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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

A Theoretical and Experimental Study of the Evolution of Surface Ice

Unduche, Fisaha Solomon 24 January 2012 (has links)
The theory of the formation of surface ice is not fully understood, yet its presence on water surface affects the operation of many different kinds of hydraulic structures. It is known that when the water surface temperature reaches supercooling ice particles are formed at the water surface. If the intensity of turbulence is low these ice particles will be suspended on the surface forming a surface ice. Herein, an experimental study that incorporates the effect of depth, velocity, roughness and air temperature is carried out in a counter-rotating flume to investigate the formation of surface ice in relation to these parameters. The experimental result is supported by a mathematical model based on the theory of formation of surface ice. A mathematical model is developed based on a comprehensive theory and is solved using Matlab. The mathematical model integrates the effect of the heat balance on the water surface with the degree of turbulent mixing and the rate of growth of surface ice. The two calibrating parameters for the model are the magnitude of the initial seeding and the surface heat loss coefficient. The developed mathematical model is calibrated for the different types of surface ice formations for the data obtained from the experiment. The experimental analysis shows that there are four main types of surface ice formations and their formation is mainly dependent on the degree of turbulence. It is also found that the types of these ice formations are dependent on the minimum supercooling temperature at an average depth. Moreover, it is demonstrated that skim ice particles are those ice particles that nucleate on the water surface during low to medium turbulent intensities and can have four different types of shapes, namely needle shapes, finger shapes, hexagonal shapes and irregular shapes. The sizes and relative quantities of these ice particles on the water surface are also dependent on the degree of turbulence.
2

A Theoretical and Experimental Study of the Evolution of Surface Ice

Unduche, Fisaha Solomon 24 January 2012 (has links)
The theory of the formation of surface ice is not fully understood, yet its presence on water surface affects the operation of many different kinds of hydraulic structures. It is known that when the water surface temperature reaches supercooling ice particles are formed at the water surface. If the intensity of turbulence is low these ice particles will be suspended on the surface forming a surface ice. Herein, an experimental study that incorporates the effect of depth, velocity, roughness and air temperature is carried out in a counter-rotating flume to investigate the formation of surface ice in relation to these parameters. The experimental result is supported by a mathematical model based on the theory of formation of surface ice. A mathematical model is developed based on a comprehensive theory and is solved using Matlab. The mathematical model integrates the effect of the heat balance on the water surface with the degree of turbulent mixing and the rate of growth of surface ice. The two calibrating parameters for the model are the magnitude of the initial seeding and the surface heat loss coefficient. The developed mathematical model is calibrated for the different types of surface ice formations for the data obtained from the experiment. The experimental analysis shows that there are four main types of surface ice formations and their formation is mainly dependent on the degree of turbulence. It is also found that the types of these ice formations are dependent on the minimum supercooling temperature at an average depth. Moreover, it is demonstrated that skim ice particles are those ice particles that nucleate on the water surface during low to medium turbulent intensities and can have four different types of shapes, namely needle shapes, finger shapes, hexagonal shapes and irregular shapes. The sizes and relative quantities of these ice particles on the water surface are also dependent on the degree of turbulence.
3

Design of multifunctional materials with controlled wetting and adhesion properties

Chanda, Jagannath 24 March 2016 (has links)
Ice accretion on various surfaces can cause destructive effect of our lives, from cars, aircrafts, to infrastructure, power line, cooling and transportation systems. There are plenty of methods to overcome the icing problems including electrical, thermal and mechanical process to remove already accumulated ice on the surfaces and to reduce the risk of further operation. But all these process required substantial amount of energy and high cost of operation. To save the global energy and to improvement the safety issue in many infrastructure and transportation systems we have to introduce some passive anti-icing coating known as ice-phobic coating to reduce the ice-formation and ice adhesion onto the surface. Ice-phobic coatings mostly devoted to utilizing lotus-leaf-inspired superhydrophobic coatings. These surfaces show promising behavior due to the low contact area between the impacting water droplets and the surface. In this present study we investigate systematically the influence of chemical composition and functionality as well as structure of surfaces on wetting properties and later on icing behavior of surfaces. Robust anti-icing coating has been prepared by using modified silica particles as a particles film. Polymer brushes were synthesized on flat, particle surfaces by using Surface initiated ATRP. We have also investigated the effect of anti-icing behavior on the surfaces by varying surface chemistry and textures by using different sizes of particles. This approach is based on the reducing ice accumulation on the surfaces by reducing contact angle hysteresis. This is achieved by introducing nano to micro structured rough surfaces with varying surface chemistry on different substrates. Freezing and melting dynamics of water has been investigated on different surfaces by water vapour condensation in a high humidity (80%) condition ranging from super hydrophilic to super hydrophobic surfaces below the freezing point of water. Kinetics of frost formation and ice adhesion strength measurements were also performed for all samples. All these experiments were carried out in a custom humidity and temperature controlled chamber. We prepared a superhydrophobic surface by using Poly dimethyl siloxane (PDMS) modified fumed silica which display very low ice-adhesion strength almost 10 times lower than the unmodified surface. Also it has self-cleaning behavior after melting of ice since whole ice layer was folded out from the surface to remove the ice during melting. Systematic investigation of the effect of three parameters as surface energy, surface textures (structure, geometry and roughness) and mechanical properties of polymers (soft and stiff) on icing behavior has also been reported.
4

Experimental and computational investigation into light scattering by atmospheric ice crystals

Collier, Christopher Thomas January 2015 (has links)
An investigation was carried out into light scattering by Gaussian rough ice crystals. Gaussian rough crystal geometries were generated using roughness parameters derived from mineral dust grains, which have been reported to be suitable proxies for rough ice crystals. Light scattering data for these geometries was computed using the discrete dipole approximation (DDA) method. Phase functions, 2D scattering patterns, degree of linear polarisation patterns and asymmetry parameters were computed for smooth, moderately rough and highly rough crystals with a variety of orientations and size parameters. A sodium fluorosilicate ice analogue crystal with three partially roughened prism facets was created using focused ion beam (FIB) milling and 2D scattering patterns were collected from it using the small ice detector (SID) 3 cloud probe. It was found that roughness reduces features in the phase function compared to scattering by smooth hexagonal prisms, particularly when the roughness features were horizontally much larger than the wavelength. However, the most effective roughness model also takes account of horizontal features whose size is closer to that of the wavelength. Horizontal features smaller than the wavelength have very little effect.

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