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Turbulence Measurements in:
Natural Convection
Boundary Layer
Swirling Jet
by
Abolfazl Shiri
Thesis Supervisor
William K. George
Abolfazl SHIRI
Feb. 19th, 2010
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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.
Abolfazl SHIRI
Feb. 19th, 2010
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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
Wakes
Abolfazl SHIRI
Jets
Plumes
Shear Layer Flows
Feb. 19th, 2010
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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
• Characteristic jet width
Uc(x)
δ1/2(x)
Asymptotic behaviour of flow at
self-preserving region:
U ( x, r )
r
 f(
)
U c ( x)
1/ 2
Abolfazl SHIRI
Feb. 19th, 2010
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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 can’t be categorized as universal self-similar, point-source of momentum jets.
• Virtual origin (x0) and jet growth rate (dδ/dx) are the parameters characterizing the initial condition.
• Azimuthal velocity component (swirl) modifies the initial condition.
Abolfazl SHIRI
Feb. 19th, 2010
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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?
Abolfazl SHIRI
Not a top-hat anymore!
Feb. 19th, 2010
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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.
Abolfazl SHIRI
Feb. 19th, 2010
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Summary of the Swirling Jet Experiment
• The far swirling jet is self-similar (like the non-swirling jet).
• For S < 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)
•
U c  1/12  x 1

Wmax  1/ 22  x  2
• The role of each term (production, advection, diffusion and dissipation) is similar in
both swirling and non-swirling jet.
Abolfazl SHIRI
Feb. 19th, 2010
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Natural Convection Experiment
Heat Transfer Modes
Conduction
Very Slow Process
Forced Convection
Convection
+
Natural Convection
• Natural convection flows are among the least well undersood.
No need for a medium
to tranferthere
the heat
Radiation
• Although
they are the most commonly occuring method of convective heat transfer,
is a
lack of controlled and reliable experimental studies because of the difficulties.
Abolfazl SHIRI
Feb. 19th, 2010
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Natural Convection Applications
Natural-draft
cooling tower
Heat-sink
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Reactor
heat exchanger
Radiator
Feb. 19th, 2010
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Some Definitions
GrL 
g  Ts  T  L3
2

Buoyancy forces
Viscous forces
GrL
GrL
Natural
Natural

1

1
convection can Re 2
convection
Re 2L
L
be neglected
dominates
Ra L  GrL  Pr
For vertical surface, transition to turbulence at RaL  109
For a wall at T=70 C in air, transition starts at L  0.6 m
Abolfazl SHIRI
Feb. 19th, 2010
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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 B.L. equation
separation
equations of the flow
Inner layer → Viscous and conduction terms are dominating
Outer layer → Viscous and conduction terms are negligible
→ 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.
Abolfazl SHIRI
Feb. 19th, 2010
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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
Abolfazl SHIRI
Feb. 19th, 2010
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Experimental Rig Schematic
Abolfazl SHIRI
Feb. 19th, 2010
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Measurement Methods
Velocity measurement:
Laser Doppler Anemometry
(LDA)
Thermocouple
Temperature measurement:
mean temperature
Cold-wire thermometry
instantaneous temperature
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Feb. 19th, 2010
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Temperature Measurement Errors
• Prongs temperature gradient.
• Wall temperature measurement errors.
• Calibration uncertainities.
• Temperature measurement errors in very low
velocity fluids.
Abolfazl SHIRI
Feb. 19th, 2010
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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.
Abolfazl SHIRI
Feb. 19th, 2010
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In Memory of
Professor Rolf Karlsson
(1945 – 2005)
Abolfazl SHIRI
Feb. 19th, 2010