Upon depressurization, confined liquid undergoes discharge through cracks/nozzles accompanied by complex phase change phenomena, which govern the safety and/or performance of fluidic systems in bioengineering, chemical engineering, energy/power, and aerospace applications. In spite of the extensive studies on phase-change and dynamics of liquids in an open system (e.g., droplets) or large-scale loop (e.g., nuclear reactors), limited studies have been devoted to a confined liquid system with small apertures. Particularly, the recent development of micro nuclear reactors and microfluidics in space has necessitated the fundamental understanding of the fluid discharge dynamics mediated by mini-channels/nozzles undergoing a rapid depressurization process. Here, we study the discharge charact...[ Read more ]

Upon depressurization, confined liquid undergoes discharge through cracks/nozzles accompanied by complex phase change phenomena, which govern the safety and/or performance of fluidic systems in bioengineering, chemical engineering, energy/power, and aerospace applications. In spite of the extensive studies on phase-change and dynamics of liquids in an open system (e.g., droplets) or large-scale loop (e.g., nuclear reactors), limited studies have been devoted to a confined liquid system with small apertures. Particularly, the recent development of micro nuclear reactors and microfluidics in space has necessitated the fundamental understanding of the fluid discharge dynamics mediated by mini-channels/nozzles undergoing a rapid depressurization process. Here, we study the discharge characteristics of fast-depressurized fluidic systems by exposing a water reservoir having mini-channels/nozzles to a vacuum chamber. Through high-speed optical and thermal imaging combined with thermodynamic measurements, we demonstrate the mini-channel-mediated fluid discharge is characterized by the dramatic flashing of the bulk liquid and two-phase spray, followed by steady evaporation and cooling of the bulk liquid until freezing. Interestingly, icing induced by evaporative cooling of the retained fluid may lead to blocking of the mini-channels (or termed self-healing of the discharge system), thus periodically interrupting the discharge and flashing process. By regulating the nozzle/channel sizes (inner diameters of 0.7 mm to 1.6 mm), we show that a finer mini-channel tends to suppress flashing, reduce evaporation, and prevent/delay freezing. Furthermore, we demonstrate that the discharge behavior is highly dependent on the initial temperature of the confined water, and a higher initial temperature (e.g., 50ºC) results in extended and faster flashing/two-phase-flow spray. Our study not only presents a physical picture of the phase change phenomena under depressurization, but also provides guidelines for the design of robust and high-performance fluidic systems for aerospace applications among others.