Abstract
Accidental releases of superheated liquids are featured by depressurisation across the outlet, leading to liquid flashing within the upstream container and the downstream jet. The resulting superheated liquid jet breaks up into massive droplets when released into the ambient with lower saturated pressure, driven by thermal nonequilibrium and the accompanying mechanical forces. Such a phenomenon facilitates fuel atomisation in energy utilisation while fostering beneficial conditions for fire, explosion, and toxic diffusion in the case of accidental release. The aim is to explore how thermal non-equilibrium and mechanical forces affect jet breakup and how the formed droplets evaporate and travel in the resulting sprays. Motivated by the aim, this project is divided into four parts based on experimental and theoretical studies.
An experimental tank with a value of 20 L was built to examine the depressurised releases of superheated liquids via a high-speed camera and phase Doppler anemometry. Because the upstream dynamics and thermodynamics determine the resulting jet, dynamic behaviours of in-tank subcooled liquid with phase change were first investigated throughout depressurised releases of superheated liquids. The proposed thermodynamic dimensionless number had a good capacity in characterising the in-tank variation. Subsequently, the main driving forces for flashing jet breakup, such as thermodynamic and mechanical effects, were clarified. Breakup-related dimensionless numbers were characterised. Further exploration elucidated the coupling between the two driving forces during depressurised releases and the quantitative relationship (f(Ja, ρv/ρl, Rp, ηp,Wev, Oh, Pr, Ec)) at the stable phase. Following that, the breakup regime of flashing jets and the corresponding jet characteristics were qualitatively and quantitatively revealed. Specifically, non-flashing, partially flashing, and fully flashing breakups coincided with RpJa(WevOh)1/7<41, 41≤RpJa(WevOh)1/7<223, and 223≤RpJa(WevOh)1/7, respectively. Finally, a liquid-gas coupling cell model was developed to explore the evaporation and motion of droplets formed by the breakup of flashing jets. The model predictions agreed well with the experimental results.
These findings have promoted the knowledge of the coupling breakup regime and characteristics of flashing jets at depressurised releases and the motion, heat transfer, and mass transfer of the resulting droplets. Such a more in-depth understanding can contribute to better preventing, controlling, and responding to unwanted release of superheated liquids, as well as design processes to generate desired sprays.