Browsing by Subject "numerical"
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Item A numerical study of convection in a channel with porous baffles(Texas A&M University, 2005-02-17) Miranda, Bruno Monte Da SilvaThe effects on heat transfer in a two-dimensional parallel plate channel with sixteen porous baffles in a staggered arrangement with a uniform heat flux heating applied to the top and bottom walls has been numerically investigated. Developing Flow (DF) was considered for this study. The Brinkman-Forchheimer-extended Darcy model was used for modeling the heat transfer and fluid flow through the porous baffles. The flow was assumed to be laminar. A finite volume based method in conjunction with the SIMPLEC algorithm was used to solve the model equations. Calculations were made by varying several independent parameters such as Reynolds number (Re), Darcy number ⎞ (Da), thermal conductivity ratio ⎛⎜ k e kf ⎠⎟ , baffle thickness ( * ) , non-dimensional w ⎝ baffle spacing ( * ) , and non-dimensional baffle height ( * ) . w The results of the study established that porous baffles out perform solid baffles from a pressure drop point of view. However, porous baffles under perform solid baffles from a heat transfer point of view. The ratio representing increase in heat transfer per unit increase in pumping power (heat transfer performance ratio) was found to be less than unity for all cases. Increasing the Darcy number was found to produce less desirable heat transfer enhancement ratios. Increasing the non-dimensional baffle spacing (d/w) and the baffle aspect ratio (H/w) were found to enhance heat transfer.Item Enhancement of the Response Range and Longevity of Microparticle-based Glucose Sensors(2011-08-08) Singh, SaurabhLuminescent microspheres encapsulating glucose oxidase and an oxygensensitive lumophore have recently been reported as potential implantable sensors for in vivo glucose monitoring. However, there are two main issues that must be addressed for enzymatic systems such as these to realize the goal of minimally-invasive glucose monitoring. The first issue is related to the short response range of such sensors, less than 200 mg/dL, which must be extended to cover the full physiological range (0-600 mg/dL) of glucose possible for diabetics. The second issue is concerning the short operating lifetime of these systems due to enzyme degradation (less than 7 days). Two approaches were considered for increasing the range of the sensor response; nanofilm coatings and particle porosity. In the first approach, microparticle sensors were coated with layer-by-layer deposited thin nanofilms to increase the response range. It was observed that, a precise control on the response range of such sensors can be achieved by manipulating different characteristics (e.g., thickness, deposition condition, and the outermost capping layer) of the nanofilms. However, even with 15 bilayers of poly(allylamine hydrochloride)/poly(styrene sulfonate) (PAH/PSS) nanofilm, limited range was achieved (less than 200 mg/dL). By performing extrapolation on the data obtained for the experimentally-determined response range versus the number of PAH/PSS bilayers, it was predicted that a nanofilm coating comprising of more than 60 PAH/PSS bilayers will be needed to achieve a linear response up to 600 mg/dL. Using modeling, it was realized that a more effective method for achieving a linear response up to 600 mg/dL is to employ microparticles with higher porosity. Sensors were prepared from highly porous silica microparticles (diameter = 7 mu m, porosity = 0.6) and their experimental response was determined. Not surprisingly, the experimentally determined response range of such sensors was found to be higher than 600 mg/dL. To improve the longevity of these sensors, two approaches were employed; incorporation of catalase and increasing the loading of glucose oxidase. Catalase was incorporated into microparticles, which protects the enzyme from peroxide-mediated deactivation, and thus improves the stability of such sensors. Sensors incorporating catalase were found to ~5 times more stable than the GOx-only sensors. It was theoretically predicted, that by maximizing the loading of glucose oxidase within the microparticles, the longevity of such sensors can be substantially improved. Based on this understanding, sensors were fabricated using highly porous microparticles; response range did not vary even after one month of continuous operation under normal physiological conditions. Modeling predicts that 1 mM of glucose oxidase and 1 mM of catalase would extend the operating lifetime to more than 90 days.Item Multiscale numerical methods for some types of parabolic equations(2009-05-15) Nam, DukjinIn this dissertation we study multiscale numerical methods for nonlinear parabolic equations, turbulent diffusion problems, and high contrast parabolic equations. We focus on designing and analysis of multiscale methods which can capture the effects of the small scale locally. At first, we study numerical homogenization of nonlinear parabolic equations in periodic cases. We examine the convergence of the numerical homogenization procedure formulated within the framework of the multiscale finite element method. The goal of the second problem is to develop efficient multiscale numerical techniques for solving turbulent diffusion equations governed by celluar flows. The solution near the separatrices can be approximated by the solution of a system of one dimensional heat equations on the graph. We study numerical implementation for this asymptotic approach, and spectral methods and finite difference scheme on exponential grids are used in solving coupled heat equations. The third problem we study is linear parabolic equations in strongly channelized media. We concentrate on showing that the solution depends on the steady state solution smoothly. As for the first problem, we obtain quantitive estimates for the convergence of the correctors and some parts of truncation error. These explicit estimates show us the sources of the resonance errors. We perform numerical implementations for the asymptotic approach in the second problem. We find that finite difference scheme with exponential grids are easy to implement and give us more accurate solutions while spectral methods have difficulties finding the constant states without major reformulation. Under some assumption, we justify rigorously the formal asymptotic expansion using a special coordinate system and asymptotic analysis with respect to high contrast for the third problem.Item Numerical Investigation of Thermal Hydraulic Behavior of Supercritical Carbon Dioxide in Compact Heat Exchangers(2012-02-14) Fatima, RomaThe present work seeks to investigate the thermal hydraulic (heat transfer and fluid dynamics) behavior of supercritical (Sc) fluids at both the fundamental and applied levels. The thermal hydraulics of these fluids is not very well known although they have been used in various applications. There are drastic changes in the thermal and hydraulic properties of fluids at supercritical conditions. There has been a lot of focus to effectively utilize these properties changes in many applications such as heat exchangers. This work focuses on studying the forced convective heat transfer of Sc-CO2 in a series of mini semi-circular horizontal tubes and a zig-zag shaped horizontal channel. The problems were investigated numerically by second-order finite volume method using a commercial software FLUENT. Three dimensional Computational Fluid Dynamics (CFD) models were developed to simulate the flow and heat transfer for three different geometries ? a single semi-circular channel, a series of nine parallel semi-circular channels and a zig-zag channel. Grid and accuracy refinement studies were carried out to assess numerical errors. All the computational meshes developed for this study incorporated the first node cell within the viscous sub-layer i.e. y <1. Since the flow is turbulent, an appropriate choice of turbulence model is highly desirable. Henceforth, various turbulence models were used to study their impact on the heat transfer solution for these problems. The present numerical work focuses on improving the CFD model and methodologies in order to capture the experimental data of the heat transfer spike at the super critical conditions. Local and average heat transfer coefficients near the critical point were determined from measured wall temperatures and calculated local bulk temperatures. The numerical results are compared with the experiments. The numerical predictions do not convincingly agree with the experiments. This could be because of the incapability of turbulent models to capture the flow physics accurately due to the rapid changes in the fluid properties near critical conditions.