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.