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.

No description available.

This research is focused on formulating a new model of entropy production for two-phase flow, including cavitating turbulent flow. In particular, it focuses on the following aspects of the fluid dynamics and the potential contribution of the model to fluid device design. It includes (i) developing a new turbulent entropy model, (ii) a new formula of entropy production rate for two-phase flow including cavitating turbulent flow based on the second law, (iii) applying the technique to study a NACA hydrofoil, and (iv) conducting associated performance analysis of a propeller using post-processing of the CFD results and demonstrating that entropy production of two-phase cavitating flow around the propeller can be correlated to the loss of power output. The first stage consists of formulating the entropy production for laminar channel flow using Gibb’s free energy. This model is validated through the analytically solved Navier-Stokes equations. Subsequently, the single-phase turbulent flow is formulated in a similar manner, but the validations are carried out by comparing the prediction of the model with DNS results. Then, the model of entropy production for two-phase turbulent flow is derived from Gibb’s equation and a version of the Reynolds averaged Navier-Stokes (RANS) equations. The k- ε model is employed to represent the turbulent properties of single phase and two phase flows. A developed inter-phase slip algorithm mixture model is applied to control over coupling of phases. The Rayleigh-Plesset equation is used to model the rate of mass generation of vapour at the inter phase. The standard k-ε turbulence equations are used to describe turbulence in the cavitation flow. The validations of CFD predictions include exploring the force and cavitation characteristics of the NACA 4412 hydrofoil section. The application of this entropy production model in engineering design is presented via the comparisons between CFD results and the experimental data for the velocity distributions behind propeller P5168.

Energy shortages around the world necessitated research into alternative energy sources especially for domestic applications to reduce the load on conventional energy sources. This resulted in research done on the possibility of integrating solar energy with an aqua-ammonia diffusion absorption cycle specifically for domestic applications. The bubble pump can be seen as the heart of the diffusion absorption cycle, since it is responsible, in the absence of a mechanical pump, to circulate the fluid and to desorb the refrigerant (ammonia) from the mixture. It is thus of paramount importance to ensure that the bubble pump is designed efficiently. Various bubble pump simulation models have been developed over the years, but it was found that none of the existing models served as a good basis for application-specific design. Most of the models constrained too many parameters from the outset which made the investigation of the effects of certain parameters on the bubble pump’s performance impossible. According to the research, no bubble pump model investigated the effect of such a wide variety of factors including tube diameter, heat flux, mass flux, generator heat input and system pressure on the bubble pump’s lift height. A simulation model for a bubble pump for integration with a solar-driven aqua-ammonia diffusion absorption cycle was developed. It serves as a versatile design model to optimise the bubble pump for a large variety of conditions as well as changes in parameters. It was achieved by constraining the bubble pump dimensions and parameters as little as possible. A unique feature of this model was the fact that the bubble pump tube was divided into segments of known quality which made the length of the pipe completely dependent on the flow inside the pipe. It also made the demarcation of the flow development inside the tube easier. The model attempted to incorporate the most appropriate correlations for pressurised two-phase aqua-ammonia flow. The most appropriate void fraction correlation was found to be Abstract The design and optimisation of a bubble pump for an aqua-ammonia diffusion absorption heat pump the Rouhani-Axelsson (Rouhani I) correlation. It was mainly due to its exclusive use of thermophysical properties and the vapour quality. The most appropriate heat transfer coefficient that predicted the most realistic wall temperature, was the correlation from Riviera and Best (1999) which was the only correlation found in the literature developed with aqua-ammonia in mind. It was found that the published correlation could not reproduce their experimental results, and a modification of their correlation was made after which the simulation model’s results correlated well with the experimental values of Riviera and Best (1999). The main goal of the simulation model was to determine the height that the bubble pump was capable of lifting at the slug to churn flow transition under various conditions. The effect of varying a variety of parameters on the bubble pump lift height was also investigated. The results from Shelton & White Stewart (2002) were compared to the outputs of the simulation model, and it was found that their constraining of the submergence ratio limited their outputs, and that their heat inputs under different conditions was a bit optimistic. The simulation model’s outputs correlated well at higher tube diameters with the results from Shelton & White Stewart (2002), but at the lower diameters which was used in their study it was impossible to compare data, since their diameters was already in mini flow and micro flow regions. The temperatures also correlated well, all within 2% of the results from Shelton & White Stewart (2002). It was found that there couldn’t be just one set of optimised conditions and values for the bubble pump, but that each cycle with differing specifications and operating conditions would yield a unique set of optimised parameters. It was for that reason very important not to constrain parameters beforehand without investigating its effect on the bubble pump first. / Thesis (MIng (Mechanical Engineering))--North-West University, Potchefstroom Campus, 2013.

Energy shortages around the world necessitated research into alternative energy sources especially for domestic applications to reduce the load on conventional energy sources. This resulted in research done on the possibility of integrating solar energy with an aqua-ammonia diffusion absorption cycle specifically for domestic applications. The bubble pump can be seen as the heart of the diffusion absorption cycle, since it is responsible, in the absence of a mechanical pump, to circulate the fluid and to desorb the refrigerant (ammonia) from the mixture. It is thus of paramount importance to ensure that the bubble pump is designed efficiently. Various bubble pump simulation models have been developed over the years, but it was found that none of the existing models served as a good basis for application-specific design. Most of the models constrained too many parameters from the outset which made the investigation of the effects of certain parameters on the bubble pump’s performance impossible. According to the research, no bubble pump model investigated the effect of such a wide variety of factors including tube diameter, heat flux, mass flux, generator heat input and system pressure on the bubble pump’s lift height. A simulation model for a bubble pump for integration with a solar-driven aqua-ammonia diffusion absorption cycle was developed. It serves as a versatile design model to optimise the bubble pump for a large variety of conditions as well as changes in parameters. It was achieved by constraining the bubble pump dimensions and parameters as little as possible. A unique feature of this model was the fact that the bubble pump tube was divided into segments of known quality which made the length of the pipe completely dependent on the flow inside the pipe. It also made the demarcation of the flow development inside the tube easier. The model attempted to incorporate the most appropriate correlations for pressurised two-phase aqua-ammonia flow. The most appropriate void fraction correlation was found to be Abstract The design and optimisation of a bubble pump for an aqua-ammonia diffusion absorption heat pump the Rouhani-Axelsson (Rouhani I) correlation. It was mainly due to its exclusive use of thermophysical properties and the vapour quality. The most appropriate heat transfer coefficient that predicted the most realistic wall temperature, was the correlation from Riviera and Best (1999) which was the only correlation found in the literature developed with aqua-ammonia in mind. It was found that the published correlation could not reproduce their experimental results, and a modification of their correlation was made after which the simulation model’s results correlated well with the experimental values of Riviera and Best (1999). The main goal of the simulation model was to determine the height that the bubble pump was capable of lifting at the slug to churn flow transition under various conditions. The effect of varying a variety of parameters on the bubble pump lift height was also investigated. The results from Shelton & White Stewart (2002) were compared to the outputs of the simulation model, and it was found that their constraining of the submergence ratio limited their outputs, and that their heat inputs under different conditions was a bit optimistic. The simulation model’s outputs correlated well at higher tube diameters with the results from Shelton & White Stewart (2002), but at the lower diameters which was used in their study it was impossible to compare data, since their diameters was already in mini flow and micro flow regions. The temperatures also correlated well, all within 2% of the results from Shelton & White Stewart (2002). It was found that there couldn’t be just one set of optimised conditions and values for the bubble pump, but that each cycle with differing specifications and operating conditions would yield a unique set of optimised parameters. It was for that reason very important not to constrain parameters beforehand without investigating its effect on the bubble pump first. / Thesis (MIng (Mechanical Engineering))--North-West University, Potchefstroom Campus, 2013.

Modern combustion systems benefit from constant technological advanceswhich aim at reducing the emissions of chemical pollutants and at wideningregimes of stable operation. Further progress in the combustion field requiresa better understanding and modelling of the combustion dynamics. In thesesystems, the combustible is often injected as a liquid polydisperse spray. Experimentaldata are thus required to validate simulation tools in configurationswith flames interacting with controlled structures in multi-phase flows.This thesis aims at studying some of these fundamental interactions in wellcontrolledlaminar flows submitted to upstream modulations. Two experimentalconfigurations are investigated comprising counterflow flames and free inertjets, fed with gaseous or liquid combustibles. The flows may be submittedto upstream velocity modulations to reproduce effects of unsteadiness. Dependingon the pulsation frequency, vortices of controlled sizes are shed fromthe burner lips and convected with the flow, while interacting with the sprayand the flame.In the first part of this thesis, the dynamics of a premixed stretched flameis analysed in a stagnation flow. The study focuses on determining the flowand flame structures under upstream modulations, and principally on studyingthe dynamics of flame/vortex interactions. Different responses of the flameare identified and analysed relative to the size of the vortex ring generated atthe burner outlet. Two propagation modes for the velocity perturbations areidentified, corresponding to a bulk oscillation of the entire reaction zone orto a flame perturbed only at its periphery. This leads to a discussion on thechoice of velocity boundary conditions to conduct 1D simulations of theseconfigurations. Comparisons between simulations and measurements of thevelocity field illustrate these conclusions. Flame transfer functions betweenheat release rate and velocity perturbations imposed at the burner outlet areestablished for different flow conditions. These measurements relying on localand global chemiluminescence of the flame show again a distinct behaviourof the emission originating from the flame region close to the burner axis andthe whole flame. Mechanisms of sound production by partially and perfectlypremixed flames are also identified and analysed relative to flame/vortex interactions.In the second part, the dynamics of a spray convected by a free inert jet or impinginga diffusion flame submitted to velocity modulations is analysed. Theoriginality of this work consists in characterizing the flow and spray dynamicsusing a set of advanced diagnostics. Phase-conditioned images at different instantsin the modulation cycle are used to analyse the interactions between thegaseous phase and the spray. The spatial distribution of combustible vapourand liquid phases is determined using Laser Induced Exciplex Fluorescence(LIEF). Velocities and sizes distribution of droplets from the spray are determinedlocally by Phase Doppler Anemometry (PDA) and in a plane by InterferometricParticle Imaging (IPI). Laser Doppler Velocimetry (LDV) andParticle Image Velocimetry (PIV) are also used to determine the response ofgaseous phase. These phase-conditioned analysis highlight some interactionsbetween the gaseous and liquid phases and constitute an interesting databasefor detailed simulation of these two-phase flows.

No description available.

In this work, a microfluidics approach is applied to two fuel cell related projects; the study of deformation and contact angle hysteresis on water invasion in porous media and the introduction of bubble fuel cells. This work was carried out as collaboration between the microfluidics and CFCE groups in the Department of Mechanical Engineering at the University of Victoria. Understanding water transport in the porous media of Polymer Electrolyte Membrane fuel cells is crucial to improve performance. One popular technique for both numeric simulations and experimental micromodels is pore network modeling, which predicts flow behavior as a function of capillary number and relative viscosity. An open question is the validity of pore network modeling for the small highly non-wetting pores in fuel cell porous media. In particular, current pore network models do not account for deformable media or contact angle hysteresis. We developed and tested a deformable microfluidic network with an average hydraulic diameter of 5 μm, the smallest sizes to date. At a capillary number and relative viscosity for which conventional theory would predict strong capillary fingering behavior, we report almost complete saturation. This work represents the first experimental pore network model to demonstrate the combined effects of material deformation and contact angle hysteresis. Microfluidic fuel cells are small scale energy conversion devices that take advantage of microscale transport phenomena to reduce size, complexity and cost. They are particularly attractive for portable electronic devices, due to their potentially high energy density. The current state of the art microfluidic fuel cell uses the laminar flow of liquid fuel and oxidant as a membrane. Their performance is plagued by a number of factors including mixing, concentration polarization, ohmic polarization and low fuel utilization. In this work, a new type of microfluidic fuel cell is conceptualized and developed that uses bubbles to transport fuel and oxidant within an electrolyte. Bubbles offer a phase boundary to prevent mixing, higher rates of diffusion, and independent electrolyte selection. One particular bubble fuel cell design produces alternating current. This work presents, to our knowledge, the first microfluidic chip to produce bubbles of alternating composition in a single channel, class of fuel cells that use bubbles to transport fuel and oxidant and fuel cell capable of generating alternating current. / Graduate

Fibre-optic sensing techniques for application in polymer electrolyte fuel cells (PEMFC) are presented in this thesis. Temperature, relative humidity (RH) and air-water two-phase flow sensors are developed and demonstrated based on optical fibre Bragg gratings (FBG). Bragg gratings offer the following characteristics that warrant their development for application in PEMFCs: small size, environmental compatibility and the possibility of multiplexed multi-parameter sensing. Contributions of this work are in novel sensor development and implementation strategies. Important installation design considerations include the sensor proximity to the catalyst layer, sensor strain relief and minimal bending of the fibre. With these considerations, the dynamic and steady-state performance of FBG temperature sensors distributed throughout the flow-field of a single cell PEMFC was validated with a co-located micro-thermocouple. In the development of FBGs for in situ measurement of relative humidity, a polyimide-coated FBG based RH sensor is presented with significantly improved response time and sensitivity over previously reported designs. The RH inside a PEMFC under transient operating conditions is monitored. Step increases in current induce significantly larger increases in RH near the outlet than near the inlet of the cell, and associated transients within the fuel cell are found on a time scale approaching the sensor response time. Finally, to complete the suite of FBG sensors for water management in PEMFCs, an evanescent field based FBG sensor embedded in a microchannel for the measurement of two-phase flow dynamics is presented. Using high speed video for validation, it is established that the novel sensor enables the measurement of droplet average velocity and size in flow regimes representative of an operating fuel cell. / Graduate

Twin-screw pumps are positive displacement machines. Two meshing screws connected by timing gears convey the fluid trapped in the screw chambers axially from suction to discharge and force it out against the back pressure. Because of the screw geometry, the circumferential pressure field around the screws is not balanced, resulting in net dynamic and static pressures applied on the rotors. The research work presented here aims at building and verifying a model to predict both: (1) the exciting lateral hydrodynamic forces produced by the unbalanced pressure field, and (2) the rotor response due to those forces. The model rests on the screw pump hydraulic models for predicting the pressure in the screw chambers as a function of the discharge pressure. These models are extended to predict the steady state dynamic pressure field as a function of the rotational angle of the rotor. The dynamic force resulting from the dynamic pressure field is calculated and applied to the rotor as a set of super-synchronous periodic forces. The structural model of the screw, although nonsymmetrical, was found to be accurately represented by an axisymmetric equivalent structure. The rotor response to the dynamic super-synchronous forces is calculated to predict the pump rotordynamic behavior. The work in this dissertation presents: (1) the axisymmetric structural model of the rotors (2) the proposed dynamic pressure model, (3) the screw pump rotor response, (4) the experimental validation of the dynamic pressure model and rotor response. The topic of twin-screw pump rotordynamics is absent from the literature. The original contribution of the work presented in this dissertation to the field of rotordynamics includes: (1) demonstrating the adequacy of an axisymmetric model for modeling the screw section, (2) developing a model for predicting the dynamic pressure field around the screws, (3) characterization of the dynamic forces (synchronous and its harmonics) applied at the screw pump rotors, (4) predicting the dynamic response of twin-screw pump rotors due to hydrodynamic forces, (5) measuring the axial dynamic pressure in two circumferential planes around the screws to verify pressure predictions, (6) measuring the dynamic response of twin-screw pump rotor.