A fluid that contains species in the presence of a concentration gradient generates unusual phenomena when it is forced into pulsatile motion. For example, each species in the fluid has an enhanced mass transport due to pure molecular diffusion. This enhancement takes place even if there is no net total flow over a cycle of the pulsatile motion. When more than one species in dilute amounts is present in an otherwise pure fluid, called a carrier, each species is transported at a different rate thereby causing a partial separation of the species. This idea traces back to hyperventilation studies done over 40 years ago and to the implementation of the technology in hospital environments to provide life-support for patients under anesthesia. However, it is only in recent years that the underlying physics of oscillatory flow as applied to mass transfer have been understood and this may lead to promising application of the technique to novel means of enhancing separation in life support applications and for detection purposes. In this thesis, results from three-dimensional time accurate studies carried out using the commercial computational fluid dynamics code FLUENT are presented. These results simulate the separation of CO2 from He in an N2 environment (carrier). The model consists of two reservoirs/mixing chambers, an oscillating piston wall, and a connecting tube. Several cases are carried out reporting on separation enhancement as a function of the Womersley number and the ratio of tidal displacement to connecting tube diameter. Unlike previous studies which were undertaken using asymptotic analysis, the present models and results incorporate full entrance effects and 3D interactions. Results of this study will be useful as a guide for the design and miniaturization of an oscillating device for species separation in further research projects at the University of Central Florida. Observations showed that a molar fraction increase occurs during the species transport in the presence of a thermal boundary layer. This was accompanied by an imposed external forced temperature condition on the surface of the cylinder to create thermal diffusion, also known as the Soret or thermal diffusion effect, which refers to the separation of mixtures in a temperature gradient as means of change on the concentration gradient of the species ratio. Calculations were performed to analyze the effect of the heat transfer on the molar fraction of the species at a specific region of the model, called the measurement point. Various mathematical models and correlations were incorporated into a MATLAB computer code that predicted the concentration of the species in an entire cycle after steady state is reached and data can be exported from FLUENT.

Microfluidic chips have drawn great attention and interest due to their broad applications in chemical, biological and biomedical fields. These kinds of miniaturized devices offer many advantages over the traditional analysis instruments, such as reduced cost, shortened time, increased throughput, improved integration/automation/portability, etc. However, the concept of integrating multiple labs on a single chip to perform micro total analysis hasn’t been realized yet because of the lack of fundamental knowledge and systematic design of each component, especially for some particular applications. A thorough understanding and grasp of the basic physical phenomenon is the theoretical basis to develop functional devices to utilize them. In this study, we intend to investigate the electrokinetic fluid flow and coherent species transport processes in microchannels, and then try to effectively control them for designing related lab-on-a-chip devices. Rather than expensive experiments, numerical studies are performed to simulate the different processes involved in various electrokinetic chip applications. In the theoretical models, applied potential field, flow field and species concentration field are considered and corresponding governing equations with initial/boundary conditions are numerically solved by computational fluid dynamics techniques. The flow field is obtained by the developed SIMPLE algorithm and a slip-wall velocity boundary condition is applied in simulating electroosmotic flow. Grid independence tests and convergence studies are performed to ensure economic computation with adequate accuracy and stability. For every application with typical channel layout, parametric studies are performed to investigate different effects through the controlling parameters linked to them. For surface patterning or microfabrication using laminar flows, various operational parameters are investigated to explore the optimized configurations for multi-stream flow and mass transport control in cross-linked microchannels. Through a series of numerical simulations, it is found that applied potentials, electroosmotic mobilities of solutions and channel dimensions have significant effects on the flow and mass transport after converging in the intersection of channel network. Diffusion coefficient has less influence than the other parameters due to the presence of high Peclet number for such applications. For the microwashing with two different electrolyte solutions, a three-dimensional model is numerically solved to reveal the flow structure change. In a straight microchannel with a rectangle cross section, KCl solution and LaCl3 solution are mainly employed for tests. Displacement processes between two solutions in both orders are tested and analyzed. The observed flow structures such as back flow in channel center and distortion of plug-like velocity profile are noticed and discussed. The distortion of the flow field results from the induced pressure gradient, which is due to the non-uniformity of electroosmotic mobilities and electrical conductivities of two replaced solutions. The bigger difference two solutions have in chemical properties, the stronger effects on flow they have. Effect of applied potential field strength is also studied and the approximate linear influences are concluded. Finally, the unsteady on-chip sample injection and separation processes involved in microchip capillary electrophoresis are studied. Species’ electrophoretic migration effect is included and the theoretical model is non-dimensionalized in a unique manner with the key fundamental parameters: the Re Sci , species’ non-dimensional electrophoretic mobility and applied potentials. The species transport characteristics are revealed numerically and well understood for future effective control and innovative chip design. Species front movement during injection and sample plug development in separation are examined with diffusion effect; results include concentration profiles and contour plots over a range of injection and separation time. The influence of i Re Sc which characterizes the relative role of convection versus diffusion is examined over the commonly encountered range and the diffusion effect is found to have an essentially negligible effect. Through three species, the electrophoretic mobilities difference is demonstrated to be the reason for separation. Real-time monitoring of different species’ movements is performed for injection guidance.

Microfluidic chips have drawn great attention and interest due to their broad applications in chemical, biological and biomedical fields. These kinds of miniaturized devices offer many advantages over the traditional analysis instruments, such as reduced cost, shortened time, increased throughput, improved integration/automation/portability, etc. However, the concept of integrating multiple labs on a single chip to perform micro total analysis hasn’t been realized yet because of the lack of fundamental knowledge and systematic design of each component, especially for some particular applications. A thorough understanding and grasp of the basic physical phenomenon is the theoretical basis to develop functional devices to utilize them. In this study, we intend to investigate the electrokinetic fluid flow and coherent species transport processes in microchannels, and then try to effectively control them for designing related lab-on-a-chip devices. Rather than expensive experiments, numerical studies are performed to simulate the different processes involved in various electrokinetic chip applications. In the theoretical models, applied potential field, flow field and species concentration field are considered and corresponding governing equations with initial/boundary conditions are numerically solved by computational fluid dynamics techniques. The flow field is obtained by the developed SIMPLE algorithm and a slip-wall velocity boundary condition is applied in simulating electroosmotic flow. Grid independence tests and convergence studies are performed to ensure economic computation with adequate accuracy and stability. For every application with typical channel layout, parametric studies are performed to investigate different effects through the controlling parameters linked to them. For surface patterning or microfabrication using laminar flows, various operational parameters are investigated to explore the optimized configurations for multi-stream flow and mass transport control in cross-linked microchannels. Through a series of numerical simulations, it is found that applied potentials, electroosmotic mobilities of solutions and channel dimensions have significant effects on the flow and mass transport after converging in the intersection of channel network. Diffusion coefficient has less influence than the other parameters due to the presence of high Peclet number for such applications. For the microwashing with two different electrolyte solutions, a three-dimensional model is numerically solved to reveal the flow structure change. In a straight microchannel with a rectangle cross section, KCl solution and LaCl3 solution are mainly employed for tests. Displacement processes between two solutions in both orders are tested and analyzed. The observed flow structures such as back flow in channel center and distortion of plug-like velocity profile are noticed and discussed. The distortion of the flow field results from the induced pressure gradient, which is due to the non-uniformity of electroosmotic mobilities and electrical conductivities of two replaced solutions. The bigger difference two solutions have in chemical properties, the stronger effects on flow they have. Effect of applied potential field strength is also studied and the approximate linear influences are concluded. Finally, the unsteady on-chip sample injection and separation processes involved in microchip capillary electrophoresis are studied. Species’ electrophoretic migration effect is included and the theoretical model is non-dimensionalized in a unique manner with the key fundamental parameters: the Re Sci , species’ non-dimensional electrophoretic mobility and applied potentials. The species transport characteristics are revealed numerically and well understood for future effective control and innovative chip design. Species front movement during injection and sample plug development in separation are examined with diffusion effect; results include concentration profiles and contour plots over a range of injection and separation time. The influence of i Re Sc which characterizes the relative role of convection versus diffusion is examined over the commonly encountered range and the diffusion effect is found to have an essentially negligible effect. Through three species, the electrophoretic mobilities difference is demonstrated to be the reason for separation. Real-time monitoring of different species’ movements is performed for injection guidance.

The aim of the present research was to gain a better understanding of the species generation and transport from enclosure fires. The species generation experiments were conducted with a half-scale ISO 9705 enclosure with three different ventilation conditions and heat release rates ranging from 50 kW to 500 kW. The transport study was conducted with a 6.1 m long hallway connected to the compartment in a head-on configuration. All measurements were performed at the compartment or hallway exit plane during the steady-state period of the fire. Measurements included species mole fractions of oxygen, carbon dioxide, carbon monoxide, and unburned hydrocarbons, along with gas pressure (used to determine gas velocities) and gas temperatures. Species mappings performed at the exit plane of the compartment indicated that the exiting species are not spatially uniform. Horizontal and vertical gradients in the species mole fractions were observed for all ventilation conditions and heat release rates examined. Predictive techniques developed previously were applied to the data obtained in the present study and were determined to be inappropriate for situations were the plume equivalence ratio was not equal to the global equivalence ratio. A new methodology for predicting species levels at the exit plane of an enclosure was developed. The proposed methodology correlates the species yields based on the combustion within the compartment as a function of a non-dimensional heat release rate. The non-dimensional heat release rate is based on the fuel load and geometrical parameters of the enclosure. The present methodology in applicable to situations where a well-mixed uniform layer is not present and the overall global conditions are of interest. Species transport to remote locations was also examined. Experiments were conducted with the baseline ventilation at x = 0 m (the compartment/hallway interface) and three different ventilation conditions at x = 6.1 m (end of hallway). The three ventilation conditions consisted of the narrow, baseline, and wide doorways. Experiments were conducted for heat release rates of 85 kW, 127 kW, and 150 kW. The results from the tests indicated that, for over-ventilated compartment fires, the hallway and hallway ventilation had no impact on the species generation within the compartment. This allows the correlations developed from the compartment study to be applied to more complex scenarios. Differences in species mole fractions between x = 0 m and x = 6.1 m were shown to be a result of air entrainment into the upper layer within the hallway, which acted as a dilutent or as a source of oxygen for further oxidation reactions. For non-dimensional heat release rates less than 1.0, the reduction in carbon monoxide levels along the hallway was a result of dilution, while for non-dimensional heat release rates greater than or equal to 1.0 the reduction in carbon monoxide levels along the hallway was a combination of dilution and further oxidation reactions. / Ph. D.

The objective of the present study was to acquire a better understanding of parameters controlling the species generation and transport from compartment fires. The experiments were performed in a half-scale ISO 9705 compartment and a 6.1 m long hallway connected in a head-on configuration. The buoyancy driven propane fire was provided by a burner and a continuous gaseous fuel supply system. All the measurements were obtained during the steady state of the fire. The ventilation conditions were fixed and three different fire source elevations were studied for heat release rates ranging from 20 kW to 150kW. The species yields were obtained from performing detailed mapping measurements at the compartment and hallway exit planes. The measurements included local specie mole fractions of oxygen, carbon dioxide, carbon monoxide and unburned hydrocarbons. The local temperature and the local pressure (for local gas velocity calculations) were also measured. In addition, visual observations of the flow dynamic were performed through a window and the vents to give useful insights and lead to a better understanding of the combustion process. The data obtained from the species generation study was analyzed using previously developed methods. The method based on equivalence ratio was presented and determined inappropriate for the present study where the global equivalence ratio was not equal to the plume equivalence ratio due to the complexity of the fire dynamic taking place. The method consisting of correlating the species yields based on the combustion within the compartment as a function of a non dimensional heat release rate allowed qualitative conclusions to be made. The non-dimensional heat release rate was based on the fuel load and the geometric parameters of the compartment. This methodology revealed similarities in the species production between the three fire source elevations investigated. A correlation of the data was obtained based on experimental data. The transport of species to remote locations was studied for the three fire source elevations and fixed ventilation conditions. Species mole fractions and yields were obtained at the compartment exit plane (compartment/hallway interface) and at the hallway exit plane. The results were compared for various heat release rates and showed differences for some scenarios attributed to mixing along the hallway and oxidation reactions outside the compartment. / Master of Science

Doctor of Philosophy / Department of Mechanical and Nuclear Engineering / M.H. Hosni / Z.C. Zheng / In order to study the capability of computational methods in investigating the mechanisms associated with disease and contaminants transmission in aircraft cabins, the Computational Fluid Dynamics (CFD) models are used for the simulation of turbulent airflow, tracer gas diffusion, and particle dispersion in a generic aircraft cabin mockup. The CFD models are validated through comparisons of the CFD predictions with the corresponding experimental measurements. It is found that using Large Eddy Simulation (LES) with the Werner-Wengle wall function, one can predict unsteady airflow velocity field with relatively high accuracy. However in the middle region of the cabin mockup, where the recirculation of airflow takes place, the accuracy is not as good as that in other locations. By examining different k-ε models, the current study recommends the use of the RNG k-ε model with the non-equilibrium wall function as a Reynolds Averaged Navier Stokes (RANS) model for predicting the steady-state airflow velocity data. It is also found that changing the cabin air-inlet nozzle height has a significant effect on the flow behavior in the middle and upper part of the cabin, while the flow pattern in the lower part is not affected as much. Through the use of LES and species transport model in simulating tracer gas diffusion, very good agreement between predicted and measured tracer gas concentration data is observed for some monitoring locations, but the agreement level is not uniform for all the sampling point locations. The reasons for the deviations between predictions and measurements for those locations are discussed. The Lagrange-Euler approach is invoked in the particle dispersion simulations. In this approach, the equation of motion for the discrete phase is coupled with the continuous phase governing equations through the calculation of drag and buoyancy forces acting on particles. The continuous phase flow is turbulent and RANS is employed in order to calculate the continuous phase velocity field. A complete study on grid dependence for RANS simulation is performed through a controllable regional mesh refinement scheme. The grid dependence study shows that using unstructured grid with tetrahedral and hybrid elements in the refinement region are more efficient than using structured grid with hexahedral elements. The effect of turbulence on the particle dispersion is taken into account by using a stochastic tracking method (Discrete Random Walk model). One of the significant features of this study is the investigation of the effect of the number of tries on the accuracy of particle concentration predictions when Discrete Random Walk is used to model turbulent distribution of particles. Subsequently, the optimum number of tries to obtain the most accurate predictions is determined. In accordance with the corresponding experimental data, the effect of particle size on particle distribution is also studied and discussed through the simulation of two different sizes of mono-disperse particles in the cabin with straight injection tube, i.e., 3µm and 10µm. Due to the low particle loading, neglecting the effect of particles motion on the continuous phase flow-field seems to be a reasonable, simplifying assumption in running the simulations. However, this assumption is verified through the comparison of the results from 1-way and 2-way coupling simulations. Eventually through the simulations for the particle injection using the cone diffuser, the effects of cabin pressure gradient as well as the particle density on particles dispersion behavior are studied and discussed. In the last part of this dissertation, the turbulent airflow in a full-scale Boeing 767 aircraft cabin mockup with eleven rows of seats and manikins is simulated using steady RANS method. The results of this simulation cannot only be used to study the airflow pattern, but also can be used as the initial condition for running the tracer gas diffusion and particle dispersion simulations in this cabin mockup.

Two computational fluid dynamics models are developed to predict the performance of a solid oxide fuel cell stack, a detailed and a simplified model. In the detailed model, the three dimensional momentum, heat, and species transport equations are coupled with electrochemistry. In the simplified model, the diffusion terms in the transport equations are selectively replaced by rate terms within the core region of the stack. This allows much coarser meshes to be employed at a fraction of the computational cost. Following the mathematical description of the problem, results for single-cell and multi-cell stacks are presented. Comparisons of local current density, temperature, and cell voltage indicate that good agreement is obtained between the detailed and simplified models, verifying the latter as a practical option in stack design. Then, the simplified model is used to determine the effects of utilization on the electrochemical performance and temperature distributions of a 10 cell stack. The results are presented in terms of fluid flow, pressure, species mass fraction, temperature, voltage and current density distributions. The effects of species and flow distributions on electrochemical performance and temperature are then analyzed for a 100 cell stack. The discussion highlights the importance of manifold design on performance and thermal management of large stacks. / Thesis (Master, Mechanical and Materials Engineering) -- Queen's University, 2013-09-30 15:55:18.627