Turbulence Measurements in:

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Turbulence Measurements in
Natural Convection Boundary Layer
Swirling Jet
by Abolfazl Shiri Thesis Supervisor William K.
George
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Turbulence Measurements in
Natural Convection Boundary Layer
Swirling Jet

• Why we did these two experiments?
• They were both turbulent flows and we aimed to
  measure the turbulence parameters.
• There is a lack of reliable experimental data in
  both flows.
• The velocity measurement method in both
  experiments was laser Doppler anemometry.
• Both have axisymmetric nature which simplifies
  the three-dimensionality of the flow.
• Doing a related experimental study while
  designing and installing the other experimental
  facility.

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Swirling Jet Experiment

• What is a jet flow?

Jet flow represent a class of free shear flows
that evolve in the absence of walls.
Free Shear Flows
Jets
Wakes
Shear Layer Flows
Plumes
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Anatomy of the Jet Flow

• Regions
• Potential core ( X/D 1 )
• Mixing layer
• Developing flow ( X/D 20 )
• Self-preserving flow

• Characteristic velocity scale Uc(x)
• Characteristic jet width d1/2(x)

Asymptotic behaviour of flow at self-preserving
region
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Entrainment
Jet mass flow

• Self-preserved region
• When the mass entrained by the
• turbulence overwhelms the
• added mass at the source of jet.

• Main application of jet flows in industry for
  mixing due to entrainment.
• Laboratory jets cant be categorized as
  universal self-similar, point-source of momentum
  jets.
• Virtual origin (x0) and jet growth rate (dd/dx)
  are the parameters characterizing the initial
  condition.
• Azimuthal velocity component (swirl) modifies
  the initial condition.

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Swirling Jet Flow

• Two cases of low and moderate swirl (S 0.15
  0.25) were compared with a non-swirling jet.
• Geometry of the nozzle and the velocity profile
  at the nozzle changes the initial condition.
• How the additional swirl effects the nozzle
  velocity profile?

Not a top-hat anymore!
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Jet Facility

• 1 inch jet nozzle diameter
• Six injectors for tangential flow derived by
  different blower
• 3.5m X 3.5m X 10m enclosure
• Solid-body rotation for tangential velocity
  distribution
• Reynolds number at nozzle 40,000

• Same facility which used in Hussein, Capp
  George 1994 for axisymmetric jets study.
• Brought from university of Buffalo by George and
  modified to add the swirl components.

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Summary of the Swirling Jet Experiment

• The far swirling jet is self-similar (like the
  non-swirling jet).
• For S lt 0.2, the effect of initial swirl is
  negligible.
• There is no considerable effect of swirl on
  growth rate, consistent with the theory.
• The change in the virtual origin of these jets
  are slight (consistent with the relatively low
  swirl number)
• The role of each term (production, advection,
  diffusion and dissipation) is similar in both
  swirling and non-swirling jet.

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Natural Convection Experiment
Conduction
Very Slow Process
Forced Convection
Convection
Heat Transfer Modes

Natural Convection
No need for a medium to tranfer the heat

• Natural convection flows are among the least
  well undersood.
• Although they are the most commonly occuring
  method of convective heat transfer, there is a
  lack of controlled and reliable experimental
  studies because of the difficulties.

Radiation
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Natural Convection Applications
Natural-draft cooling tower
Reactor heat exchanger
Radiator
Heat-sink
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Some Definitions
For vertical surface, transition to turbulence at
RaL ? 109
For a wall at T70 C in air, transition starts at
L ? 0.6 m
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Theory of the NCBL

• Turbulent natural convection boundary layer flow
  next to a cylindrical surface
• Axisymmetric flow homogeneous in tangential
  direction.
• Newtonian, Incompressible flow.
• Temperature gradient in the flow cause the
  density, viscosity and other thermodynamics
  properties variation.
• Buoyancy as the source of momentum.

To simplify the momentum and energy equations of
the flow
Inner layer ? Viscous and conduction terms are
dominating Outer layer ? Viscous and conduction
terms are negligible
B.L. equation separation
? For an acceptable seperation between the scales
we need a really big Grashof number flow...
This was primary reason for the large experimetal
facility at Chalmers.
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Experimental Rig

• Previous experiments
• Most of the experiments were carried out next to
  a vertical flat plate Tsuji Nagano (1988)
• Measurements on vertical cylinder by Persson
  Karlsson (1996) were problematic
• Low Grashof number
• Boundary conditions were not controlled.

New experimental facility was built to modify
the rig used by Persson Karlsson
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Experimental Rig Schematic
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Measurement Methods
Laser Doppler Anemometry (LDA)
Velocity measurement
Thermocouple
mean temperature
Temperature measurement
Cold-wire thermometry
instantaneous temperature
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Temperature Measurement Errors

• Prongs temperature gradient.
• Wall temperature measurement errors.
• Calibration uncertainities.
• Temperature measurement errors in very low
  velocity fluids.

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Summary of the NCBL Experiment

• The experiments were carried out in three
  different heights 1.5m, 3m and 4m corresponding
  to the Rayleigh numbers Ra 1.0 1010 , 7
  1010 and 1.7 1011 respectively.
• Simultaneous two components velocity and
  temperature measured across boundary layer in
  turbulent region.
• Temperature measurement methods were not
  suitable for this flow, but lack of any other
  alternative method with the necessary accuracy
  forced us to use them, considering the short
  comings.
• A comprehensive theoritical foundation was
  established for future investigations.

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In Memory of

• Professor Rolf Karlsson
• (1945 2005)