This research presents the results of numerical study of heat transfer and pressure drop in a heat exchanger that is designed with different shape pin fins. The heat exchanger used for this research consists of a rectangular duct fitted with different shape pin fins, and is heated from the lower plate. The pin shape and the compact heat exchanger (CHE) configuration were numerically studied to maximize the heat transfer and minimize the pressure drop across the heat exchanger. A three dimensional finite volume based numerical model using FLUENT© was used to analyze the heat transfer characteristics of various pin fin heat exchangers. The simulation applied to estimate the heat transfer coefficient and pressure drop for a wide range of Reynolds numbers with different pin fins. Circular pin configuration variations included changes in pin spacing, axial pitch and pin height ratio. Rectangular and drop-shaped pin variations also included changes in length and aspect ratio. Correlations for Nusselt number and friction factor were developed. The optimum drop shaped pin array was shown to match the heat transfer rates obtained by the optimum circular pin configuration while incurring less than one third the specific fluid friction power losses. The data and conclusions of this study can be applied to the optimization of different heat exchangers which are used in industry, especially oil cooler in power transformers which are currently working with low cooling efficiency. It can also be used in the design of electronic components, turbine blade cooling or in other high heat flux dissipation applications requiring a low-profile, high area-density based micro-heat exchanger design. This study also shows that numerical models backed with experimental analysis can reduce both the time and money required to create and evaluate engineering concepts, especially those that deal with fluid flow and heat transfer. In the following chapters, first the problems which are encountered by power transformer suppliers are described. Then pin fin technology is studied with more details as a novel solution to the oil cooling problem. Some studies on behavior of power transformer coolers are also conducted to make their problems more clear. Available experimental data in the Iran Transfo company have been used for validation of these studies. They are presented as separated papers at the end of thesis. Finally the results of pin fin studies are presented and horizontal continuous casting (HCC) is explained as a manufacturing method for pin fins production. A separate paper which is based on experimental study on HCC is also included at the end of thesis. / Forskningen presenterad är ett resultat av en numerisk studie av värmeöverföring och tryckfall i en värmeväxlare designad med olika former av Kylflänsar. Värmeväxlaren består av ett rektangulärt kanal utrustat med olika former av Kylflänsar och är uppvärmd underifrån. Kylflänsar forma och den kompakta värmeväxlarens utformning är studerade numeriskt för att maximera värmeöverföringen och minimera tryckfallet över värmeväxlaren.En tredimensionell finit volym baserad på en numerisk modell i FLUENT© användes för att analysera värmeöverföringsegenskaper för olika Kylflänsar konfigurationer. Genom simuleringar uppskattades värmegenomgångstalet och tryckfallet för olika Reynolds tal och Kylflänsar konfigureringar. Cirkulära Kylflänsar konfigurationer inkluderar variation av avstånd mellan Kylflänsar, och förhållandet mellan axiellt avstånd och höjd. Rektangulära och droppformade Kylflänsar inkluderade även variation för längd och aspekt förhållande. Korrelation mellan Nusselts tal och friktionsfaktor utvecklades. Optimal matris för hur droppformade Kylflänsar placerades visades överensstämma med optimal överföring för cirkulära Kylflänsar men bara med en tredjedel av friktionsförlusterna för fluiden. Data och slutsatser från studien kan användas inom för optimering av värmeväxlare använda i industrin, speciellt oljekylda högspänningstransformatorer som har låg effektivitet i kylningen. Resultaten kan även användas inom design av elektronikkomponenter, kylning av turbinblad eller andra komponenter med högt värmeflöde där låg profil, och stor ytdensitet behövs. Studien visar att kombinationen av numeriska modeller som valideras genom experiment kan reducera både tid och kostnad vid utveckling och utvärdering av ingenjörsverktyg, speciellt inom fluidmekanik och värmeöverföring. I följande kapitel beskrivs först problem som identifierats av tillverkare av högspänningstransformatorer. Kylflänsar studeras i detalj som en ny lösning till de identifierade problemen med oljekylning. Några studier har genomförts för att ytterligare belysa problemen kring högspänningstransformatorers kylning. Tillgängliga data från Iran Transfo company har använts för validering av resultat från studierna. Studierna presenteras som separata artiklar i slutet av avhandlingen. Avslutningsvis presenteras resultat från studierna av Kylflänsar och en horisontell kontinuerlig gjutprocess (HCC) för tillverkning av Kylflänsar. HCC-studien presenteras som en separat artikel inkluderad sist i avhandlingen.

An important factor in forecast models today is cirrus clouds, but not much are known about their dynamics which makes them hard to parameterize. In this study a new theory was derived to enable a more correct way to describe the interplay between radiative heating and dynamical motions in these clouds. This hypothesis was tested by performing three dimensional simulations of cirrus clouds, using the University of Utah Large Eddy Simulator (UULES). Eleven clouds of varying initial radius and ice water mixing ratio were examined, with the aim of finding a pattern in their dynamical features. The model was set up without short wave radiation from the sun, and without any precipitation affecting the clouds, leaving only terrestrial heating and atmospheric cooling to create motions in the clouds. Two categories of initial dynamics could be seen: • Isentropic adjustment: The isentropes within the cloud are adjusting to the environment due to rising of the cloud. Causes horizontal spreading through continuity. • Density current: A dominating initial feature is spreading in small mixed layers at the cloud top and bottom. Caused by the density difference between the cloud and its environment. An interesting phenomenon showing up in the simulations was mammatus clouds, which were visible in two of the cases. The only instability available to create these clouds was the radiative heating difference, which does not agree with present theories for how they form. Two dimensionless numbers S and C were derived to describe the nature of the spreading motions and convection in the cloud. Both these numbers agreed with results. / Cirrusmoln har en viktig roll i dagens prognosmodeller, men är svåra att parametrisera på ett bra sätt eftersom man inte har tillräcklig kunskap om deras dynamik och utveckling. I denna studie togs en ny teori fram för att göra det möjligt att på ett mer korrekt sätt beskriva samspelet mellan strålningsuppvärmning och dynamiska rörelser i dessa moln. Hypotesen testades sedan genom att utföra tredimensionella simuleringar av cirrus moln med hjälp av University of Utah Large Eddy Simulator (UULES). Elva moln med varierande initiella radier och isvatteninnehåll undersöktes, med målet att finna ett mönster i dynamik och utveckling. UULES ställdes in så att miljön där molnen simulerades varken innehöll kortvågsstrålning från solen eller nederbörd. Således fanns det bara en resterande faktor för att skapa rörelser i molnen; skillnaden i den infraröda strålningsuppvärmningen mellan molntopp och molnbas. Två kategorier av initiella rörelser uppstod i molnen: • Justering av isotroper: Molnen stiger i höjd vilket gör att isotroperna inuti dem justeras till omgivningen. Detta orsakar horisontell spridning genom kontinuitet. • Densitets ström: Horisontell spridning av molnen koncentrerad till mixade skikt i de övre och undre delarna. Orsakas av skillnad i densitet mellan moln och omgivning. Ett intressant fenomen som visade sig i två av simuleringarna var mammatusmoln. Den enda instabiliteten tillgänglig för att skapa dessa moln var skillnaden i strålningsuppvärmning mellan molntopp och -bas. Detta stämmer inte överrens med nuvarande teorier för hur dessa moln skapas. Två dimensionslösa tal, S och C togs fram för att indikera vilken av de initiella rörelserna som dominerar i molnet, samt vilken typ av konvektion som dominerar. Båda dessa tal stämde väl överrens med resultat.

At the microscopic level, many materials are made of smaller and randomly oriented grains. These grains are separated by grain boundaries which tend to decrease the electrical and thermal conductivity of the material. The motion of grain boundaries is an important phenomenon controlling the grain growth in materials processing and synthesis. Mathematical modeling and simulation is a powerful tool for studying the motion of grain boundaries. The research reported in this thesis is focused on the numerical simulation and analysis of a coupled surface and grain boundary motion which models the evolution of grain boundary and the diffusion of the free surface during the process of grain growth. The “quarter loop” geometry provides a convenient model for the study of this coupled motion. Two types of normal curve velocities are involved in this model: motion by mean curvature and motion by surface diffusion. They are coupled together at a triple junction. A front tracking method is used to simulate the migration. To describe the problem, different formulations are presented and discussed. A new formulation that comprises partial differential equations and algebraic equations is proposed. It preserves arc length parametrization up to scaling and exhibits good numerical performance. This formulation is shown to be well-posed in a reduced, linear setting. Numerical simulations are implemented and compared for all formulations. The new formulation is also applied to some other related problems. We investigate numerically the linear stability of the travelling wave solutions for the quarter loop problem and a simple grain boundary motion problem for both curves in two dimensions and surfaces in three dimensions. The numerical results give evidence that they are convectively stable. A class of high order three-phase boundary motion problems are also studied. We consider a region where three phase boundaries meet at a triple junction and evolve with specified normal velocities. A system of partial differential algebraic equations (PDAE) is proposed to describe this class of problems by extending the discussion for the coupled surface and grain boundary motion. The linear well-posedness of the system is analyzed and numerical simulations are performed.

No description available.

No description available.

This work studies the current collected by the spherical Langmuir probes to be mounted on the ESA Swarm satellites in order to quantify deviations from idealized cases caused by non-ideal probe geometry. The finite-element particle-in-cell code SPIS is used to model the current collection of a realistic probe, including the support structures, for two ionospheric plasma conditions with and without drift velocity. SPIS simulations are verified by comparing simulations of an ideal sphere at rest to previous numerical results by Laframboise parametrized to sufficient accuracy. It is found that for probe potentials much above the equivalent electron temperature, the deviations from ideal geometry decrease the current by up to 25 % compared to the ideal sphere case and thus must be corrected if data from this part of the probe curve has to be used for plasma density derivations. In comparison to the non-drifting case, including a plasma ram flow increases the current for probe potentials around and below the equivalent ion energy, as the contribution of the ions to the shielding is reduced by their high flow energy.

Fluid velocities and Brownian effects at nanoscales in the near-wall region of microchannels can be experimentally measured in an image plane parallel to the wall using, for example, evanescent wave illumination technique combined with particle image velocimetry [R. Sadr extit{et al.}, J. Fluid. Mech. 506, 357-367 (2004)]. The depth of field of this technique being difficult to modify, reconstruction of the out-of-plane dependence of the in-plane velocity profile remains extremely challenging. Tracer particles are not only carried by the flow, but they undergo random fluctuation imposed by the proximity of the wall. We study such a system under a particle based stochastic approach (Langevin) and a probabilistic approach (Fokker-Planck). The Langevin description leads to a coupled system of stochastic differential equations. Because the simulated data will be used to test a statistical hypothesis, we pay particular attention to the strong order of convergence of the scheme developing an appropriate Milstein scheme of strong order of convergence 1. Based on the probability density function of mean in-plane displacements, a statistical solution to the problem of the reconstruction of the out-of-plane dependence of the velocity profile is proposed. We developed a maximum likelihood algorithm which determines the most likely values for the velocity profile based on simulated perfect particle position, simulated perfect mean displacements and simulated observed mean displacements. Effects of Brownian motion on the approximation of the mean displacements are briefly discussed. A matched particle is a particle that starts and ends in the same image window after a measurement time. AS soon as the computation and observation domain are not the same, the distribution of the out-of-plane distances sampled by matched particles during the measurement time is not uniform. The combination of a forward and a backward solution of the one dimensional Fokker-Planck equation is used to determine this probability density function. The non-uniformity of the resulting distribution is believed to induce a bias in the determination of slip length and is quantified for relevant experimental parameters.

According to the previous experimental fact that an interdigitated single PEMFC has a better performance than other flow type single PEMFC, therefore this research is aimed to predict a two-cell stack interdigitated PEMFC via a numerical simulation. Investigation the effects of the cell temperature, the cell operating pressure, the fuel flow rate and the air flow rate are performed. This research can provide design reference for application of interdigitated PEMFC stack.

Current high power LEDs are used popularly, energy saving can be achieved if the heat transfer performance of a high power LED is increased. Numerical analysis is carried out herein to study the parameters effect on the cooling performance for the heat sinks of the LED projector. The parameters include fin spacing, fin depth, fin thickness, base thickness and flow speed. The numerical results reveal that the parameters of the heat sinks significantly affects the average Nusselt Number. The results of this study can provide design references for LED projector.

The power supply for electronic devises is demanded to be lighter and smaller in nowadays market. Therefore, the cooling problem becomes the major design challenge due to reduced heat transfer area. In this thesis, a numerical computation method is employed to numerically simulate the natural convection heat transfer field for a small power supply placed on the ground or table in atmospheric conditions. The effects of parameters are studied including internal heat sink structure, shell structure, heat rate of generation, body size and ground material. The results of the present study can provide design reference.