The present thesis is a result of three year research study aimed at understanding the wettability effects on interface dynamics and phasechange. The project was performed in between University of Bergamo, University of Mons, CNR-ISTEC Institute of Science and Technology for Ceramics (Faenza, Italy), Politecnico di Milano and Italian Space Agency (ASI). The scope was to add a piece of the puzzle in understanding the basic physics knowledge on the wettability influence on: 1) pool boiling heat transfer; 2) liquid-surface interaction. Energy saving and research for the development of alternative energy sources are the major concerns of the twenty-first century. The solution is twofold: - dependence on traditional energy sources should be eliminated and it serves to found renewable energy sources; - serves to determine innovative methods to increase the efficiency of the current energy systems in order to save conventional energy resources, increasing their sustainability. Researches shown that the miniaturization in the production of electromechanical systems is a key factor in the development of energy-efficient systems. The requests, for example, of electronic devices, such as smartphones, tablet and laptop, more and more small and thin, but with increasingly high performance, lead to a denser associated electronics packaging, in which the heat generated per unit area is increased. Heat that must be removed and that is dissipated through the use of innovative heat exchange devices, such as microfluidic devices. The increase of the performance of these devices involves focusing on the heat exchange phenomena that occur inside these systems, and in particular on the boiling process, that is the evaporation at the solid-liquid interface when the surface temperature is higher than the temperature of the fluid in saturation condition. Research also show that the heat exchange performance of a surface is in function of its roughness and wettability. For this reason, pool boiling experiments were carried out with degassed water on stainless steel substrates with different surface topography and wettability. Boiling curves have been measured together with visual high-speed observations of the boiling process. The onset temperature of nucleate boiling has been measured and the influence of surface roughness and wettability has been quantified for different surfaces, with the aim to better understand the effect of superhydrophobicity on pool boiling. The original finding is that the boiling curve shape is rather different between hydrophilic and superhydrophobic case, keeping the same surface roughness: superhydrophobic surfaces show a peculiar behaviour similar to an early “quasi-Leidenfrost” regime for low superheat, i.e. once boiling is initiated, the boiling process immediately enters the film boiling regime. Also, for the roughness range analysed, the wettability has a predominant effect with respect to the roughness, when the contact angle exceeds the typical value for superhydrophobic surfaces (receding contact angle θrec > 135°). The theme of the energy saving also concerns the impact of drops on dry solid surfaces, since the process is involved in many industrial processes, i.e. spray cooling, ink-jet printing, spray painting, fuel injection, raindrop erosion. The investigation and the comprehension of the dynamics of a single drop impact is the first step to understand and control the liquid-solid interaction of more complex phenomena, such as the formation of fouling in the marine environment, the formation of ice on aircraft wings, the pressure drops in the ducts and the oxidation in the pipe-line. For this reason, a considerable part of the work has been then devoted to study experimentally, using a highspeed camera, the normal impact of water and hexadecane liquid drops onto dry, rigid surfaces with different wettabilities. The results highlighted that it is not possible to easily correlate contact angles (receding, advancing and hysteresis) and drop impact outputs on different surfaces. In order to explain the observed phenomena, physical and chemical characteristics of both the liquid drops and the surfaces have to be taken into account. Surface morphology is crucial in defining the critical velocity over which impalement occurs. Also surface chemistry is relevant, as dipolar interactions between surface functional groups and molecules in the liquid phase can favour impalement. As far as the liquid properties are concerned, with increasing viscosity and lower fluid surface tension the Cassie-to-Wenzel transition, i.e. impalement of the liquid meniscus into the texture, shifts to smaller Weber. These results emphasize how an accurate design of the surface properties must be pursued in the future research towards dynamically amphiphobic surfaces. Moreover, the construction of an experimental apparatus for the evaluation of the heat exchange coefficients of an oscillating interface is reported. This part of the work is included in the appendix as such apparatus has been designed and built, however no experiments were performed.

(2016). Wettability effects on interface dynamics and phase-change . Retrieved from http://hdl.handle.net/10446/222101

Wettability effects on interface dynamics and phase-change

MALAVASI, Ileana
2016

Abstract

The present thesis is a result of three year research study aimed at understanding the wettability effects on interface dynamics and phasechange. The project was performed in between University of Bergamo, University of Mons, CNR-ISTEC Institute of Science and Technology for Ceramics (Faenza, Italy), Politecnico di Milano and Italian Space Agency (ASI). The scope was to add a piece of the puzzle in understanding the basic physics knowledge on the wettability influence on: 1) pool boiling heat transfer; 2) liquid-surface interaction. Energy saving and research for the development of alternative energy sources are the major concerns of the twenty-first century. The solution is twofold: - dependence on traditional energy sources should be eliminated and it serves to found renewable energy sources; - serves to determine innovative methods to increase the efficiency of the current energy systems in order to save conventional energy resources, increasing their sustainability. Researches shown that the miniaturization in the production of electromechanical systems is a key factor in the development of energy-efficient systems. The requests, for example, of electronic devices, such as smartphones, tablet and laptop, more and more small and thin, but with increasingly high performance, lead to a denser associated electronics packaging, in which the heat generated per unit area is increased. Heat that must be removed and that is dissipated through the use of innovative heat exchange devices, such as microfluidic devices. The increase of the performance of these devices involves focusing on the heat exchange phenomena that occur inside these systems, and in particular on the boiling process, that is the evaporation at the solid-liquid interface when the surface temperature is higher than the temperature of the fluid in saturation condition. Research also show that the heat exchange performance of a surface is in function of its roughness and wettability. For this reason, pool boiling experiments were carried out with degassed water on stainless steel substrates with different surface topography and wettability. Boiling curves have been measured together with visual high-speed observations of the boiling process. The onset temperature of nucleate boiling has been measured and the influence of surface roughness and wettability has been quantified for different surfaces, with the aim to better understand the effect of superhydrophobicity on pool boiling. The original finding is that the boiling curve shape is rather different between hydrophilic and superhydrophobic case, keeping the same surface roughness: superhydrophobic surfaces show a peculiar behaviour similar to an early “quasi-Leidenfrost” regime for low superheat, i.e. once boiling is initiated, the boiling process immediately enters the film boiling regime. Also, for the roughness range analysed, the wettability has a predominant effect with respect to the roughness, when the contact angle exceeds the typical value for superhydrophobic surfaces (receding contact angle θrec > 135°). The theme of the energy saving also concerns the impact of drops on dry solid surfaces, since the process is involved in many industrial processes, i.e. spray cooling, ink-jet printing, spray painting, fuel injection, raindrop erosion. The investigation and the comprehension of the dynamics of a single drop impact is the first step to understand and control the liquid-solid interaction of more complex phenomena, such as the formation of fouling in the marine environment, the formation of ice on aircraft wings, the pressure drops in the ducts and the oxidation in the pipe-line. For this reason, a considerable part of the work has been then devoted to study experimentally, using a highspeed camera, the normal impact of water and hexadecane liquid drops onto dry, rigid surfaces with different wettabilities. The results highlighted that it is not possible to easily correlate contact angles (receding, advancing and hysteresis) and drop impact outputs on different surfaces. In order to explain the observed phenomena, physical and chemical characteristics of both the liquid drops and the surfaces have to be taken into account. Surface morphology is crucial in defining the critical velocity over which impalement occurs. Also surface chemistry is relevant, as dipolar interactions between surface functional groups and molecules in the liquid phase can favour impalement. As far as the liquid properties are concerned, with increasing viscosity and lower fluid surface tension the Cassie-to-Wenzel transition, i.e. impalement of the liquid meniscus into the texture, shifts to smaller Weber. These results emphasize how an accurate design of the surface properties must be pursued in the future research towards dynamically amphiphobic surfaces. Moreover, the construction of an experimental apparatus for the evaluation of the heat exchange coefficients of an oscillating interface is reported. This part of the work is included in the appendix as such apparatus has been designed and built, however no experiments were performed.
28
2014/2015
TECNOLOGIE PER L'ENERGIA E L'AMBIENTE
MARENGO, Marco
Malavasi, Ileana
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