Graduate Thesis Or Dissertation
 

Thermal Actuation and Atomization for Desalination Systems

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/vx021p034

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  • Desalination stands for removing salt from saline water, and produce freshwater. Desalination technologies have been envisioned as a potential solution for freshwater production from oceans as an immense source of water acquiring 97% earth’s water, and 75% of earth’s surface. However, one third of world’s population face water scarcity every year due to high cost of energy, and large volumes of concentrated brine discharge of conventional desalination technologies. In this regard, a novel desalination technology is developed at Water and Energy Technologies Laboratory providing a dual solution for high energy cost, and large discharge: STEWARD (Solar Thermal Extraction of Water by Atomization and Recuperative Desalination). It uses cheap, and clean low-grade heat from abundantly available solar energy or industrial waste heat to operate the cycle, and address high cost of energy; air humidification-dehumidification (HDH) to produce freshwater, and dry salt, ensuring zero liquid discharge. Along with these features, STEWARD is designed to be a modular, scalable, and portable desalination technology. This dissertation articulates a systematic study of development of this technology, and its underlying science. A dual-fluid thermal actuation system was developed for driving and heating, air and saline streams simultaneously using low-grade heat, with efficiencies comparable to an electrically actuated heating system. The dual-fluid thermal actuation system was operated for 118℃ boiler temperature producing discharge rates of 209-277 ml/min, discharge pressures from 139-172 kPa, and overall system efficiencies of 32-56% for 1 kW, and for 4-12 s cycle time. Operating system for smaller cycle time reduced system size to one fourth of the size of a conventional thermally actuated water heating system. Next, a perforated plate airblast atomizer was developed for desalination systems to facilitate enhanced atomization, excellent evaporation, and complete suppression of fouling at high temperatures, and salinities. Effects of atomizer design, water mass flow rate, and air mass flux were assessed on spray characteristics to map its performance over 18-31ᵒ spray cone angle, and 237-643 µm droplet size. Empirical models were developed for spray cone angle, and average droplet diameter to optimize the atomizer design, and operating parameters to achieve desired spray characteristics. For fouling tests, a conventional plain-jet airblast atomizer was tested at 100℃ air, and 50℃ saline, which fouled severely (spray cone angle varying from 44ᵒ-57ᵒ for 35,000 ppm, and 44ᵒ-67ᵒ for 100,000 ppm over 2 hours); the perforated plate atomizer demonstrated a complete suppression of fouling (steady spray cone angle). Low-pressure region in the vicinity of air orifice due to Venturi suction of fast moving air in the outer layer of spray was identified as the root cause of fouling in conventional airblast atomizers (plain-jet, and pre-filming). Novel liquid-film airblast atomization mechanism as in perforated plate atomizer facilitates liquid annulus around the air-jet, and suppresses low-pressure regions around it. Complete suppression of fouling was observed up to 100,000 ppm salinity, and 175℃ air, and 65℃ saline over 14 hours of operation, against a maintenance time of 35 minutes for the conventional airblast atomization mechanisms. The liquid-film atomization can be adopted in conventional atomizers to completely eliminate fouling. This development brings the high evaporation airblast atomizers from jet engines to highly contaminated liquid atomization processes such as desalination, with improved performances. The dual-fluid thermal actuation can be used in existing off-grid technologies, heat storage, and green-buildings for its high efficiencies, and smaller size; the perforated plate atomizer/liquid-film atomization can be implemented in powder metallurgy, fuel spray, spray drying and cooling.
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