Tracer Particles and Seeding for PIV (PowerPoint)

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					Tracer Particles and Seeding for PIV
    Seeding particles for PIV
 Proper tracer must be small enough to follow
  (trace) fluid motion and should not alter fluid or
  flow properties.
 Proper tracer must be large enough to be visible by
  the camera.
 Uniform seeding is critical to the success of
  obtaining velocity field. No seed particles, no data.
    The seeding source must be placed cleverly so that the
     particles mix with the flow well.
    Particles with finite inertia are known to disperse non-
     uniformly in a turbulent flow, preferential concentration
   Seeding particles for PIV (cont’d)
The tracing ability and the dispersion
 characteristics depends on the
 aerodynamical characteristics of particles
 and the continuous medium;
The visibility depends on the scattering
 characteristics of particles.
The choice of optimal diameter for seeding
 particles is a compromise between two
 aspects.
   Scattering characteristics of particles

Laser sheet leads to a low energy density –
 particle scattering efficiency is important;
Light scattering capability - scattering cross
 section Cs is defined as the ratio of the total
 scattered power Ps, to the laser intensity I0
 incident on the particle
Example of scattering cross section (1)




   The scattering cross section as a function of
   the particle size (refractive index m=1.6).
Example of scattering cross section (2)

  Diameter dp      Scattering cross section Cs

 Molecule                          10-33m2

 1m              Cs(dp/)4       10-12m2

 10m             Cs( dp/)2      10-9m2



   Scattering cross section as a function of the
   particle size
 Mie scattering of small particle (1)




Light Scattering by an oil particle in air when refractive
index m ~ 1.4. Left: 1m diameter, right: 10m
diameter
Mie scattering of small particle (2)




                      Light scattering by a 1 m,
                      10 m, and 30 m glass
                      particle in water.
                      Refractive index m = 1.52
     Summary of particle light scattering
     for PIV
 The ratio Is90/Is0 decreases with increasing size parameter
    dp/, with values roughly in the range 10-1-10-3 for scattering
    particles useful in PIV.
   The resulting intensity of the scattered light for a given light
    sheet intensity will depend on the combined influences of Cs
    and Is90/Is0, which exhibit opposing tendencies with
    increasing particle size. In general, larger particles will still
    give stronger signals.
   The ratio Is90/Is0 increases with increasing refractive index
    m. Hence particles in air gives stronger 90o scattering than in
    water.
   Tracking characteristics of particles

The tracking ability depends on
   Particle shape – assumed spherical –
    aerodynamically equivalent diameter - dp
   Particle density p
   Fluid density f and fluid dynamic viscosity  or
    kinematic viscosity = /f
Newton’s Law governing the motion of a
 single particle:
    General governing equation



     Meaning of each term:
    I. Viscous drag according to the Stokes’ law
    II. Acceleration force
    III. Force due to a pressure gradient in the vicinity of the
         particle
    IV. Resistance of an inviscid fluid to the acceleration of the
         sphere (“added mass”)
    V. Basset history integral – resistance caused by the
         unsteadiness of the flow field.
   Stokes’ drag law
 The Stokes’ drag law is considered to apply when
  the particle Reynolds number Rep is smaller than
  unity, where Rep is defined as



 In a typical PIV experiment with 10m particles
  and 20 cm/s mean velocity,
  Rep=10x10-6 x 0.2 / 1.46x10-5 = 0.13 (air);
  Rep=10x10-6 x 0.2/1.0x10-6 = 2 (water).
    Particle parameter
    - the particle response time tp
 Velocity lag of a particle in a continuously
  accelerating fluid:

 The particle velocity response to the fluid velocity
  if heavy particles (p>>f) in a continuously
  accelerating flow is:



 Particle response time:
    Particle parameter
    - the Stokes number St
 Stokes number St as the ratio of the particle
  response time to the Kolmogorov time scale:



 St: the degree of coupling between the particle
  phase and the fluid.
    St0 the particles behave like tracers
    St the particles are completely unresponsive to the
     fluid flow.
   Particle parameter
   - the characteristic frequency C
In the case of gas flow where p>>f,
  characteristic frequency of the particle
  motion



Tracing ability in turbulence, c=2fc
Figure of characteristic frequency




The response of particles in turbulence flow. (From Haetig J,
Introductory on particle behavior ISL/AGRAD workshop on laser
anemometry (Institute Saint Louis) report R 117/76, 1976)
   Particle size vs. Turbulence scale

Seeding particles need to be smaller than the
  smallest turbulence scale if one wants to
  identify all the structures in the vicinity of
  the flow. The smallest fluid length scale is
  called the Kolmogorov length scale, and it is
  related to the size of the smallest eddy.
   Additional Considerations

Particle seeding uniformity
     Additional Considerations (cont’d)
   Secure sufficient spatial detail in the flow field a higher
    concentration of particles is generally needed with PIV
    than with LDV, with which it is possible to wait indefinitely
    for the arrival of a scattering particle in the probe volume.
   A uniform particle size is desirable in order to avoid
    excessive intensity from larger particles and background
    noise, decreasing the accuracy, from small particles.
   Particles that naturally exist in the flow seldom meet the
    above requirements. Hence, in PIV applications, it is often
    necessary to seed the flow with a chosen tracer particle. The
    particles are either premixed with the whole fluid (e.g.,
    stirred ) or released in situ by a seeding source.
    Imaging of small particles
 Relation between real particles and particle image recorded
  in the camera can be analyzed by the diffraction limited
  imaging of a small particle
                                For a given aperture
                                diameter Da and wavelength ,
                                the Airy spot size
    Imaging of small particles (cont’s)

 With an imaging lens, the
  diffraction-limited size:




 Estimate of the particle
  image diameter:



   dp: original particle diameter
Seeding particles for PIV (liquid flow)
Seeding particles for PIV (gas)
    Commercial seeding particles - TSI
    (http://www.tsi.com)
 Silicon Carbide: Suitable for measurements in liquids and
  gases, silicon carbide particles have a narrow particle size
  distribution (mean diameter of 1.5m). Their high refractive
  index is useful for obtaining good signals in water, even in
  backscatter operation. They can also be used in high
  temperature flows. Supplied as a dry powder, they can be
  mixed in liquid to form a suspension before dispersing.
 Titanium Dioxide: Titanium dioxide particles (mean
  diameter of 0.2m) are usually dispersed as a dry powder
  for gas flow measurement applications. The smaller particle
  size makes titanium dioxide attractive for high-speed flows.
  It can also be used for high temperature flows.
     Commercial seeding particles - TSI
     (http://www.tsi.com) (cont’d)
 Polystyrene Latex: With an extremely narrow size
    distribution (nominal diameter of 1.0m), polystyrene latex
    (PSL) particles are useful in many different measurements.
    Supplied in water, they are not recommended for high
    temperature applications.
   Metallic coated: Metallic coated particles (mean diameter
    of 9.0m) are normally used to seed water flows for LDV
    measurements due to their lower density and higher
    reflectivity. They cannot be used where salt is present. Salt
    reacts with the metal coating, causing the particles to
    agglomerate and drop out of the flow.
Commercial seeding particles - TSI
(http://www.tsi.com) (cont’d)
      Commercial seeding particles - Dantec
      (http://www.dantecmt.com)
   Polyamide seeding particles (PSP): These are produced by
    polymerisation processes and therefore have a round but not exactly
    spherical shape. They are microporous and strongly recommended for
    water flow applications.
   Hollow glass spheres and silver-coated hollow glass spheres (HGS,
    S-HGS): Intended primarily for liquid flow applications, these are
    borosilicate glass particles with a spherical shape and a smooth surface.
    A thin silver coating further increases reflectivity.
   Fluorescent polymer particles (FPP): These particles are based on
    melamine resin. Fluorescent dye (Rhodamine B:) is homogeneously
    distributed over the entire particle volume. In applications with a high
    background light level, fluorescent seeding particles can significantly
    improve the quality of vector maps from PIV and LDV measurements.
    The receiving optics must be equipped with a filter cantered on the
    emission wavelength (excitation max.: 550 nm; emission max.: 590 nm).
Commercial seeding particles - Dantec
(http://www.dantecmt.com) (cont’d)
   Particle generation
 Liquid flow
   Simple, select proper powder then mix w/ liquid
 Gas flow
   liquid droplets
      Atomization or Condensation
   solid particles
      Atomization or Fluidization
 Requirement for PIV
   Nearly monodisperse size distribution
   High production rate
  Liquid droplets
Advantage
  Steady production rate;
  Inherently spherical shape;
  Known refractive index
Problem
  Form non-uniform liquid films on window
Generator
  Laskin atomizer
  Commercial atomizer (e.g., TSI)

				
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posted:11/30/2011
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