<p>Heat sinks have the capability of increasing operating heat flux limits for improved thermal management in the immersion cooling of electronics using dielectric fluids. However, even for arrays of simple, straight fins, the generation of vapor between and along fins during pool boiling lead to performance effects that are not well understood. Further investigation of the heat-flux-dependent variation of boiling modes that can manifest along the fin height is required. Although methods for the prediction of fin boiling heat transfer exist that incorporate a variable heat transfer coefficient determined from a flat surface, they have been developed and assessed for single, isolated fins under the assumption that the sides of the fin at any location behave like that of a flat surface. As a result, when applied to fin arrays, these methods may not always be accurate for the full range of heat flux operation along boiling curve up to the critical heat flux, due to the fins interfering with each other when arranged in arrays of differing spacing and height. To establish when the fins in an array can be described as isolated and having the flat surface boiling behavior, pool boiling experiments are performed using copper heat sinks in two fluids with vastly different properties: HFE-7100 and water. The spacing and height of the longitudinal fins are varied across a range from much larger to less than half of the scale of the capillary length scale of both fluids, <em>L</em>_<em>b</em>. High-speed visualizations enable the identification of different boiling regimes to identify correspondence between flow observations and the boiling performance, such as when there is bubble confinement from fin interference. Trends in the pool boiling data are also compared, noting changes in superheat at various heat fluxes to establish when fin height or spacing affects boiling behavior. The experimental boiling performance is compared to predictions developed assuming isolated fins so as to identify the spacings and heights for which the fin arrays follow this behavior. Overall, the data from both fluids strongly support a hypothesis that <em>L</em>_<em>b</em> is the key length scale. Heat transfer from fin array heat sinks with heights and spacings above <em>L</em>_<em>b</em> are shown to be accurately predicted in both fluids. However, spacings smaller than <em>L</em>_<em>b</em> lead to bubble confinement which affects the superheat, particularly at low heat fluxes, while heights shorter than <em>L</em>_<em>b</em> are unable to support multiple boiling regimes along the fin sidewall. This work identifies the capillary length as the key length scale at which confinement and height effects need to be considered for accurate predictions of immersion cooling applications.</p>

We introduce an elastic constitutive model of gravity that enables the interpretation of cosmological observations in terms of established ideas from Solid Mechanics and multiscale modeling. The behavior of physical space is identified with that of a material-like medium called "cosmic fabric," which exhibits constitutive behavior. This cosmic fabric is a solid hyperplate that is broad in the three ordinary spatial dimensions and thin in a fourth hyperspatial dimension. Matter in space is treated as fabric inclusions that prescribe in-plane (three-dimensional) strain causing the transverse bending of the fabric into the fourth hyperspatial dimension. The linearized Einstein-Hilbert action, which governs the dynamics of physical space, is derived from postulating Hooke’s Law for the fabric, and the Schwarzschild metric is recovered from investigating matterabric interactions. At the continuum length scale, the Principle of Relativity is shown to apply for both moving and stationary observers alike, so that the fabric’s rest reference frame remains observationally indistinguishable at such a length scale. Within the Cosmic Fabric paradigm, the structural properties of space at different hierarchical length scales can be investigated using theoretical notions and computational tools from solid mechanics to address outstanding problems in cosmology and fundamental physics. For example, we propose and offer theoretical support for the "Inherent Structure Hypothesis", which states that the gravitational anomalies currently attributed to dark matter may in fact be manifestations of the inherent (undeformed) curvature of space. In addition, we develop a numerical framework wherein one can perform numerical "experiments" to investigate the implications of said hypothesis.

This Thesis work focused on the acquisition and evaluation of data on mean velocity, turbulence intensity and length scales obtained via hot wire measurements in a high pressure single-stage land-based turbine available at the Department of Heat and Power Technology in KTH. Turbulence was generated by the use of grids and twoprobes, upstream and downstream of the turbine stage, were used. Different cases with different features were taken into consideration in this work: data were acquiredfor three different grids (I, II and III), two different blisks (5 and 6) and two operatingpoints. After post-processing all the information obtained from the anemometer, interesting comparisons between sub-cases could be made, and conclusions could bedrawn. In particular, it was found that Grid III generates a higher turbulence intensityupstream (around 13 %) with respect to Grid II (6 %) and I (3 %). Also, turbulenceintensity downstream did not seem to be affected by the conditions upstream butonly from the blisk type: turbulence levels were very similar when the same bliskwas mounted regardless of the type of grid upstream. Furthermore, the presence ofturbulence resulted in a slight decrease of the stage total-to-static efficiency ηT −S ofless than 0.5 % for Grid III while almost no change in efficiency was noticed when GridII or I were mounted. Furthermore, three different length scales were computed andanalyzed: Integral Length Scale, Taylor Microscale and Kolmogorov Microscale. Whilethe first two were found to be both in the order of 10−3 m, the last one -as expected- ismuch smaller, around 10−6 m. / Detta examensarbete fokuseradepå insamling och utvärdering av data om medelhastighet, turbulensintensitet och längdskalor erhållna via varmtrådsmätningar i ett högtryck turbinsteg tillgänglig påavdelningen för Kraft- och värmeteknologi på KTH. Turbulens genererades medelstanvändningen av olika perforerade plåtar i inloppet och två sonder, uppströms och nedströms om turbinsteget, användes för mätningar. Olika driftpunkter ochkonfigurationer med olika egenskaper togs i beaktande i detta arbete: data inhämtadesför tre olika inloppturbulensfall, Grid (I, II och III), två olika rotorbliskar (5 och 6)och två driftpunkter. Efter att ha efterbehandlat all information som erhållits frånmätningarna, kan intressanta jämförelser mellan delfall göras och slutsatser dras. I synnerhet fann man att Grid III genererar en högre turbulensintensitet uppströms (cirka 13 %) med avseende på Grid II (6 %) och I (3 %). Dessutom verkar turbulensintensitet nedströms inte påverkas av förhållandena uppströms men endast av typ avrotorblisk: turbulensnivåerna nedströms var mycket lika när samma blisk monteradesoavsett typ av galler uppströms. Dessutom resulterade förekomsten av turbulens i enliten minskning av stegets total-till-statiska verkningsgrad ηT−S med mindre än 0,5 %-enheter för Grid III medan nästan ingen förändring i verkningsgrad märktes när GridII eller I var monterade. Vidare beräknades tre olika längdskalor och analyserades: Integral Length Scale, Taylor Microscale och Kolmogorov Microscale. Medan de två första visade sig vara båda i storleksordningen 10−3 m, är den sista, som förväntat, är mycket mindre, cirka 10−6m.

Beams are among the most commonly used structural members that are encountered in virtually all systems of structural design at various scales. Mathematical models used to determine the response of beams under external loads are deduced from the three-dimensional elasticity theory through a series of assumptions concerning the kinematics of deformation and constitutive behavior. The kinematic assumptions exploit the fact that such structures do not experience significant trans- verse normal and shear strains and stresses. For example, the solution of the three- dimensional elasticity problem associated with a straight beam is reformulated as a one-dimensional problem in terms of displacements whose form is presumed on the basis of an educated guess concerning the nature of the deformation. In many cases beam structures are subjected to compressive in-plane loads that may cause out-of-plane buckling of the beam. Typically, before buckling and during compression, the beam develops internal axial force that makes the beam stiffer. In the linear buckling analysis of beams, this internal force is not considered. As a result the buckling loads predicted by the linear analysis are not accurate. The present study is motivated by lack of suitable theory and analysis that considers the nonlinear effects on the buckling response of beams. This thesis contains three new developments: (1) the conventional beam theories are generalized by accounting for nonlinear terms arising from εzz and εxz that are of the same magnitude as the von K´arm´an nonlinear strains appearing in εxx. The equations of motion associated with the generalized Euler–Bernoulli and Timoshenko beam theories with the von K´arm´an type geometric nonlinear strains are derived using Hamilton’s principle. These equations form the basis of investigations to determine certain microstructural length scales on the bending, vibration and buckling response of beams used in micro- and nano-devices. (2) Analytical solutions of the conventional Timoshenko beam theory with the von K´arm´an nonlinearity are de- veloped for the case where the inplane inertia is negligible when compared to other terms in the equations of motion. Numerical results are presented to bring out the effect of transverse shear deformation on the buckling response. (3) The development of a nonlinear finite element model for post-buckling behavior of beams.

No description available.

Although a number of definitions of mixing have been proposed in the literature, no single definition accurately and clearly describes the full range of problems in the field of industrial mixing. Based on the review of mixing and segregation characterization techniques in chemical engineering, spatial statistics and population studies, a definition of industrial mixing is proposed in this thesis, based on three separate dimensions of segregation. The first dimension is the intensity of segregation which quantifies the uniformity of concentration; the second dimension is the scale of segregation or clustering; and the last dimension is the exposure or the potential to reduce segregation. The first dimension focuses on the instantaneous concentration variance; the second on the instantaneous length scales in the mixing field; and the third on the driving force for change, i.e. the mixing time scale, or the instantaneous rate of reduction in segregation. The definition is introduced using concepts, theory and mathematical equations. This definition provides a theoretical framework for the rigorous analysis of mixing problems, encompassing all industrial mixing processes and allowing a clear evaluation of experimental methods. In this work, the three dimensions of segregation are presented and defined in the context of previous definitions of mixing, and then applied to a range of industrial mixing problems to test their accuracy and robustness. Suitable quantities for direct measurement of the dimensions of segregation are then investigated in detail. The result is a toolkit of ready-to-use methods for the measurement of the intensity (CoV) and the scale of segregation (maximum striation thickness on a transect, point-to-nearest neighbour distributions and variogram), provided as Matlab codes. The chosen methods are thoroughly investigated by testing their applicability, limitations, sampling strategies and meaningfulness of the results using selected sets of mixing data, resulting in creation of guidelines for the use of each of the provided methods. The developed definition of mixing, together with tools and guidelines for measurement of mixing will help researches to further develop the field of mixing, engineers to solve practical industrial mixing problems, and instructors of chemical engineering courses to introduce mixing concepts more easily. / Chemical Engineering

No description available.