Evaporation enhancement for evaporative cooling systems

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Cooling is of critical importance for a wide range of fields. Presently, there are two fields where cooling is essential for realizing performance and growth potential: (1) microelectronic devices, which have components that are continually decreasing in size while facing an increase in power demands and corresponding need for heat removal, and (2) air conditioning for residential and commercial spaces, which is correlated with productivity and energy demand. There are relatively limited methods for cooling, most of which require refrigerants that can damage the environment. Current technology is not keeping pace with the rising cooling demands so there is a need for efficient and environmentally friendly technology. We take our inspiration for efficient cooling from nature, where human perspiration serves as an example of using evaporating droplets for cooling. Evaporation can provide very efficient cooling by exploiting the large amount of energy dissipated during a liquid to vapour phase change. Sessile droplets offer advantages over film and pool boiling due to their high surface area to volume ratios, which are beneficial for interfacial processes such as evaporation. There is great potential for evaporative cooling technology using sessile droplets; however, there is still an inadequate understanding of the evaporation process and the role of interfacial effects. Better understanding of the influence of interfacial effects on evaporation will lead to improved performance and efficiency, which is especially crucial as technology decreases in size to the micro- and nano-scales where interfacial effects become more important. This research program investigates an interfacial effect known as Marangoni convection, which is caused by an imbalance of surface tension forces at a liquid-vapour interface, and results in fluid flow within a droplet. In certain circumstances Marangoni convection has been shown to substantially increase evaporation rates and a majority of the energy required for evaporation is transported by Marangoni convection along the surface of a droplet. The enhanced evaporation rates can be used to improve cooling technology performance; however, this behaviour is still poorly understood and fluctuates based on the properties of the droplet (size, chemical mixture) and the substrate material. The objectives of this research are to: (1) quantify the influence of Marangoni convection on evaporation rates using experimental techniques for various sessile droplet sizes, fluid mixtures, and substrate materials, (2) develop and validate mathematical and numerical models of the evaporation process, which include the effects of Marangoni convection as a method for increasing the evaporation rates, and (3) apply the findings to develop energy efficient evaporative cooling technology by generating designs and performing simulations with our models. This research program aims to develop energy efficient, high capacity, and environmentally friendly evaporative cooling technology. Cooling living and working spaces using current technology places high demands on energy requirements, particularly in developing countries, but also in warm and humid Canadian summers. Cooling in electronic devices presents an opportunity for new technology to enter the market since power density and performance are limited by the overheating of components. We will generate an understanding and quantification of the influence of Marangoni convection on evaporation rates in sessile droplets, which will be valuable for a broad range of applications. This research will produce highly qualified personnel who are proficient in the thermal management and energy fields, which are significant for many expanding Canadian industries.