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Browsing by Subject "Mass transfer"

Now showing 1 - 7 of 7
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    Biomedical therapy delivery by fluid-mechanic means
    (2014-01) Weiler, James Michael
    The efficacy of many biomedical therapies can be improved when the physical processes which underlie the treatment modality are thoroughly understood. Many treatments make use of transport processes that are deeply embedded in mechanical engineering theory and practice. The research documented in this thesis is firmly based on fluid-mechanic, heat-transfer, mass-transfer, and particle transport theory. The thesis encompasses three categories of biomedical applications: drug distribution, thermal-based surgery, and drug delivery by means of particle transport.The first application dealt with a drug-eluting stent and with the distribution of the drug both into the artery wall by diffusion as well as into the blood flowing in the lumen via advection. This conjugate problem was redefined in dimensionless form and solved by numerical simulation to yield universal solutions. The solutions revealed the existence of a mass transfer boundary layer adjacent to the surface of the stent. Upstream diffusion, opposite to the direction of the advection, occurred. The results showed that the mass transfer into the flowing blood was orders of magnitude larger than the diffusive transfer into the artery walls. The focus of the second application was an in-depth, a fundamentals-based investigation of a new, minimally invasive treatment for menorrhagia. The involved physical processes include vapor transport into the uterine cavity, heat liberated by phase-change, and heat penetration into human tissue by means of conduction and blood perfusion. Cell necrosis was achieved by elevated temperatures sustained for a sufficient period of time. The outcome of this work was the depth of tissue necrosis corresponding to a given duration of the treatment. The predicted depths of necrosis compared favorably with clinical results. The final focus was the creation of a new methodology for the accurate delivery to targeted sites of drug particulates administered either through the mouth or the nose. The drug particles are carried through the respiratory system by an air stream. A numerical-based solution process was developed utilizing the laws of fluid mechanics, the physics of particle transport, and impaction theory. The final solution proved capable of predicting the landing locations of particles based on their respective sizes.
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    Bubbly Flow Physics for Applications in Aerated Hydroturbines and Underwater Transport
    (2016-05) Karn, Ashish
    Bubbly flows occur in a wide variety of situations and are frequently employed in industry and other engineering applications. The current work deals with two such examples of bubbly flows: speed enhancement and controllability of next-generation high–speed underwater vehicles for naval defense applications and environment-friendly power generation through aerating hydroturbines. A complete understanding of the bubbly flow physics is imperative to leverage it for these different applications. These investigations into bubbly flow physics are accomplished by studying the fundamental fluid dynamics of bubbles at different size scales. A large bubble is used to envelop an underwater vehicle causing tremendous reduction in flow resistance while the small bubbles are employed for aeration applications in a hydroturbine. Our experiments have provided critical insights into the design and development of operational strategies and models for these novel technologies. For the dispersed bubbly flow regime, bubble size characteristics in the wake of a ventilated turbine blade is measured using shadow imaging and a newly developed image processing approach. Simultaneous mass transfer measurements in the wake have shown an interrelationship with the bubble size and high speed imaging of the bubbles reveal the physical mechanisms of bubble breakup and coalescence and its effect upon the bubble sizes in the wake. In the supercavity bubble regime, systematic studies are carried out to investigate the supercavity closure mechanisms in detail, and a unified theory is proposed to predict different closure modes. Further insights are provided into the interrelationship between supercavity closure, ventilation demand and gas entrainment behaviors of supercavity. The effect of ocean waves on the stability of supercavity bubbles and its closure are also investigated by replicating the ocean waves in the high speed water tunnel. Specifically, a novel aspect of the current research pertains to the visualization and quantification of the internal flows inside supercavity bubbles and drops.
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    The effects of fluid flow and epiphytes on submerged aquatic vegetation
    (2012-05) Hansen, Amy Therese
    The intent of this research was to investigate the effects of fluid flow characteristics and epiphyte colonization on submerged aquatic vegetation (SAV) photosynthesis and dissolved material uptake. SAV, with its stems and leaves completely submerged in the water column, is strongly affected by both the physical characteristics of the water, such as dissolved material concentrations and fluid motion, and by factors that alter its interaction with the water, such as epiphyte colonization of SAV surfaces. The nature of these interactions was investigated through a series of four separate studies. First, through a laboratory mesocosm experiment, epiphyte uptake and SAV uptake of a dissolved contaminant (nickel) were shown to occur at different rates and due to different mechanisms. Second, a model of photosynthetic rates, based on mass transfer theory, was developed requiring only three parameters that accounted for the effect of water motion on photosynthetic rates. This model was experimentally validated with dissolved oxygen and velocity profiles over blades of giant kelp, Macrocystis pyrifera. Third, using two separate microscale velocity imaging methods, photosynthesis was shown to alter fluid motion near the surface of a Cladophora spp. filament by more than doubling velocity gradients and thus surface shear stress. In this investigation, bacterial epiphytes had no effect on shear stresses but assemblages consisting primarily of diatom epiphytes strongly decreased the surface shear stress from what would have been experienced during photosynthesis without epiphytes present; indicating a harmful interaction with epiphytes. Fourth, in agreement with the microscale results in the third study, epiphyte removal was shown to increase local dissolved oxygen concentrations throughout the water column as well as decrease water column soluble reactive phosphorus concentrations due to higher photosynthetic rates in field research in a constructed wetland. In a related laboratory study, epiphyte detachment rates were functionally related to water velocity. Overall, I have shown through laboratory and field experiments that SAV photosynthesis is closely linked to fluid flow characteristics, SAV and epiphyte uptake are not equally affected by flow conditions, and epiphyte colonization decreases SAV photosynthetic rates.
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    Heat and mass transfer during the melting process of a porous frost layer on a vertical surface
    (2013-05) Mohs, William Francis
    An important problem in the refrigeration industry is the formation and removal of frost layers on sub-freezing air coolers. The frost layer, a porous structure of ice and air, directly diminishes the performance and efficiency of the entire cooling system by presenting resistances to air flow and heat transfer in the air cooler. To return the system to pre-frosted performance the layer must be removed through a defrost cycle. The most common defrost cycle uses heat applied at the heat exchanger surfaces to melt the frost. Current methods of defrosting are inherently inefficient, with the majority of the heat being lost to the surrounding environment. Most studies have concentrated on the formation of the frost layer, and not the melting phenomena during the defrost cycle. In this study, direct measurements and a fundamental model to describe the melting process of a frost layer on a vertical heated surface are presented. The experimental facility provides the first direct measurements of heat and mass transfer during defrost. The measurements confirmed the multistage nature of defrost. The multistage model characterized the different thermal and mass transport processes that dominate each stage. The first stage is dominated by sensible heating of the frost layer. Both the experiment and model showed that heat and mass transfer through sublimation during the initial stages are insignificant, accounting for less than 1% of the total energy transfer. The second stage of defrost is dominated by the melting of the frost layer. The melt rate model generally predicts the front velocity within 25% of the velocity determined using the digital image analysis technique. Higher heat transfer rates resulted in faster melt velocity, and thus shortened defrost times. Evaporation of the melt liquid from the surface dominates the final stage. The heat transfer model for this stage predicts the heat transfer coefficient within ±25% of the experiment. The overall defrost efficiency was found to be primarily dependent on the initial frost thickness, with thicker layer having less heat lost to the ambient space and a higher efficiency.
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    Local Variation of Heat and Mass Transfer for Flow Over a Cavity and on a Flat Plate
    (2017-09) Taliaferro, Matthew
    Boundary layer theory for flat plates is fundamental to our understanding of fluid flow and heat transfer. However, most of the experimental and analytical work for thermal boundary layers focus on streamwise effects. Lateral changes of heat and mass transfer near a lateral singularity in the surface boundary conditions have not been as extensively studied. Lateral heat transfer is studied using OpenFOAM to run numerical simulations for heated strips of varying width, fluids with varying thermal properties, separation lengths, and unheated starting lengths. Turbulent mass transfer is studied using the naphthalene sublimation technique for heated strips of varying depths, widths, and freestream velocities. The lateral edge effect is found to scale with the conduction thickness for both turbulent and laminar boundary layer flows. For laminar boundary layer flow the lateral edge effect extends approximately three conduction thicknesses into the flow, while for turbulent boundary layer flow it extends approximately ten conduction thicknesses into the flow. The results are useful for modeling heat transfer from discrete electronic components. In addition, the results should serve as useful benchmarks for numerical fluid models and computations where lateral transport is important.
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    Study of mass transfer across hydrofoils for use in aerating turbines
    (2013-09) Monson, Garrett M.
    Hydroelectric projects often have a low tailwater dissolved oxygen (DO) concentration. Low DO levels negatively impact the biota of the water body and are often regulated. Auto-Vented Turbines (AVTs) are one form of DO mitigation that is typically successful and cost-effective. Saint Anthony Falls Laboratory (SAFL) at the University of Minnesota (UMN) is partnering with the Department of Energy (DoE) and Alstom Engineering to conduct research developing a conventional hydropower turbine aeration test-bed for computational routines and a software tool for predicting the DO uptake of AVTs. The focus of this thesis is on the development of the test-bed through the conduct of physical experiments focused on measuring mass transfer across bubbles in various flow conditions. This test-bed will be a valuable database for verification of numerical models of DO uptake. Numerical models can simulate the parameters of the water tunnel and experimental set-up, then verify their accuracy by simulating the air entrainment rate, bubble size and mass transfer of the test-bed. The findings presented herein can lead to further optimization of AVTs, as well as reduce cost and regulatory uncertainty prior to hydropower relicensing or development.
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    Transition regime collisions in aerosols
    (2013-08) Gopalakrishnan, Ranganathan
    In most natural and engineered aerosol systems, particles fall in the "transition regime", intermediate to the free molecular and continuum ranges. Furthermore, while theories for transport properties of spherical particles in the free molecular and continuum ranges have been available for decades, theories that are applicable to particles of arbitrary shape are lacking. This thesis addresses the transport of neutral and charged molecules (ions) as well as coagulation with particles of arbitrary shape and size. Computational and experimental studies are performed to develop and validate models of collisional mass transfer onto aerosol particles. A broad overview of the thesis is presented in Chapter 1. Using dimensional analysis and Brownian Dynamics trajectory calculations, the collisions between spherical and nonspherical particles are analyzed in Chapters 2 & 3. This approach is extended to include the effect of Coulombic potential interactions in Chapter 4, along with a critical assessment of existing theories. Further, the unipolar charging of arbirary shaped aerosol particles is studied in Chapter 5. Different from previous chapters, an approach to directly calculate the steady state charge distribution of particles exposed to arbitrary bipolar ion populations is developed in Chapter 6. Experiments are conducted with spherical and cylindrical particles to better understand momentum transfer (Chapter 7) and bipolar charging (Chapter 8) in the transition regime. Finally, conclusions derived from this research and future directions are discussed in Chapter 9.