Turbulence Measurements in:

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

2. 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.

3. 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 Jets Plumes Shear Layer Flows

4. 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:

5. 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.

6. 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!

7. 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.

8. 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.

9. 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

10. Natural Convection Applications Natural-draft cooling tower Reactor heat exchanger Heat-sink Radiator

11. Natural convection can be neglected Natural convection dominates Some Definitions 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

12. 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.

13. 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

14. Experimental Rig Schematic

15. Measurement Methods Laser Doppler Anemometry (LDA) Velocity measurement: Thermocouple mean temperature Temperature measurement: Cold-wire thermometry instantaneous temperature

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

17. 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.

18. In Memory of Professor Rolf Karlsson (1945 – 2005)