Abolfazl SHIRI Feb. 19 th, Turbulence Measurements in: Natural Convection Boundary Layer Swirling Jet by Abolfazl Shiri Thesis Supervisor William K. George

Abolfazl SHIRI Feb. 19 th, 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. 19 th, Swirling Jet Experiment What is a jet flow? Jet flow represent a class of free shear flows that evolve in the absence of walls. JetsWakesShear Layer FlowsPlumes Free Shear Flows

Abolfazl SHIRI Feb. 19 th, Anatomy of the Jet Flow Regions: Potential core ( X/D ~ 1 ) Mixing layer Developing flow ( X/D ~ 20 ) Self-preserving flow Characteristic velocity scale U c (x) Characteristic jet width δ 1/2 (x) Asymptotic behaviour of flow at self-preserving region:

Abolfazl SHIRI Feb. 19 th, 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 (x 0 ) and jet growth rate (dδ/dx) are the parameters characterizing the initial condition. Azimuthal velocity component (swirl) modifies the initial condition. Jet mass flow

Abolfazl SHIRI Feb. 19 th, 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!

Abolfazl SHIRI Feb. 19 th, 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

Abolfazl SHIRI Feb. 19 th, 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.

Abolfazl SHIRI Feb. 19 th, Natural Convection Experiment Conduction Convection Radiation Forced Convection Natural Convection Heat Transfer Modes 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. +

Abolfazl SHIRI Feb. 19 th, Natural Convection Applications Natural-draft cooling tower Radiator Heat-sink Reactor heat exchanger

Abolfazl SHIRI Feb. 19 th, Some Definitions For vertical surface, transition to turbulence at Ra L  10 9 Natural convection dominates Natural convection can be neglected For a wall at T=70 C in air, transition starts at L  0.6 m

Abolfazl SHIRI Feb. 19 th, 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.

Abolfazl SHIRI Feb. 19 th, 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. 19 th, Experimental Rig Schematic

Abolfazl SHIRI Feb. 19 th, Measurement Methods Velocity measurement: Laser Doppler Anemometry (LDA) Temperature measurement: Cold-wire thermometry Thermocouple mean temperature instantaneous temperature

Abolfazl SHIRI Feb. 19 th, Temperature Measurement Errors Prongs temperature gradient. Wall temperature measurement errors. Calibration uncertainities. Temperature measurement errors in very low velocity fluids.

Abolfazl SHIRI Feb. 19 th, 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 × 10 10, 7 × and 1.7 × 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. 19 th, In Memory of Professor Rolf Karlsson (1945 – 2005)