abolfazl shiri feb. 19 th, 2010 1 turbulence measurements in: natural convection boundary layer...

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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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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.

JetsWakes Shear Layer FlowsPlumes

Free Shear Flows

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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 δ1/2(x)

Asymptotic behaviour of flow at self-preserving region:

1/ 2

( , )( )

( )c

U x r rf

U x

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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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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

• 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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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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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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Natural Convection Applications

Natural-draft cooling tower

RadiatorHeat-sink

Reactorheat exchanger

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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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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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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

Velocity measurement: Laser Doppler Anemometry (LDA)

Temperature measurement:

Cold-wire thermometry

Thermocouple

mean temperature

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)

abolfazl shiri feb. 19 th, 2010 1 turbulence measurements in: natural convection boundary layer...

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