Theoretical and numerical studies of chaotic mixing are performed to circumvent the difficulties of efficient mixing, which come from the lack of turbulence in microfluidic devices. In order to carry out efficient and accurate parametric studies and to identify a fully chaotic state, a spectral element algorithm for solution of the incompressible Navier-Stokes and species transport equations is developed. Using Taylor series expansions in time marching, the new algorithm employs an algebraic factorization scheme on multi-dimensional staggered spectral element grids, and extends classical conforming Galerkin formulations to nonconforming spectral elements. Lagrangian particle tracking methods are utilized to study particle dispersion in the mixing device using spectral element and fourth order Runge-Kutta discretizations in space and time, respectively. Comparative studies of five different techniques commonly employed to identify the chaotic strength and mixing efficiency in microfluidic systems are presented to demonstrate the competitive advantages and shortcomings of each method. These are the stirring index based on the box counting method, Poincare sections, finite time Lyapunov exponents, the probability density function of the stretching field, and mixing index inverse, based on the standard deviation of scalar species distribution. Series of numerical simulations are performed by varying the Peclet number (Pe) at fixed kinematic conditions. The mixing length (lm) is characterized as function of the Pe number, and lm ∝ ln(Pe) scaling is demonstrated for fully chaotic cases. Employing the aforementioned techniques, optimum kinematic conditions and the actuation frequency of the stirrer that result in the highest mixing/stirring efficiency are identified in a zeta potential patterned straight micro channel, where a continuous flow is generated by superposition of a steady pressure driven flow and time periodic electroosmotic flow induced by a stream-wise AC electric field. Finally, it is shown that the invariant manifold of hyperbolic periodic point determines the geometry of fast mixing zones in oscillatory flows in two-dimensional cavity.

<p>The work is focused on experimental study and modelling of spacer grid influence on single- and two-phase flow. In the experimental study a mock-up of a realistic fuel bundle with five spacer grids of thin plate spring construction was investigated. A special pressure measuring technique was used to measure pressure distribution inside the spacer. Five pressure taps were drilled in one of the rods, which could exchange position with other rods, in this way providing a large degree of freedom. Laser Doppler Velocimetry was used to measure mean local axial velocity and its fluctuating component upstream and downstream of the spacer in several subchannels with differing spacer part. The experimental study revealed an interesting behaviour. Subchannels from the interior part of the bundle display a different effect on the flow downstream of the spacer compared to subchannels close to the box wall, even if the spacer part is the same. This behaviour is not reflected in modern correlations. The modelling part, first, consisted in comparing the present experimental data to Computational Fluid Dynamics calculations. It was shown that stand-alone subchannel models could predict the local velocity, but are unreliable in prediction of turbulence enhancement due to spacer. The second part of the modelling consisted in developing a deposition model for increase due to spacer. In this study Lagrangian Particle Tracking (LPT) coupled to Discrete Random Walk (DRW) technique was used to model droplet movements through turbulent flow. The LPT technique has an advantage to model the influence of turbulence structure effect on droplet deposition, in this way presenting a generalized model in view of spacer geometry change. The verification of the applicability of LPT DRW method to model deposition in annular flow at Boiling Water Reactor conditions proved that the method is unreliable in its present state. The model calculations compare reasonably well to air-water deposition data, but display a wrong trend if the fluids have a different density ratio than air-water.</p>

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No description available.

The circulation and gradual transformation in properties of oceanic water masses is a matter of great interest for short-term weather and biological forecasting, as well as long-term climate change. It is usually agreed that the Nordic Seas between Greenland and Norway are key to these transformations since they are an important producer of dense water, a process central to the theory of the global thermohaline circulation. In this study, one component of this deep water is examined – that formed in the Nordic Seas themselves from the inflowing North Atlantic Current. Using Lagrangian particle tracking applied to a 50-year global ocean hindcast simulation, it is concluded that only about 6% of the inflowing North Atlantic Current is thus transformed, and that most of these transformations occur in boundary currents. Furthermore, it is found that the densified North Atlantic water attains only medium depths instead of joining the deep overflows. The model’s poor representation of vertical mixing, however, limits the applicability of this study to deep water formation.

The work is focused on experimental study and modelling of spacer grid influence on single- and two-phase flow. In the experimental study a mock-up of a realistic fuel bundle with five spacer grids of thin plate spring construction was investigated. A special pressure measuring technique was used to measure pressure distribution inside the spacer. Five pressure taps were drilled in one of the rods, which could exchange position with other rods, in this way providing a large degree of freedom. Laser Doppler Velocimetry was used to measure mean local axial velocity and its fluctuating component upstream and downstream of the spacer in several subchannels with differing spacer part. The experimental study revealed an interesting behaviour. Subchannels from the interior part of the bundle display a different effect on the flow downstream of the spacer compared to subchannels close to the box wall, even if the spacer part is the same. This behaviour is not reflected in modern correlations. The modelling part, first, consisted in comparing the present experimental data to Computational Fluid Dynamics calculations. It was shown that stand-alone subchannel models could predict the local velocity, but are unreliable in prediction of turbulence enhancement due to spacer. The second part of the modelling consisted in developing a deposition model for increase due to spacer. In this study Lagrangian Particle Tracking (LPT) coupled to Discrete Random Walk (DRW) technique was used to model droplet movements through turbulent flow. The LPT technique has an advantage to model the influence of turbulence structure effect on droplet deposition, in this way presenting a generalized model in view of spacer geometry change. The verification of the applicability of LPT DRW method to model deposition in annular flow at Boiling Water Reactor conditions proved that the method is unreliable in its present state. The model calculations compare reasonably well to air-water deposition data, but display a wrong trend if the fluids have a different density ratio than air-water.

The mixing and evaporation of Diesel Exhaust Fluid (DEF) inside an Urea Selective Catalyst Reduction (SCR) chamber has been numerically investigated. The first task in this work has been to first look into the numerical framework and assess the models available in a commercial CFD software (ANSYS Fluent 14.5). Secondly the knowledge inherited from the model sensitivity analysis will be applied on the practical case of an Urea-SCR mixing chamber. Mass flow rate and temperature effects of the exhaust gas on the mixing and evaporation of the DEF spray has been investigated. The results indicate that evaporation rates inside the mixing chamber are dependent on the mass flow rate of the exhaust gas but not on the temperature due to compressibility effects of the exhaust gas. For a constant mass flow rate an increase in temperature decreases the residence time of droplets (due to compressibility) with a similar order of magnitude as the individual droplet evaporation rate increases (due to higher temperature) thus the two effects balances each other. The results could potentially contribute to the development and optimization of current SCR systems.

Gas turbines for military applications, when operating in harsh environments like deserts often encounter unexpected operation faults. Such performance deterioration of the gas turbine decreases the mission readiness of the Air Force and simultaneously increases the maintenance costs. Some of the major factors responsible for the reduced performance are ingestion of debris during take off and landing, distorted intake flows during low altitude maneuvers, and hot gas ingestion during artillery firing. The focus of this thesis is to study ingestion of debris; specifically sand. The region of interest being the internal cooling ribbed duct of the turbine blade. The presence of serpentine passages and strong localized cross flow components makes this region prone to deposition, erosion, and corrosion (DEC) by sand particles. A Lagrangian particle tracking technique was implemented in a generalized coordinate multi-block Navier-Stokes solver in a distributed parallel framework. The developed algorithm was validated by comparing the computed particle statistics for 28 microns lycopodium, 50 microns glass, and 70 microns copper with available data [2] for a turbulent channel flow at Ret=180. Computations were performed for a particle-laden turbulent flow through a stationary ribbed square duct (rib pitch / rib height = 10, rib height / hydraulic diameter = 0.1) using an Eulerian-Lagrangian framework. Particle sizes of 10, 50, and 100 microns with response times (normalized by friction velocity and hydraulic diameter) of 0.06875, 1.71875, and 6.875 respectively are considered. The calculations are performed for a nominal bulk Reynolds number of 20,000 under fully developed conditions. The carrier phase was solved using Large Eddy Simulation (LES) with Dynamic Smagorinsky Model [1]. Due to low volume fraction of the particles, one-way fluid-particle coupling was assumed. It is found that at any given instant in time about 40% of the total number of 10 micron particles are concentrated in the vicinity (within 0.05 Dh) of the duct surfaces, compared to 26% of the 50 and 100 micron particles. The 10 micron particles are more sensitive to the flow features and are prone to preferential concentration more so than the larger particles. At the side walls of the duct, the 10 micron particles exhibit a high potential to erode the region in the vicinity of the rib due to secondary flow impingement. The larger particles are more prone to eroding the area between the ribs and towards the center of the duct. At the ribbed walls, while the 10 micron particles exhibit a fairly uniform propensity for erosion, the 100 micron particles show a much higher tendency to erode the surface in the vicinity of the reattachment region. The rib face facing the flow is by far the most susceptible to erosion and deposition for all particle sizes. While the top of the rib does not exhibit a large propensity to be eroded, the back of the rib is as susceptible as the other duct surfaces because of particles which are entrained into the recirculation zone behind the rib. / Master of Science

The focus of this study is the prediction of trajectories of solid particles injected into either a cylindrically- shaped solid rocket motor (SRM) or a bidirectional vortex chamber (BV). The Lagrangian particle trajectory is assumed to be governed by drag, virtual mass, Magnus, Saffman lift, and gravity forces in a Stokes flow regime. For the conditions in a solid rocket motor, it is determined that either the drag or gravity forces will dominate depending on whether the sidewall injection velocity is high (drag) or low (gravity). Using a one-way coupling paradigm in a solid rocket motor, the effects of particle size, sidewall injection velocity, and particle-to-gas density ratio are examined. The particle size and sidewall injection velocity are found to have a greater impact on particle trajectories than the density ratio. Similarly, for conditions associated with a bidirectional vortex engine, it is determined that the drag force dominates. Using a one-way particle tracking Lagrangian model, the effects of particle size, geometric inlet parameter, particle-to-gas density ratio, and initial particle velocity are examined. All but the initial particle velocity are found to have a significant impact on particle trajectories. The proposed models can assist in reducing slag retention and identifying fuel injection configurations that will ensure proper confinement of combusting droplets to the inner vortex in solid rocket motors and bidirectional vortex engines, respectively.

Particulate emissions from road transportation are known to have an adverse impact on human health as well the environment. As the effects become more palpable, stricter legislation have been proposed by regulating bodies. This puts forward a challenge for the automotive industry to develop after treatment technologies to fulfil the progressively stricter legislation. At present, the most common after-treatment technologies used for particulates are the diesel and gasoline particulate filters. The typical size distribution of the particles is such that the smallest particles in terms of size are in numbers the largest, although they are not influencing the total particle mass significantly. The most recent legislation have included restrictions on the particle number as well as particle mass. In this thesis numerical tools for studying the transport and interaction of particles in an exhaust flow are evaluated. The specific application is particle agglomeration as a mean to reduce the number of particles and manipulate the size distribution. As particles agglomerate the particle number distribution is shifted and larger sized agglomerates of particles are created reducing the number of ultra-fine particles. The particle agglomeration is obtained by forcing sudden acceleration and deceleration of the host gas carrying the particles by variations in the cross sectional areas of the geometry it is passing through. Initially, a simplified one dimensional model is utilized to assess the governing parameters of particle grouping. Grouping here means that the particles form and are transported in groups, thus increasing the probability for agglomeration. The lessons learned from the 1D-model are also used to design the three dimensional geometry: an axisymmetric corrugated pipe. Two different geometries are studied, they both have the same main pipe diameter but different diameter on the corrugations. The purpose is to find the potential onset of flow instabilities and the influence of 3D-effects such as recirculation on the agglomeration. The CFD simulations are performed using DES methodology. First the simulations are run without particles in a non pulsatile flow scenario. Later particles are added to the setup in a one way coupled approach (no particle-particle interaction). The main results were: 1) An additional criterion for grouping to the ones given in previous work on the 1D model is proposed. It is found that grouping is more likely if the combination of the pulse frequency and geometric wavelength is large. Furthermore, smooth pulse forms (modelling the modulation in the flow due to the geometry) yielded more grouping than other more abrupt pulse shapes. However, idealised inlet pulses underestimate the extent of grouping compared to actual engine pulses. 2) For the geometry with larger maximum cross sectional area stronger flow separation was observed along with higher turbulent kinetic energy. 3) Particles were added in the flow field and a reduction in the particle count was observed in the initial simulations for particles going from the first corrugated segment to the last. Natural extensions of the present work would be to consider pulsatile flow scenarios, particle-particle interaction and a polydisperse setup for the particles / <p>QC 20181008</p>