Browsing by Subject "Heat transfer"

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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.
Boiling of Dilute Emulsions.
(2010-06) Roesle, Matthew Lind
Although boiling in pure liquids has been studied thoroughly, boiling in other circumstances is less well understood. One area that has received little attention is boiling of dilute emulsions in which the dispersed component has a lower boiling point than the continuous component. These mixtures exhibit several surprising behaviors that were unknown until the 1970's. Generally, boiling of the dispersed component enhances heat transfer over a wide range of surface temperatures without transition to film boiling, but a high degree of superheat is required to initiate boiling. In single-phase convection the dispersed component has little effect on heat transfer. These behaviors appear to occur in part because few droplets in the emulsion contact nucleation sites on the heated surface. No detailed and physically consistent model of boiling in dilute emulsions exists at present. The unusual behavior of boiling dilute emulsions makes them potentially useful for high heat flux cooling of electronics. High-power electronic devices must be maintained at temperatures below ~85 °C to operate reliably, even while generating heat fluxes of 100 W/cm2 or more. Current research, generally focusing on single phase convection or flow boiling in small diameter channels, has not yet identified an adequate solution. An emulsion of refrigerant in water would be well-suited to this application. The emulsion retains the high specific heat and thermal conductivity of water, while boiling of the refrigerant enhances the heat transfer coefficient at temperatures below the saturation temperature of water. To better understand boiling dilute emulsions and expand the experimental database, an experimental study of boiling heat transfer from a horizontal heated wire, including visual observations, is performed. Emulsions of pentane in water and FC-72 in water are studied. These emulsions have properties suitable for practical use in high heat flux cooling applications, unlike most emulsions that have previously been studied. The range of the experimental study is extended to include enhanced boiling of the continuous component, which has not previously been observed, in addition to boiling of the dispersed component. In both regimes the heat transfer coefficient is enhanced compared to that of water. Visual observations reveal the presence of large attached bubbles on the heated wire, the formation of which coincides with the inception of boiling in the heat transfer data. At very low dispersed component fractions and low temperatures, boiling of individual dispersed droplets is not observed. The large attached bubbles represent a new boiling mode that has not been reported in previous studies and is, under some circumstances, the dominant mode of boiling heat transfer. A model of boiling dilute emulsions is developed based upon the Euler-Euler model of multiphase flows. The general balance equations as developed by Drew and Passman are applied to the present situation, thus providing a rigorous and physically consistent framework. The model contains three phases that represent the continuous component, liquid droplets of the dispersed component, and bubbles that result from boiling of individual droplets. Mass, momentum, and energy transfer between the phases are modeled based upon the behavior of and interaction between individual elements of the dispersed phases. One-dimensional simulations of a single boiling droplet in superheated liquid are also performed, and the results are used to develop the closure equations of the larger model. Droplet boiling is assumed to occur when a droplet contacts a heated surface or a vapor bubble. Collisions between droplets and bubbles and chain-boiling of closely-spaced droplets are considered. The model is limited to the dispersed component boiling regime, and thus it does not account for phase change of the continuous component. The model also does not include the large attached bubbles revealed in the visualization experiments. However, simulations of boiling match several trends observed in the experimental data. The model thus provides a physically consistent and partially validated platform for future analytical and numerical work.
Determination of Optimum Time for the Application of Surface Treatments to Asphalt Concrete Pavements - Phase II
(Minnesota Department of Transportation, 2008-06) Marasteanu, Mihai; Velasquez, Raul; Herb, William; Tweet, John; Turos, Mugur; Watson, Mark; Stefan, Heinz G.
Significant resources can be saved if reactive type of maintenance activities are replaced by proactive activities that could significantly extend the pavements service lives. Due to the complexity and the multitude of factors affecting the pavement deterioration process, the current guidelines for applying various maintenance treatments are based on empirical observations of the pavement surface condition with time. This report presents the results of a comprehensive research effort to identify the optimum timing of surface treatment applications by providing a better understanding of the fundamental mechanisms that control the deterioration process of asphalt pavements. Both traditional and nontraditional pavement material characterization methods were carried out. The nontraditional methods consisted of X-Ray Photoelectron Spectroscopy (XPS) for quantifying aging, while for microcracks detection, electron microprobe imaging test (SEM) and fluorescent dyes for inspection of cracking were investigated. A new promising area, the spectral analysis of asphalt pavements to determine aging, was also presented. Traditional methods, such as Bending Beam Rheometer (BBR), Direct Tension (DTT), Dynamic Shear Rheometer (DSR) and Fourier Transform Infrared Spectroscopy (FTIR) for asphalt binders and BBR and Semi-Circular Bending (SCB) for mixtures were used to determine the properties of the field samples studied in this effort. In addition, a substantial analysis of measured pavement temperature data from MnROAD and simulations of pavement temperature using a one-dimensional finite difference heat transfer model were performed.
The Effects of Agitation on convective heat transfer with applications to electronics cooling
(2014-01) Agrawal, Smita
With continuously increasing number of chips and smaller and smaller CPU sizes, heat fluxes that need to be dissipated from computers are on a rapid increase. CPU cooling being critical to the performance of electronic devices, this field demands considerable research focus. Researchers have been pushing existing computer cooling technologies to their limits and also developing new cooling techniques. Forced convection, spray jet cooling, boiling heat transfer are a few to name. Different technologies have their respective merits and limits in terms of their cooling capability, reliability, ease of manufacturing and durability. Forced convection using air has always been preferred due to its cost effectiveness and reliability. The traditional way of employing this technique has been by using a blower fan that cools the heat sink that dissipates heat from the chips. However, the current heat removal demands better performance than that can be provided by a blower fan alone. Agitation is a strong mixing mechanism that can disturb the near-wall flow, thin the thermal boundary layer and enhance the convective heat transfer. This thesis study carries a detailed study of agitation alone through a Large Scale Mock Up (LSMU) unit which is dynamically similar to a single channel of a heat sink. The LSMU has a translationally oscillating plate (agitator) inside the channel cavity. Time averaged heat transfer coefficients and time resolved velocity measurements have been made along different regions of the channel to characterize the convective cooling performance of the agitator. The ensemble-averaged mean velocity variations show periods of acceleration, deceleration and flow reversal during a cycle as a result of agitator movement. Turbulence is found to increase toward the end of the acceleration phase and persist through the deceleration phase. A parametric study has been done to explore the effects of agitator frequency (f), amplitude (A) and agitation velocity (2πAf) on heat transfer and flow mechanism. The heat transfer coefficient increases with the increase in frequency and amplitude. At a fixed agitation velocity, heat transfer coefficient is mainly governed by the agitation velocity irrespective of the value of amplitude or frequency.
Experimental strategies for frost analysis
(2013-12) Janssen, Daniel D.
An area of increasing importance in the field of refrigeration is the study of frosting and defrosting. Frosting poses a concern to many refrigeration systems, as frost growth both obstructs airflow through low temperature heat exchangers and increases heat transfer resistance. Drastic decreases in system efficiency result from the compounding of these problems, and because it is difficult to prevent the frosting process, refrigeration systems must be defrosted periodically to restore optimal operating conditions. A deeper understanding of the complex physical processes of frosting and defrosting will lead to more efficient refrigeration system designs; an idea which has driven a rise in frost growth research over recent decades. Although research has shown great progress, there remain significant challenges associated with predicting the frosting and defrosting processes accurately under wide ranges of conditions. The equations governing such behavior still remain insoluble by exact analytical methods. Numerical approaches have shown the most promising results, but are yet in an early stage of development. Most research has instead been concerned with developing correlations for frost properties and growth, though few are applicable to varying conditions. The most commonly used correlations are shown to have widely different results, perhaps owing to different experimental methods used to acquire data and a lack of deeper level analysis. A new thickness correlation is proposed which attempts to reconcile to some degree the gap between theory and application. Broader ranges of data are used for fitment which enables the application of the correlation to a wider range of conditions. To improve the consistency of results in frost research, it is suggested that new forms of data acquisition be explored. Proposed alternative methods utilize high magnification imaging equipment in combination with computer based measurements, which are shown to be capable of improving accuracy by an order of magnitude in some areas (specifically frost thickness measurement) when calibrated appropriately. In addition to improving measurement accuracy such methods make possible the rapid calculation of droplet geometry during defrosting, an area which has seen little research until recently. The influence of the experimental apparatus on results is also investigated, and a variety of different setups used in past and recent research are categorized according to capability and functionality. Pros and cons of related parameters are discussed with an emphasis on goals. Opportunities for future work include the further development of computer based measurement methods, the acquisition of data over wider ranges of conditions and improvements on the experimental apparatus required to achieve those conditions reliably.It is clear from this research that frost growth is a developing field where much progress is yet to be made. Experimental setups of types ranging from small enclosed tests to wind tunnels on industrial evaporators have provided a clearer understanding of the phenomenon in many aspects. Research presented in this thesis shows that small scale experiments are preferable at this point in time to reach deeper understanding of the frost growth process. It is shown here that many current methods of measurement for important frost growth parameters can be greatly improved upon by the use of computer based algorithms. Faster and more accurate measurement opportunities mean that larger data sets spread across wider ranges of testing conditions can be obtained, setting the stage for more advanced correlation development. Currently, most correlations are only applicable to specific conditions and are still not highly accurate. An attempt is made to show that larger collections of reliable data can be used to develop more robust correlations. To do so a new correlation is proposed which fits a wide range of conditions well. Finally it is shown that the defrosting process may be understood more fully by the use of digital analysis of visual data during defrosting.
Experiments on Film Cooling of Gas Turbine Vane Passage Surfaces: The Effects of Various Distributions of Combustor Coolant and Endwall Injection Coolant
(2019-08) Nawathe, Kedar Prasad
The efficiency of gas turbines is known to increase with the exit temperature of the combustor gases. However, this temperature is limited by the melting point of various equipment downstream of the combustor. To increase this limit, coolants injected at different locations form low-temperature films on the surfaces of these regions to avoid melting and thermal stress damage. This injection significantly changes the flow field in the vane passage. Therefore, there is a need to study the aerodynamic and thermal effects of combustor, transition duct and passage coolant injection, which can assist the designers of gas turbines to employ better cooling schemes with minimal use of coolant, thus increasing the efficiency and durability of turbines. The study presented in this thesis discusses experimental tests performed to understand the coolant effectiveness for cooling the endwall and vane surfaces of a nozzle guide vane cascade. The test section contains an engine representative combustor-turbine interface along with a contoured endwall. High turbulence intensity as well as high Reynolds numbers are achieved in the facility to closely simulate engine conditions. In addition to recording the surface effectiveness values, in-field thermal and aerodynamic measurements were taken. It has been previously discovered that the effusion and louver coolants, used to cool the combustor section, can also be credited in cooling the endwall and the vane surfaces. Therefore, this study is helpful to understand the effects of changing the mass flow ratios of different coolants injected upstream of the passage. Especially, louver coolant injection in the combustor transition duct region, due to its location and injection angle, is suspected to provide significant passage endwall cooling. In-field measurements provide insight to the coolant transport through the passage and its interaction with the mainstream. As the net combustor coolant momentum is higher than the film coolant momentum, the changes in the flow field due to its injection are more significant and are sustained to the end of the passage, giving more streamwise coverage than with a conventional film cooling setup. The film coolant mass flow ratio is varied in this study to see how the interaction of different coolants changes with changes of their injected momentum. The measurements reveal that the upstream coolant flow helps in keeping the film coolant attached to the endwall for most of the passage and also keeping the film thickness fairly uniform in the pitchwise direction. An increase in louver coolant mass flow rate shows higher endwall cooling effectiveness. The velocity contours show that a dominant vortex is present due to combustor coolant injection and that the passage vortex was, although present, diminished due to this other vortex. While changing the louver coolant mass flow rate did not change the intensity of these vortices, changes in the film coolant mass flow rate increased the intensity of the dominant vortex near the pressure surface. Also, the presence of this new vortex helps in film cooling the pressure surface, a region where, conventionally, the least amount of coolant coverage is recorded.
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.
Heat transfer to droplets in developing boundary layers at low capillary numbers
(2014-08) Wenzel, Everett
This thesis describes the heating rate of a small liquid droplet in a developing boundary layer wherein the boundary layer thickness scales with the droplet radius. Surface tension modifies the nature of thermal and hydrodynamic boundary layer development, and consequently the droplet heating rate. A physical and mathematical description precedes a reduction of the complete problem to droplet heat transfer in an analogy to Stokes' first problem, which is numerically solved by means of the Lagrangian volume of fluid methodology.For Reynolds numbers of order one, the dispersed phase Prandtl number significantly influences the droplet heating rate only in the transient period when the thermal boundary layer first reaches the droplet surface. As the dispersed phase Prandtl number increases, so does the duration of the transient. At later times, when the the droplet becomes fully engulfed by the boundary layer, the heating rate becomes a function of only the constant heat flux boundary condition. This characteristic holds for all Peclet and Weber numbers, but the spatial behavior of the droplet differs for small and large Peclet and Weber numbers.Simulation results allow for the development of a predictive tool for the boiling entry length of dilute systems in channel flow. The tool relies on an assumption of temperature equivalency between the droplet and the thermal boundary layer evaluated in absence of the dispersed phase, which is supported by the computational results. Solutions for plug and fully developed flow do not differ appreciably, suggesting a precise description of the fluid mechanics is not necessary for an approximation of the boiling entry length. Future experimental work is required to validate the predictive models derived in this thesis.
Interactions Between Fluid Flow, Heat Transfer, And Particle Transport In The Presence Of Jet-Axis Switching And Realistic Fluid Movers
(2014-12) Gorman, John
The overarching goal of this thesis is to identify and quantify new processes and phenomena related to fluid flow, heat transfer, and particle transport interacting in unique modes. The research can be categorized into three modes of interaction: (a) heat transfer processes governed by the complex patterns of fluid flow provided by real-world fluid-moving devices, (b) heat transfer processes which are governed by a naturally occurring, extraordinary fluid-flow phenomenon, and (c) interacting fluid flow, particle transport, and heat transfer all of which are governed by the aforementioned extraordinary fluid-flow phenomenon. These categories are respectively treated in individual chapters of the thesis. The traditional approach to convective heat transfer is virtually devoid of realistic fluid flow models. As a consequence, traditional convective heat transfer analysis is oversimplified to the point of being out of step with reality. This assertion is proven here, and a new fundamentals-based model of high fidelity involving realistic fluid movers of is created. Next, the extraordinary fluid flow phenomenon designated as jet-axis switching is introduced and illustrated quantitatively. This phenomenon occurs whenever a non-circular jet passes into and through an unrestricted space. When the jet is involved in a process called jet-impingement heat transfer, the zone of jet incidence is highly altered due to the axis-switching process. The ignoring of the switching process, which has been standard in all previous work on non-circular-jet impingement heat transfer, has been shown here to be highly error prone. The major part of the thesis encompasses jet-axis-switching fluid mechanics, convective heat transfer, and particle transport. An all-encompassing simulation model was created which took account of fluid-particle, particle-particle, fluid--impingement plate, and particle--impingement plate interactions, all with heat transfer. It was found that jet-axis switching exerted a major effect on the trajectories of the particles, with a corresponding impact on the particle collection efficiency of the impactor plate. The transfer of heat between the fluid and the impingement plate was little affected by any alterations in the pattern of fluid flow caused by the presence of particles. On the hand, direct particle-to-plate heat transfer is substantial.
Investigation of the thermal parameters of reclaimed asphalt materials with applications to asphalt recycling
(2014-08) DeDene, Christopher D.
Asphalt concrete is the third most widely used resource in the world, next to Portland Cement Concrete and water. In the United States alone, over 550 million tons of hot mix asphalt (HMA) are produced at more than 4,000 asphalt plants across the country. With over 94% of the paved roads in the United States surfaces with asphalt concrete, it's safe to say asphalt pavement is what America drives on. However, a majority of today's pavement projects are geared towards rehabilitation and reconstruction of existing pavements, rather than construction of new roads. While it is true that asphalt pavement is 100% recyclable and it is the most recycled material in America, the reality is most roads contain no more than 20% recycled material. There are many factors that prohibit new road construction in excess of 20% recycled content, and this thesis aims to explore just one of those factors - the thermodynamics of hot mix asphalt pavement recycling. Most research that is investigating the use of high amounts of Reclaimed Asphalt Pavement (RAP) have been based on empirical trials. This work has approached the issue of pavement recycling by measuring the thermal properties of recycled asphalt, examining the thermodynamic limits of asphalt drum mixing, and by modeling asphalt mixing drums using finite element techniques to determine the amount of time required to achieve full melting inside of asphalt drums. It was found that for many different drum configurations, there is insufficient retention time for RAP to reheat. This insufficient heating could cause premature failures in asphalt pavements using high percentages of RAP. A secondary goal of this thesis is to explore the benefits of using the waste mining material, taconite tailings, in new asphalt pavements. This research shows there is thermodynamic benefit gained by using taconite tailings because they can be heated faster than traditional aggregates. This heating supplies more heat to RAP, which in turn, may allow for more of the recycled asphalt pavement to be incorporated into new asphalt pavements.
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.
Mixed convection in horizontal fluid-superposed porous layers
(2013-08) Dixon, John Mark
Mixed convection in horizontal fluid-superposed porous layers is studied in the following work. Much research has been done in the field of natural, mixed, and forced convection in a porous layer. Several studies have investigated natural and forced convection in a two-domain system that includes a porous and a fluid layer, but mixed convection has not been addressed. This problem can be found in many natural and engineering applications. Some examples include beach sand, human lungs, bread, gravel, soil, rock, packed bed reactors, fiberglass insulation, thermal energy storage systems, electronic cooling, crude oil extraction, nuclear reactors, and the list goes on. The present study is motivated by the wide range of applications and seeks to fill the gap in the literature regarding mixed convection. The problem considers a long, narrow channel that is partially filled with a porous layer and has a fluid layer above the porous layer. The channel is partially heated on the bottom and cross flow along the length of the channel is added in varying degrees. The problem is studied at a fundamental level, with the governing equations being derived, non-dimensionalized, discretized, and solved numerically. The two layers are treated as a single domain and the porosity is used as a switching parameter, causing the governing equations to transition from an extended form of the Darcy-Brinkman-Forchheimer equation in the porous layer to the Navier-Stokes equations in the fluid layer. This method avoids the need for interfacial boundary conditions to be explicitly defined at the interface between the two domains. Several dimensionless numbers are varied and their effects on the overall Nusselt number of the system are documented. The parameters varied include the Peclet number, the Rayleigh number, the porous layer height ratio, the Darcy number, the Prandtl number, and the conductivity ratio between the solid and fluid phases. In addition, the impact of the various additional terms in the extended form of Darcy's law is investigated and documented as well. The conductivity ratio, Darcy number, porous layer height ratio, Rayleigh number, and Peclet number all have a strong effect on the overall Nusselt number of the system, while the Prandtl number, the Brinkman term, the Forchheimer term, and the convective terms have a negligible effect. A critical Peclet number was observed, where the Nusselt number is a minimum, and was shown to be proportional to the Rayleigh-Darcy number and inversely proportional to the porous layer height ratio. A critical porous layer height ratio was also found, where the Nusselt number is a minimum, and was shown to be proportional to the Rayleigh-Darcy number and inversely proportional to the Peclet number. The streamlines capture the transition from the natural convection regime to the forced convection regime. In the transition region the flow patterns have characteristics of both domains. The isotherms capture the plume flow and show the influence of the cross flow on the shape and character of the plume. An experimental apparatus is designed in order to collect data over a similar range of parameters as explored numerically. The average error between the numerical and experimental results is 30%, with a peak of 67%. The numerical results show good agreement with the experimental data within the bounds of uncertainty. The experimental results confirm the presence of a critical Peclet number. However, they do not show the same trends at intermediate porous layer height ratios. The effect of the porous layer height ratio, η=h_p⁄H, on the Nusselt number is shown to be small in the range of η = 0.5 to η = 1 and large in the range of η = 0 to η = 0.5. Also, the transition to the forced convection regime occurs earlier for the numerical results than it does for the experimental results. This points towards future research opportunities that focus on the lower range of porous layer height ratio values.
Modeling of Particle Engulfment during the Growth of Crystalline Silicon for Solar Cells
(2016-12) Tao, Yutao
A major challenge for the growth of multi-crystalline silicon is the formation of carbide and nitride precipitates in the melt that are engulfed by the solidification front to form inclusions. These lower cell efficiency and can lead to wafer breakage and sawing defects. Minimizing the number of these engulfed particles will promote lower cost and higher quality silicon and will advance progress in commercial solar cell production. To better understand the physical mechanisms responsible for such inclusions during crystal growth, we have developed finite-element, moving-boundary analyses to assess particle dynamics during engulfment via solidification fronts. Two-dimensional, steady-state and dynamic models are developed using the Galerkin finite element method and elliptic mesh generation techniques in an arbitrary Eulerian-Lagrangian (ALE) implementation. This numerical approach allows for an accurate representation of forces and dynamics previously inaccessible by approaches using analytical approximations. We reinterpret the significance of premelting via the definition of an unambiguous critical velocity for engulfment from steady-state analysis and bifurcation theory. Parametric studies are then performed to uncover the dependence of critical growth velocity upon some important physical properties. We also explore the complicated transient behaviors due to oscillating crystal growth conditions as well as the nonlinear nature related with temperature gradients and solute effects in the system. When compared with results for the SiC-Si system measured during ParSiWal experiments conducted by our collaborators, our model predicts a more realistic scaling of critical velocity with particle size than that predicted by prior theories. However, the engulfment growth velocity observed in the subsequent experiment onboard the TEXUS sounding rocket mission turned out to be unexpectedly higher. To explain this model discrepancy, a macroscopic model is developed in order to account for the natural convection in the terrestrial experiments. We demonstrate that the convective flows are able to keep most small particles suspended in the melt, so that the observed critical velocities and their variance are enhanced in the experiments conducted on earth. According to simulation results, some solutions, which are applicable in photovoltaic industry, to the inclusion problem are also discussed and studied.
Natural convection in water-saturated metal foam with a superposed fluid layer.
(2010-11) Wade, Aaron D.
Experimental results are presented for steady state natural convection heat transfer in an enclosure with a horizontal layer of copper foam and an overlying water layer, heated from below. The foam was placed between two parallel plates in a cylindrical enclosure adjacent to either one or both boundaries. The cylindrical copper foam disks were manufactured by ERG Aerospace with 127 mm DIA, with porosity ranging from 0.88 to 0.92, and pore densities of 5 and 10 PPI. The effective thermal conductivity of the foam was measured by running experiments at pre-convection Rayleigh numbers. The permeability and drag coefficient of the foam were determined by measuring the pressure drop across foam samples in a separate facility. With foam on the hot boundary, natural convection heat transfer experiments are run for 5 and 10 PPI foam, Rayleigh numbers, based on the total height of the enclosure and the water properties, from 2 × 106 to 5 × 108, aspect ratios, H/D, from 0.1 to 0.8, and ratios of the total foam layer height to the total height of the apparatus, η = Hp/H, of 0.25, 0.5, and 0.75. With foam on both boundaries, experiments are run for 10 PPI foam, Rayleigh numbers from 1 × 107 to 3 × 108, aspect ratios from 0.3 to 0.8, and η of 0.5 and 0.75. Natural convection heat transfer experiments were also performed for water alone over the Rayleigh number range 9 × 105 ‒ 3 × 108 and for aspect ratios 0.2 to 0.8. Heat transfer results are presented as an enhancement factor, the ratio of heat transfer with foam to that of water alone at the same Rayleigh number. The results with foam on the hot boundary were not a function of η. 10 PPI foam on the hot boundary did not enhance heat transfer with an average enhancement factor of 0.98. 5 PPI foam on the hot boundary had an enhancement factor of 1.1. The increased enhancement of 5 PPI foam over 10 PPI foam is attributed to the greater permeability of the 5 PPI foam. With foam on both boundaries, enhancement tended to increase with Rayleigh number and is a function of η. For η = 0.5 enhancement ranged from 0.93 to 1.33. For η = 0.75 enhancement ranged from 1.1 to 1.29. For the same enclosure filled with copper foam (from Kathare et al. [13]), enhancement was not a strong function of Rayleigh number and ranged from 1.2 to 1.6, though enhancement was typically above 1.35. The decreased enhancement of a partially filled enclosure, compared to a fully filled enclosure, is attributed to a lack of direct conduction path from each boundary, significantly decreasing the apparent conductivity of the enclosure, and decreased advection as the convection cells do not fully penetrate the porous layer(s). Power law correlations relating the fluid Nusselt number to the fluid Rayleigh number were determined for the three foam cases. The data for foam on the hot boundary alone and on both boundaries were also successfully correlated using porous media variables where an apparent conductivity of the composite system, considering the foam and fluid layers in series, was used as the effective conductivity. The porous media Nusselt number is related to the porous media Rayleigh number and a modified porous media Prandtl number.
Optimizing urban environmental simulations using boinc
(2013-08) Vegesna, Aditya
Urban cities are usually densely populated and have massive infrastructure. They consume a lot of energy and generate pollution. Urban form and structure interact with the environment in a complex way. There is transfer of energy between buildings and the ground layer. Winds flow through the urban street canyons, affecting evaporation, temperature and pollution dispersion. The effects of such complex interactions are still not widely known or understood. How well an urban space disperses pollution, or requires energy for heating or cooling is potentially impacted by many components, such as where the buildings are located with respect to each other, which materials the buildings are constructed from, or where trees or parks are placed. The aim of the Genusis project is to provide a tool for urban planners that they can utilize to understand such impacts and to assist them in taking design decisions accordingly. Even with just a few choices in building locations or tree types the number of possible configurations is vast. Running the simulations on many thousand of these configurations is a huge problem on its own and truly not feasible for urban planners to use in their daily routines.This thesis strives towards tackling that problem by developing a computational environment in which specifying these configurations is easy and can compute potential solutions to the problems within an acceptable time frame using multiple machines. A simple and yet powerful language is created to let urban planners control the simulations and specify the configurations. In order to reduce the computational time, Berkeley Open Infrastructure for Network Computing (BOINC) is used to harness all available computational resources. Experiments were conducted to analyze the implementation and performance of the system. The results obtained validate the implementation and indicate a significant performance gain.
Synthetic jet flow and heat transfer for electronics cooling
(2014-05) Huang, Longzhong
The progressive increase of heat dissipation from modern electronics requires more and more powerful cooling systems. Various cooling technologies have been developed such as liquid cooling, micro-channel cooling, and active cooling. The present study focuses on applying a unique device called a synthetic jet to cool electronics. A synthetic jet is able to generate an unsteady flow with a simple structure that makes it effective in convective heat transfer. This study provides both practical and fundamental view of synthetic jets in the application of electronics cooling. A mock-up synthetic jet is fabricated to study heat transfer and fluid mechanics of synthetic jet cooling. The scaled synthetic jet is geometrically and dynamically similar to the actual jet. The heat transfer performance characteristics of a synthetic jet impinging on a fin are tested with different operating frequencies and with different orifice shapes. Flow visualizations and detail flow field measurements of the impinging synthetic jet flow are documented to support the heat transfer experiment. The optimized parameters obtained from the scaled experiment are applied to the actual synthetic jet design. The actual synthetic jet is realized using a piezoelectric stack and applied on a cooling system based on a full-sized heat sink module. The cooling performance of the whole system is documented. The noise characteristics of the actual synthetic jet is tested and analyzed. A muffler with optimized parameters is found and used for noise reduction. Numerical simulation is used to find the optimal design for the synthetic jets. The computation is realized by the commercial software ANSYS Fluent. The numerical model is verified by comparing the computational results with experimental results. A parametric study of heat transfer performance of synthetic jet cooling is documented.
Thermal convection at high Rayleigh numbers in compressed gases.
(2007-12) Srinivasan, Vinod
This study focuses on the heat transfer relationship for turbulent convection in a layer of fluid heated from below. Results are presented in the form of the Nusselt number as a function of the Rayleigh number, for Rayleigh numbers ranging from 2 × 10 9 to 3.1 × 10 12 . High Rayleigh numbers are attained by pressurizing nitrogen, argon and krypton to pressures of up to 80 bar. The experimental apparatus is designed with close attention to the effects of conduction through the insulating sidewalls at low Rayleigh number, and to the effects of variable properties that may affect the Boussinesq approximation at high Rayleigh numbers. The results show a relationship between the Nusselt and Rayleigh numbers that is close to a power-law with an exponent of 1/3 for the Rayleigh number. There is no visible transition, or incipient transition to a power-law regime with an exponent of 1/2, as has been theoretically predicted by some investigators. It is argued that various other values of the exponent that are found in the literature are either due to conduction effects at low Rayleigh numbers (leading to a lower exponent) or variable properties (leading to a higher exponent) at high Rayleigh numbers. However, the precise mechanism of energy transport that leads to an exponent of 1/3 remains unclear. While a large-scale recirculation has been observed in experimental apparatuses, there remains uncertainty as to the manner in which this flow affects the stability of the thermal boundary layer. Local temperature measurements were taken using a 76μ m thermocouple probe. The temperature measurements are at significant variance from the expected temperature distribution in the thermal boundary layer. Conduction effects in the probe are shown to be significant. Temperature statistics measured with the probe show some averaging over high frequencies.
Thermal modeling and design of a solar non-stoichiometric redox reactor with heat recovery
(2013-08) Lapp, Justin L.
A promising new technology for sustainable fuel production is the splitting of water and carbon dioxide by the non-stoichiometric two-step metal oxide redox cycle. Development of oxide materials and reactors to realize the cycle is currently in infancy, with significant room for improvement over previous demonstrations. Research efforts have gone into developing and characterizing reactive metal oxide materials for the cycle, while less literature is devoted to the design and understanding of non-stoichiometric redox reactors. The work presented attempts to close the gap by exploring multiple levels of modeling analysis to determine the important considerations for designing a reactor to perform high efficiency non-stoichiometric redox cycling. Cerium oxide (ceria) is considered as the reactive material for reactors in this work. A reactor for non-stoichiometric redox cycling should allow for continuous use of the solar input and should implement heat recovery. In the first stage of the research, thermodynamic analysis is carried out on a model reactor system to quantify the potential efficiency benefits of heat recovery and to determine the effects of the reduction temperature and sweep gas flow rate. Heat recovery is found to improve reactor efficiency from 4% to 16%. The selection of reduction temperature is important to high efficiency. For many cases the heating of gases is a major source of heat loss, indicating that heat recovery should be applied to the gas flows as well as the solid metal oxide. In the second stage of the research, a reactor is presented which incorporates continuous redox cycling of ceria and heat recovery from the solid ceria by using counter-rotating hollow cylinders of ceria and inert material. Heat transfer modeling is applied to this concept to explore its performance potential and identify the important design factors for effective heat recovery. Energy conservation is applied using a finite volume method with detailed modeling of radiative heat transfer by the Monte Carlo method and the Rosseland diffusion approximation. A simplified model of the rotating cylinders and a more complete model of the full reactor geometry are applied. It is determined that the proposed design can recover over 50% of the heat from the ceria, and provide a temperature differential of 400 K between the reaction steps. Geometric and material parameters are varied in a parametric study to determine which are important forheat recovery. The important parameters for heat recovery and chemical utilization of the material are those which define the heat transport across the ceria cylinder wall. Temperatures, heat transfer rates, heat fluxes, and the chemical state of the material are predicted. Using the heat transfer model results and other analysis, values of thermal design parameters for a prototype reactor are selected as part of an effort leading to a prototype reactor to be built and tested at the University of Minnesota. Heat recovery is found to be a path with great potential for improving the efficiency of solar-driven non-stoichiometric redox cycles. The prototype reactor described has the potential to demonstrate high levels of heat recovery and unprecedented efficiency. However, a careful understanding of the properties of the reactive material and the geometric parameters of the reactor is needed to ensure that heat which is input or removed is effectively transported across the cylinder wall for heat recovery. The models described here account for the important effects and explore the complexity needed to investigate the problem. Primary future improvements to the modeling work will include coupling of heat transfer and fluid mechanics, implementation of chemical rate expressions, and the addition of high-temperature and spectral material properties as they become available.
Thermal-capillary analysis of the horizontal ribbon growth of solar silicon
(2013-12) Daggolu, Parthiv
Horizontal ribbon growth (HRG) promises the growth of crystalline silicon at rates that are orders of magnitude greater than vertical ribbon growth technologies. If successful, this process would enable the production of higher-quality, near-single-crystalline silicon wafers at fraction of the cost of current production techniques. This fascinating process was first conceived by Shockley in late 1950's for silicon growth and was practiced by Bleil in the late 1960's for germanium growth. Large-scale development efforts were sub- sequently carried out by Kudo in Japan in the late 1970's and by the Energy Materials Corporation in the US in the early 1980's. However, after encouraging early results, experimental advances and process development efforts stalled, and this technique was abandoned in favor of growth methods that were easier to develop.Unlike vertical meniscus-defined crystal growth processes, such as edge-defined film- fed growth (EFG), which are inherently stable, there are many failure modes that must be avoided in the HRG process. We argue that its successful operation will rely on a thorough understanding of system design and control-issues that are perhaps only feasibly addressed via computational modeling of the system. Towards these ends, we present a comprehensive thermal-capillary model based on finite-element methods to study the coupled phenomena of heat transfer, fluid mechanics and interfacial phenom- ena (solidification and capillarity) in the HRG process. Bifurcation analysis coupled with transient computations using this model reveals process limitations that manifest as failure mechanisms, such as bridging of crystal onto crucible, spilling of melt from the crucible, and undercooling of melt at the ribbon tip, that are consistent with prior experimental observations and suggests operating windows that may allow for stable process operation. Further, coupled impurity transport calculations reveal interesting and potentially beneficial redistribution mechanisms at the solidification interface that lead to an inherent purification of the majority of the growing crystal ribbon.
Understanding the benefits and limitations of magnetic nanoparticle heating for improved applications in cancer hyperthermia and biomaterial cryopreservation
(2013-12) Etheridge, Michael Laurence
The current work focused on the ability of magnetic nanoparticles to produce heat in the presence of an applied alternating magnetic field. Magnetic nanoparticle hyperthermia applications utilize this behavior to treat cancer and this approach has received clinical approval in the European Union, but significant developments are necessary for this technology to have a chance for wider-spread acceptance.Here then we begin by investigating some of the important limitations of the current technology. By characterizing the ability of superparamagnetic and ferromagnetic nanoparticles to heat under a range of applied fields, we are able to determine the optimal field settings for clinical application and make recommendations on the highest impact strategies to increase heating. In addition, we apply these experimentally determined limits to heating in a series of heat transfer models, to demonstrate the therapeutic impact of nanoparticle concentration, target volume, and delivery strategy.Next, we attempt to address one of the key questions facing the field- what is the impact of biological aggregation on heating? Controlled aggregate populations are produced and characterized in ionic and protein solutions and their heating is compared with nanoparticles incubated in cellular suspensions. Through this investigation we are able to demonstrate that aggregation is responsible for up to a 50% decrease in heating. However, more importantly, we are able to demonstrate that the observed reductions in heating correlate with reductions in longitudinal relaxation (T1) measured by sweep imaging with Fourier transformation (SWIFT) magnetic resonance imaging (MRI), providing a potential platform to account for these aggregation effects and directly predict heating in a clinical setting.Finally, we present a new application for magnetic nanoparticle heating, in the thawing of cryopreserved biomaterials. A number of groups have demonstrated the ability to rapidly cool and preserve tissues in the vitreous state, but crystallization and cracking failures occur upon the subsequent thaw. Magnetic nanoparticles offer a potential solution to these issues, through their ability to provide rapid, uniform heating, and we illustrate this through heating in several cryoprotectant solutions and by modeling the effects of heating at the bulk and micro-scales.

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