Some of the outflowing ions in the plasma edge have sufficient energy to access orbits which allow them to free-stream out of the confined plasma region and be lost to the wall or divertor. The effects of this ion-orbit-loss (IOL) on the poloidal distribution of ion, energy and momentum fluxes from the plasma edge into the tokamak scrape-off layer (SOL) are analyzed for a representative DIII-D H-mode discharge. IOL yields large fluxes of particle, energy and momentum, distributed poloidally over the SOL, but predominantly into the outboard SOL, significantly changing the fluxes due to transport processes for confined ions within the edge plasma. An intrinsic co-current rotation in the edge of the plasma is produced by the preferential loss of counter-current ions

Spray cooling is a high heat removal technique which has been used widely in many industries, especially metallurgical, where the control of the temperatures of metals is an important factor to obtain the desired microstructure; and also in microelectronics where is very important to obtain high heat fluxes at relatively low surface temperatures. In this study, an open loop spray cooling system has been fabricated to provide an upward-facing spray over a 12 mm diameter test surface. A full cone spray nozzle was used to deliver deionized water to the test surface at five pressures (10, 15, 20, 25 and 30 psi), and at three different distances to the test surface (3, 7 and 12 mm). The volumetric flow rate at the surface used in the experiments depended on both the pressures and the distances. For a distance of 3 mm and 7 mm, the volumetric flow rate range from 336.6 to 627 ml/min while for 12 mm, the range was from 336.6 to 484.28 ml/min. Heat fluxes of 1.92 to 451 W/cm2, 2.1 to 417.3 W/cm2 and 1.9 to 409.5 W/cm2 for distances of 3, 7 and 12 mm respectively were registered at different input power levels. For all the three distances, the volumetric flow rate affects the heat flux, especially for 3 mm; and this effect decreases for higher distances. However, the distance between the nozzle and the test surface has little effect on the heat flux at low pressures but at higher pressures, the difference in heat flux is mainly due to the fact that part of the spray does not impinge the test surface.

Spray cooling is a technique for achieving large heat fluxes at low surface temperatures by impinging a liquid in droplet form on a heated surface. Heat is removed by droplets spreading across the surface, thus removing heat by evaporation and by an increase in the convective heat transfer coefficient. The addition of nano-sized particles, like aluminum or copper, to water to create a nanofluid could further enhance the spray cooling process. Nanofluids have been shown to have better thermophysical properties when compared to water, like enhanced thermal conductivity. Although droplet size, velocity, impact angle and the roughness of the heated surface are all factors that determine the amount of heat that can be removed, the dominant driving mechanism for heat dissipation by spray cooling is difficult to determine. In the current study, experiments were conducted to compare the enhancement to heat transfer caused by using alumina nanofluids during spray cooling instead of de-ionized water for the same nozzle pressure and distance from the heated surface. The fluids were sprayed on a heated copper surface at a constant distance of 21 mm. Three mass concentrations, 0.1%, 0.5%, and 1.0%, of alumina nanofluids were compared against water at three pressures, 40psi, 45psi, and 50psi. To ensure the suspension of the aluminum oxide nanoparticles during the experiment, the pH level of the nanofluid was altered. The nanofluids showed an enhancement during the single-phase heat transfer and an increase in the critical heat flux (CHF). The spray cooling heat transfer curve shifted to the right for all concentrations investigated, indicating a delay in two-phase heat transfer. The surface roughness of the copper surface was measured before and after spray cooling as a possible cause for the delay.

The flow structure and convective heat transfer behavior of a free liquid jet ejecting from a round nozzle impinging vertically on a hemispherical solid plate and a slot nozzle impinging vertically on a cylindrical curved plate have been studied using a numerical analysis approach. The simulation model incorporated the entire fluid region and the solid hemisphere or curved plate. Solution was done for both isothermal and constant heat flux boundary conditions at the inner surface of the hemispherical plate and the constant heat flux boundary condition at the inner surface of the cylindrical shaped plate. Computations for the round nozzle impinging jet on the hemispherical plate and cylindrical plate were done for jet Reynolds number (ReJ) ranging from 500 to 2000, dimensionless nozzle to target spacing ratio (β) from 0.75 to 3, and for various dimensionless plate thicknesses to diameter nozzle ratio (b/dn) from 0.083-1.5. Also, computations for the slot nozzle impinging jet on the cylindrical plate were done for inner plate radius of curvature to nozzle diameter ratio (Ri/dn) of 4.16-16.66, plate thickness to nozzle diameter ratio (b/dn) of 0.08-1.0, and different nozzle diameters (dn), Results are presented for dimensionless solid-fluid interface temperature, dimensionless maximum temperature in the solid, local and average Nusselt numbers using the following fluids: water (H2O), flouroinert (FC-77), and oil (MIL-7808) and the following solid materials: aluminum, copper, Constantan, silver, and silicon. Materials with higher thermal conductivity maintained a more uniform temperature distribution at the solid-fluid interface. A higher Reynolds number increased the Nusselt number over the entire solid-fluid interface. Local and average Nusselt number and heat transfer coefficient distributions showed a strong dependence on the impingement velocity or Reynolds number. As the velocity increases, the local Nusselt number increases over the entire solid-fluid interface. Decreasing the nozzle to target spacing favors the increasing of the Nusselt number. Increasing the nozzle diameter decreases the temperature at the curved plate outer surface and increases the local Nusselt number. Similarly, local and average Nusselt number was enhanced by decreasing plate thickness. Numerical simulation results are validated by comparing with experimental measurements and related correlations.

This thesis is devoted to the study of spinning contacts located in bearing between the roller-end and the ring flange. The main direction of the lubricant flow may change when the contact is subjected to skew. This complex kinematics influences the contact behaviour. A dual experimental-numerical approach is proposed to study this problem. The Tribogyr test-rig allows the experimentation of the contact at the 1:1 scale. A film thickness measurement method, based on white light interferometry, was developed on Tribogyr. This method allows the measurement of film thickness between 0 and 800 nm with an accuracy of a few nanometres. The measurement of forces in the main flow direction shows similarities with classical rolling-sliding contacts. However, the friction coefficient is globally lower as soon as spin is involved. Transverse forces are of the same order of magnitude as the longitudinal forces. This is due to transverse shearing caused by the spin. A numerical model has been developed for the simulation of these spinning contacts. The finite element model, which is based on a fully-coupled solving strategy, takes into account the temperature calculation and the lubricant non-Newtonian rheology. Its validation with Tribogyr experimental results in terms of film thickness and friction has been conducted. Spin and skew effects induce high shear-thinning and thermal-thinning of the lubricant that lead to a decrease of the film thickness. Under high spinning condition, the lubricant exiting the contact may be re-injected to the contact inlet. Consequently, the heat transfers between the lubricant and the solids in contact are modified. In contact subjected to high skew, a local increase (dimple) of the film thickness may occur. Important skew may also lead to starvation conditions. Many experimental campaigns, coupled with an intensive use of the numerical model, allowed to understand the physical phenomena involved as well as to predict the efficiency, in terms of power losses, of the spinning contacts.

Bubble nucleation at subcooled flow boiling condition in a horizontal annular channel with a square cross section by the use of high-speed camera is investigated. The channel represents a scaled-down version of a single rod of CANDU reactor core. The experiments were performed by the use of water at pressures between 1-3 atm, constant heat flux of 0.124 MW/m2, liquid bulk subcooling of 32-1oC and mean flow velocities of 0.3-0.4 m/s. Bubble lift-off diameters were obtained from direct high speed videography. The developed model for the bubble lift-off diameter was obtained by analyzing the forces acting on a bubble. Furthermore, a model for the bubble growth rate constant was suggested. The proposed model was then compared to experimental data and it has shown a good agreement with the experimental data. Additionally, the effects of liquid bulk subcooling, liquid pressure and mean flow velocity on bubble lift-off diameter were investigated.

Bubble nucleation at subcooled flow boiling condition in a horizontal annular channel with a square cross section by the use of high-speed camera is investigated. The channel represents a scaled-down version of a single rod of CANDU reactor core. The experiments were performed by the use of water at pressures between 1-3 atm, constant heat flux of 0.124 MW/m2, liquid bulk subcooling of 32-1oC and mean flow velocities of 0.3-0.4 m/s. Bubble lift-off diameters were obtained from direct high speed videography. The developed model for the bubble lift-off diameter was obtained by analyzing the forces acting on a bubble. Furthermore, a model for the bubble growth rate constant was suggested. The proposed model was then compared to experimental data and it has shown a good agreement with the experimental data. Additionally, the effects of liquid bulk subcooling, liquid pressure and mean flow velocity on bubble lift-off diameter were investigated.

By using the finite element method to solve the appropriate momentum and energy equations the friction factors and Nusselt numbers for fully developed laminar flow were determined for one- and two-dimensional flow systems. The Nusselt numbers were determined for domain boundaries subjected to a constant heat flux (H1) or a constant surface temperature (T) around the computational boundaries and in the axial directions. C++ programs, that were rewritten and extended from previous programs, were used to solve the laminar flow and to determine the values. The required wall shear stresses and heat fluxes were directly obtained for a duct as part of the primary finite-element solution; these values were then used to determine the Nusselt number and friction factor for the specific duct. The computations were performed for circular-, annular-, trapezoidal-, rectangular- and triangular ducts. Special emphasis was placed on trapezoidal ducts since only a limited number of studies have been performed on trapezoidal duct shapes and none of these studies employed the finite element method. Excellent agreement was found when the determined values were compared with the values reported in the literature. In general, the agreement of the values improved as the number of elements was increased. It was, therefore, concluded that the methods used in this study yielded friction factors and Nusselt numbers that are very accurate and usable. / Thesis (M.Ing. (Mechanical Engineering))--North-West University, Potchefstroom Campus, 2009.

By using the finite element method to solve the appropriate momentum and energy equations the friction factors and Nusselt numbers for fully developed laminar flow were determined for one- and two-dimensional flow systems. The Nusselt numbers were determined for domain boundaries subjected to a constant heat flux (H1) or a constant surface temperature (T) around the computational boundaries and in the axial directions. C++ programs, that were rewritten and extended from previous programs, were used to solve the laminar flow and to determine the values. The required wall shear stresses and heat fluxes were directly obtained for a duct as part of the primary finite-element solution; these values were then used to determine the Nusselt number and friction factor for the specific duct. The computations were performed for circular-, annular-, trapezoidal-, rectangular- and triangular ducts. Special emphasis was placed on trapezoidal ducts since only a limited number of studies have been performed on trapezoidal duct shapes and none of these studies employed the finite element method. Excellent agreement was found when the determined values were compared with the values reported in the literature. In general, the agreement of the values improved as the number of elements was increased. It was, therefore, concluded that the methods used in this study yielded friction factors and Nusselt numbers that are very accurate and usable. / Thesis (M.Ing. (Mechanical Engineering))--North-West University, Potchefstroom Campus, 2009.

EFFECTS OF OFF-CENTER ANGLE ON THE HEAT TRANSFER COEFFICIENT ON VERTiCAL TIER OF MULTIPLE SPHERICAL SURFACES Kaya, Ebubekir M.S., Department of Mechanical Engineering Supervisor: Assoc. Prof. Dr. Cemil Yamali December 2004, 112 pages The purpose of this study is to investigate the laminar film condensation phenomenon of steam on a vertical tier of multiple spherical surfaces by using both analytical and experimental methods. The analytical heat transfer results were obtained by following the Nusselt type of analysis and represented graphically. In addition, in order to observe the real behavior of the film condensation, an experimental setup was manufactured and experiments were done. In analytical section / mass flow rate, (mean) velocity, film thickness, local heat flux and local heat transfer coefficient values were obtained and plotted as depending on angular position. Moreover, mean heat flux and mean heat transfer coefficient variations were presented with respect to diameter of the sphere and sub-cooling. On the other hand, for the experimental section, heat flux and mean heat transfer coefficient values were obtained and expressed as depending on sub-cooling. To see the effects of off-center angle, setup was inclined for different angles and experiments were repeated for each inclination angle. At the end of the study, mean heat transfer coefficients belong to analytical and experimental studies were compared to each other as well as to the literature.