UCLA

Water scarcity is identified as one of the global issues by the United Nations. As per WHO,2.2 billion people lack access to safe drinking water. Over 2 billion people live in countries experiencing high water stress. Almost 80% of wastewater (UNESCO, 2017) flows back into the ecosystem without being treated or reused. Fresh water demand is expected to increase drastically in the upcoming decade. Fresh water production from unconventional water sources such as seawater, brackish water, and ultra-saline water becomes crucial to achieve self-sufficiency. Desalination provides a favorable solution for coastal places like California due to easy access to seawater. Thermal desalination is suitable for treating water with high salinity due to its robustness and can directly utilize renewable sources like solar energy to adhere to sustainability. This study developed and investigated a novel thermal desalination system which encompasses the primary objective of this work. The novel system combines dynamic flash evaporation, a pressure-driven phase change phenomenon to produce vapor from liquid, along with a vapor separation process initiated through tangential injection. The inlet feed water to be treated passes through injection tubes which are connected to injection passages installed tangentially onto a separator tube. Dynamic flashing is initiated by pressure drop due to friction and acceleration in the injection tubes creating a two-phase mixture. Subsequent tangential injection separates the two-phase mixture through centrifugal force. This approach offers a compact system with vapor production and separation processes occurring on the order of several milliseconds. Tap water and seawater were tested with the system. Performance parameters of thermal conversion efficiency to analyze vapor production efficacy and phase separation efficiency to evaluate the purity of the condensate were investigated. Single-stage system was able to achieve up to 98% for thermal and phase separation efficiencies. Further improvement in the purity of the condensate was achieved through a two-stage system, where the entrained droplets along with vapor captured from the first stage undergoes a second round of separation in a stage connected in series. This resulted in condensate with over 99.9% purity. With seawater of 2.5% salt concentration by mass, the condensate obtained achieved salt concentrations lower than 0.02% by mass comparable to that of potable water. With the goal of optimizing the system for varying operating conditions, the dynamic flashing in one of the injection tubes was studied. A visualization study was performed in the injection tubes. Pressure and temperature measurements along the tube were analyzed for different inlet flowrates and liquid temperatures. The pressure and temperature values showed increasing gradients along the tube indicating an increase in vapor production for increasing flowrates and liquid temperatures. The flow regime development due to the vapor production was tracked through high-speed imagery. Visual observations informed complex flow regimes for flashing flow with numerous bubble nucleations and growth throughout the tube. Bulk nucleation showed dominance over wall nucleation. Distinctive flow regimes were observed for tap water and saltwater. Volumetric void fraction measurements were performed using the capacitance impedance technique. Measurements showed an increasing void fraction along the tube supporting visual identification of the flow regimes. Variation of local superheat along the tube showed dependence on the flow regime. The results provide experimental data to aid modeling efforts on flashing flows.