instantaneous velocities These velocity measurements are of very
importance in boundary layer studies and turbulence studies.
It uses Doppler effect principle.
INTRODUCTION
The instrument measures the velocity at a point in the fluid, flowing in a glass walled channel, by detecting the Doppler shift in the frequency of the scattered light.
Doppler EffectIt says if there is a relative motion
between source and observer then actual frequency will change by Doppler shift.
PRINCIPLE
Numerator – Receiver (observer) Toward + Away –
Denominator – Source Toward – Away + Doppler shift = f’ - f
Source and receiver moving
r
r
s S
v1 c vcf f fv c v1c
∆f = (v/µ)[( cosθ -1 )cosβ - sinθ sinβ] If the incident and scattered beams are
equally inclined to a normal to the flow direction,
β=(π/2-θ) Then ∆f = (2v/µ)sin(θ/2) so velocity of particle is v=(∆f.µ)/(2sin(θ/2))
DESIGN AND METHODOLOGY
Principle of LDV, differential beam technique
Laser
Signalprocessing
Transmittingoptics
Receiving opticswith detector
Signalconditioner
Flow
He-NeAr-IonNd-YagDiode
Beamsplitter, Achrom.Lens
Gas,LiquidParticle
Achrom. LensSpatial FilterPhotomultiplierPhotodiode
Spectrum analyserCorrelatorCounter, Tracker
AmplifierFilter
PC
The instrument has four components (i) Laser source He-Ne (ii) Beam splitter
(iii) Light pick-up unit (iv) Signal processor. He-Ne laser is generally usedThe beam inclination θ could be raised Intensity of reference beam could be
attenuated .
Light--->photodiode--->optical mixing---> weak signal(f=∆f)--->amplifier---->signal processor unit ----->filter---->amplifier---->f–v converter
The instantaneous output voltage can be recorded on a strip chart recorder or can be digitized and stored on a floppy disk or Compact disc
The mean voltage, averaged over a few seconds, is indicated by a panel meter
Sensor- photodiodeTransducer – filter and f-v converterModifier – amplifierTerminator- meter reading
COMPONENTS
There is no transfer function involvementNon contact type of measurementVery high frequency response upto megahertzVery high accuracyVery small sensing volumeSuitable in both gas and liquid flow
Why is it preferred?
Investigation of boundary layer Measurement of flow between blades of
turbineCombustion and flame phenomenon in gas
turbine and jet propulsion systemsIn measurement of blood flowWind velocities
Areas in which it is preferred
It has very high accuracy and precisionsAccurate upto 0.2%There is no need of calibration so its
measurement is very precise.
Accuracy and precision
non-intrusiveit can be used in both Laminar and turbulent
flowsSurface velocity and vibration measurementIts measurements are not afected by
temperature and terbiditywide range of velocities (~m/s – km/s)can be used in hard environments (high T,
corrosive, etc)1D, 2D, and 3D measurements
ADVANTAGES:
needs optical access (not steal walls, only transparent fluids)
single-point measurementsneeds seeding$$$$
DISADVANTAGES:
top related