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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

From Dynamical Superhydrophobicity to Thermal Diodes

Boreyko, Jonathan January 2012 (has links)
<p>The interaction between liquid drops and textured surfaces not only offers fundamental challenges in capillarity and wetting, but also enables new applications ranging from self-cleaning materials to self-sustaining condensers. The first part of this dissertation deals with the fundamental wetting and dewetting dynamics of drops on textured surfaces, and the self-propelled jumping of dropwise condensate on superhydrophobic surfaces. The second part builds upon these findings in dynamical superhydrophobicity to develop a jumping-drop thermal diode that rectifies heat flow between textured superhydrophilic and superhydrophobic surfaces. </p><p>On the fundamental side, anti-dew is an essential property of robust superhydrophobic surfaces, particularly those deployed in ambient environments or phase-change systems. A superhydrophobic lotus leaf retains water repellency after repeated condensation in nature but becomes sticky to water drops after condensation on a fixed cold plate. To solve this mystery, we first study the possible wetting states of superhydrophobic surfaces possessing two-tier surface roughness mimicking that on the lotus leaf. By incrementally increasing the ethanol concentration of water/ethanol drops, two distinct wetting transitions are observed on two-tier surfaces. Drops in the intermediate wetting state uniformly wet the microscale roughness but not the nanoscale roughness. Dew drops exhibited a similar intermediate wetting state. Our experiments show that mechanical vibration can be used to overcome the energy barrier for transition from the intermediate wetting (Partial Wenzel) state to the fully dewetted (Cassie) state, and the threshold for the dewetting transition follows a scaling law comparing the kinetic energy imparted to the drop with the work of adhesion. </p><p>Although vibration-induced dewetting is effective for removing millimetric condensate from the surface, micrometric condensate cannot be removed as surface energy dominates at small scales. We report a new discovery in which the micrometric condensate can spontaneously dewet and jump off the superhydrophobic surface. The spontaneous jumping results from the surface energy released upon drop coalescence, which leads to the rapid out-of-plane jumping motion of the coalesced drops. The jumping drops follow an inertial-capillary scaling and give rise to self-sustained dropwise condensation with a micrometric average diameter. Using two approaching Leidenfrost drops suspended on a vapor layer to simulate superhydrophobicity, we show that the out-of-plane directionality results from the impingement of the expanding liquid bridge against the heated Leidenfrost surface, which is initially formed between coalescing drops above the substrate.</p><p>On the practical side, textured surfaces offer new possibilities for phase-change heat transfer. Taking advantage of the self-propelled jumping condensate, we developed a planar phase-change thermal diode that transports heat in a preferential direction. The jumping-drop diode is composed of parallel superhydrophobic and superhydrophilic plates, and the thermal rectification is enabled by spontaneously jumping dropwise condensate which only occurs when the superhydrophobic surface is colder. The superhydrophobic surface has nanoscale surface roughness that is anti-dew, while the superhydrophilic surface consists of porous copper wick borrowed from heat pipes. Our planar thermal diode with asymmetric wettability is scalable to large areas with an orientation-independent diodicity of over a hundred. </p><p>More broadly speaking, the self-propelled jumping offers an alternative means to return liquid condensate in phase-change systems. We systematically investigate the heat transfer performance of a vapor chamber enabled by the jumping condensate. When the non-condensable gases are removed, the effective heat transfer coefficient is mainly governed by the interfacial resistance of the phase-change processes and the conduction resistance across the superhydrophilic wick. Potential routes for improving the heat transfer performance are discussed, including the optimization of the superhydrophilic wick and its separation with the opposing superhydrophobic surface. The new jumping return mechanism is unique in that it neither relies on external forces nor requires wick structures along the return path, and is expected to be applicable to a variety of phase-change heat transfer systems.</p> / Dissertation
2

Atomistic to Continuum Multiscale and Multiphysics Simulation of NiTi Shape Memory Alloy

Gur, Sourav, Gur, Sourav January 2017 (has links)
Shape memory alloys (SMAs) are materials that show reversible, thermo-elastic, diffusionless, displacive (solid to solid) phase transformation, due to the application of temperature and/ or stress (/strain). Among different SMAs, NiTi is a popular one. NiTi shows reversible phase transformation, the shape memory effect (SME), where irreversible deformations are recovered upon heating, and superelasticity (SE), where large strains imposed at high enough temperatures are fully recovered. Phase transformation process in NiTi SMA is a very complex process that involves the competition between developed internal strain and phonon dispersion instability. In NiTi SMA, phase transformation occurs over a wide range of temperature and/ or stress (strain) which involves, evolution of different crystalline phases (cubic austenite i.e. B2, different monoclinic variant of martensite i.e. B19', and orthorhombic B19 or BCO structures). Further, it is observed from experimental and computational studies that the evolution kinetics and growth rate of different phases in NiTi SMA vary significantly over a wide spectrum of spatio-temporal scales, especially with length scales. At nano-meter length scale, phase transformation temperatures, critical transformation stress (or strain) and phase fraction evolution change significantly with sample or simulation cell size and grain size. Even, below a critical length scale, the phase transformation process stops. All these aspects make NiTi SMA very interesting to the science and engineering research community and in this context, the present focuses on the following aspects. At first this study address the stability, evolution and growth kinetics of different phases (B2 and variants of B19'), at different length scales, starting from the atomic level and ending at the continuum macroscopic level. The effects of simulation cell size, grain size, and presence of free surface and grain boundary on the phase transformation process (transformation temperature, phase fraction evolution kinetics due to temperature) are also demonstrated herein. Next, to couple and transfer the statistical information of length scale dependent phase transformation process, multiscale/ multiphysics methods are used. Here, the computational difficulty from the fact that the representative governing equations (i.e. different sub-methods such as molecular dynamics simulations, phase field simulations and continuum level constitutive/ material models) are only valid or can be implemented over a range of spatiotemporal scales. Therefore, in the present study, a wavelet based multiscale coupling method is used, where simulation results (phase fraction evolution kinetics) from different sub-methods are linked via concurrent multiscale coupling fashion. Finally, these multiscale/ multiphysics simulation results are used to develop/ modify the macro/ continuum scale thermo-mechanical constitutive relations for NiTi SMA. Finally, the improved material model is used to model new devices, such as thermal diodes and smart dampers.
3

[en] A STUDY ON THERMAL CONDUCTION AND RECTIFICATION / [pt] UM ESTUDO SOBRE CONDUÇÃO E RETIFICAÇÃO TÉRMICA

ALEXANDRE AUGUSTO ABREU ALMEIDA 02 July 2021 (has links)
[pt] É um resultado conhecido na literatura que uma cadeia unidimensional de partículas, que interagem harmonicamente com seus primeiros vizinhos, não conduz calor, e forças não lineares são necessárias para reproduzir a lei de Fourier da condução de calor. Quando são introduzidas assimetrias em tal sistema condutor, se obtém um efeito retificador onde a corrente térmica apresenta magnitudes diferentes dependendo de qual lado da cadeia tem maior temperatura, tais dispositivos sendo chamados de diodos térmicos. Neste trabalho estudamos os dois fenômenos, condução de calor e retificação térmica, em uma cadeia unidimensional de partículas, com condições de contorno fixas, acopladas a dois banhos térmicos, um em cada extremidade, modelados como termostatos de Langevin. As partículas interagem com seus primeiros vizinhos harmonicamente e estão sujeitas a um potencial localizado externo não linear, para o qual estudamos dois tipos, os potenciais Frenkel-Kontorova e Ø elevado a 4. Verificamos que a lei de Fourier é observada, para ambos os casos, com o perfil de temperatura e a condutividade térmica dependendo da relação entre as amplitudes harmônica e anarmônica, e a temperatura média do sistema. Em seguida, para criar uma assimetria na cadeia, nós acoplamos dois segmentos de mesmo tamanho. Observamos um efeito retificador onde a direção preferencial difere para cada potencial localizado estudado. A forma como as temperaturas dos banhos térmicos mudam a magnitude da retificação também foi observada. Nós também investigamos o efeito de não linearidades interfaciais, por meio de uma lei de potência que acopla segmentos Ø elevado a 4. Alterando o expoente da lei de potência, nós buscamos as condições sob as quais a retificação ótima é atingida. / [en] It is a known result in the literature that a one-dimensional chain of particles that interact harmonically with its first neighbors does not conduct heat, and nonlinear forces are needed to reproduce Fourier s law of heat conduction. When asymmetries are introduced in such a conducting system, a rectifying effect is obtained where the thermal current shows different magnitudes depending on which side of the chain has higher temperature, such devices being called thermal diodes. In this work we study both phenomena, heat conduction and thermal rectification, in a onedimensional chain of particles, with fixed boundary conditions, coupled to two thermal baths, one at each end, modeled as Langevin thermostats. The particles interact with their first neighbors harmonically and have a nonlinear on-site potential, for which we study two types, Frenkel-Kontorova and Ø 4 potentials. We verify that, for both cases, Fourier s law is observed, where the temperature profile and the thermal conductivity are dependent on the relation between the harmonic and anharmonic amplitudes, and the system s average temperature. Next, to create an asymmetry in the chain, we coupled two different segments of equal lengths. We observed a rectifying effect, where the preferential direction differs for each of the two on-site potentials studied. How the heat-bath temperatures changes the magnitude of rectification was also observed. We also investigated the effect of interfacial nonlinearities through a power-law potential, coupling Ø 4 segments. By changing the power-law exponent, we looked for the conditions under which optimal rectification is achieved.

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