Tracer Particles and Seeding for PIV

1
Tracer Particles and Seeding for PIV
2
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

3
Seeding particles for PIV (contd)

• 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.

4
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

5
Example of scattering cross section (1)
The scattering cross section as a function of the
particle size (refractive index m1.6).
6
Example of scattering cross section (2)
Scattering cross section as a function of the
particle size
7
Mie scattering of small particle (1)


Light Scattering by an oil particle in air when
refractive index m 1.4. Left 1mm diameter,
right 10mm diameter
8
Mie scattering of small particle (2)
Light scattering by a 1 mm, 10 mm, and 30 mm
glass particle in water. Refractive index m 1.52
9
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.

10
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
• Newtons Law governing the motion of a single
  particle

11
General governing equation

• Meaning of each term
• Viscous drag according to the Stokes law
• Acceleration force
• Force due to a pressure gradient in the vicinity
  of the particle
• Resistance of an inviscid fluid to the
  acceleration of the sphere (added mass)
• Basset history integral resistance caused by
  the unsteadiness of the flow field.

12
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,Rep10x10-6 x 0.2 /
  1.46x10-5 0.13 (air)Rep10x10-6 x
  0.2/1.0x10-6 2 (water).

13
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 (?pgtgt?f) in a
  continuously accelerating flow is
• Particle response time

14
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.

15
Particle parameter- the characteristic frequency
C

• In the case of gas flow where ?pgtgt?f,
  characteristic frequency of the particle motion
• Tracing ability in turbulence, ?c2?fc

16
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)
17
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.

18
Additional Considerations

• Particle seeding uniformity

19
Additional Considerations (contd)

• 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.

20
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

21
Imaging of small particles (conts)

• With an imaging lens, the diffraction-limited
  size
• Estimate of the particle image diameter

dp original particle diameter
22
Seeding particles for PIV (liquid flow)
23
Seeding particles for PIV (gas)
24
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.

25
Commercial seeding particles - TSI
(http//www.tsi.com) (contd)

• 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.

26
Commercial seeding particles - TSI
(http//www.tsi.com) (contd)
27
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).

28
Commercial seeding particles - Dantec
(http//www.dantecmt.com) (contd)
29
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

30
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)