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Abolfazl SHIRI Feb. 19 th, 2010
Turbulence Measurements in:
Natural Convection Boundary Layer
Swirling Jet
by
Abolfazl ShiriAbolfazl Shiri
Thesis Supervisor
William K. GeorgeWilliam K. George
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Abolfazl SHIRI Feb. 19 th, 2010
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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Abolfazl SHIRI Feb. 19 th, 2010
Swirling Jet Experiment
• What is a jet flow?Jet flow represent a class of free shear flows that evolve in the absence of walls.
JetsWakes Shear Layer FlowsPlumes
Free Shear Flows
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Abolfazl SHIRI Feb. 19 th, 2010
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 δ1/2(x)
Asymptotic behaviour of flow at self-preserving region:
1/ 2
( , )( )
( )c
U x r rf
U x
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Abolfazl SHIRI Feb. 19 th, 2010
Entrainment
• 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.
Jet mass flow
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Abolfazl SHIRI Feb. 19 th, 2010
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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Abolfazl SHIRI Feb. 19 th, 2010
Jet Facility
• 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.
• 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
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Abolfazl SHIRI Feb. 19 th, 2010
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)
•
• The role of each term (production, advection, diffusion and dissipation) is similar in
both swirling and non-swirling jet.
222/1max
112/1
xW
xUc
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Abolfazl SHIRI Feb. 19 th, 2010
Natural Convection Experiment
Conduction
Convection
Radiation
Forced Convection
Natural Convection
Hea
t Tra
nsfe
r M
odes
Very Slow Process
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.
+
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Abolfazl SHIRI Feb. 19 th, 2010
Natural Convection Applications
Natural-draft cooling tower
RadiatorHeat-sink
Reactorheat exchanger
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Abolfazl SHIRI Feb. 19 th, 2010
Some Definitions
forces Viscous
forcesBuoyancy
2
3
LTTg
Gr sL
For vertical surface, transition to turbulence at RaL 109
Pr LL GrRa
1Re2
L
LGr1
Re2
L
LGr Natural convection dominates
Natural convection can be neglected
For a wall at T=70 C in air, transition starts at L 0.6 m
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Abolfazl SHIRI Feb. 19 th, 2010
Theory of the NCBL
→ 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.
Inner layer → Viscous and conduction terms are dominating
Outer layer → Viscous and conduction terms are negligible
To simplify the momentum and energy equations of the flow
B.L. equation separation
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.
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Abolfazl SHIRI Feb. 19 th, 2010
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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Abolfazl SHIRI Feb. 19 th, 2010
Experimental Rig Schematic
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Abolfazl SHIRI Feb. 19 th, 2010
Measurement Methods
Velocity measurement: Laser Doppler Anemometry (LDA)
Temperature measurement:
Cold-wire thermometry
Thermocouple
mean temperature
instantaneous temperature
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Abolfazl SHIRI Feb. 19 th, 2010
Temperature Measurement Errors
• Prongs temperature gradient.
• Wall temperature measurement errors.
• Calibration uncertainities.
• Temperature measurement errors in very low velocity fluids.
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Abolfazl SHIRI Feb. 19 th, 2010
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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Abolfazl SHIRI Feb. 19 th, 2010
In Memory of
Professor Rolf Karlsson
(1945 – 2005)