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Countercurrent flow limitation (CCFL) refers to an important class of gravity-induced hydrodynamic processes that impose a serious restriction on the operation of gas–liquid two-phase systems. In a nuclear power plant, CCFL may occur in the liquid level measurement system where an orifice is applied in the pipeline, which may introduce error into the level measurement system. CCFL can occur in horizontal, vertical, inclined, and even much more complicated geometric patterns, and the hot-leg channel flow passage has been widely investigated; however, a pipeline with variable cross-sections, including an orifice, has not yet been investigated. An experimental investigation has been conducted in order to identify the phenomenon, pattern, and mechanism of CCFL onset in this type of geometry. Both visual and quantified experiments were carried out. A high-speed camera was applied to capture the flow pattern. Visual experiments were implemented at atmospheric pressure, while quantified pressurizer experiments were implemented at higher pressures. It was determined that if the condensate drainage is low and the liquid level is also low, with a stable stratified flow upstream of the orifice, there is no oscillation of the differential pressure. However, at higher condensate drainage levels, when the liquid level increases, a stratified wavy flow occurs. One of these waves can suddenly rise upstream of the orifice to choke it, which subsequently gives rise to differential pressure across the orifice, with periodic variation. This pattern alternately features stratified flow, stratified wavy flow, and slug flow, which indicates the occurrence of CCFL. The CCFL occurring under these experimental conditions can be expressed as a Wallis type correlation, where the coefficients m and C are 0.682 and 0.601, respectively. [ABSTRACT FROM AUTHOR]

The Nb4AlC3 MAX phase can be regarded as a TMC structure with stacking faults, which has great potential as a novel solid hydrogen storage material. Herein, we used ab initio calculations for understanding the hydrogen incorporation into Nb4AlC3 MAX phases, including equilibrium structural characteristics, energy changes, electronic structures, bonding characteristics, and diffusion paths. According to the calculated results, H has thermal stability in the interstice of the Nb-Al layer, and the most probable insertion site is an octahedron (3-site) composed of three Nb atoms and three Al atoms. When C vacancies are introduced, the Nb-C layer has a specific storage capacity for H. In addition, Al vacancies can also be used as possible sites for H incorporation. Moreover, the introduction of vacancies significantly increase the hydrogen storage capacity of the MAX phase. According to the electronic structure and bonding characteristics, the excellent hydrogen storage ability of the Nb4AlC3 structure may be due to the formation of ionic bonds between H and Nb/Al. It is worth noting that the H-Al bond in the 1-site is a covalent bond and an ionic bond key mixture. The linear synchronous transit optimization study shows that only H diffusion in Al vacancies is not feasible. In conclusion, the Nb-Al layer in Nb4AlC3 can provide favorable conditions for the continuous insertion and subsequent extraction of H, while the vacancy structure is more suitable for H storage. Our work provides solid theoretical results for understanding the hydrogen incorporation into Nb4AlC3 MAX phases that can be helpful for the design of advanced hydrogen storage materials. [ABSTRACT FROM AUTHOR]

Objectives: To characterise the prevalence of myopia and eye diseases among school adolescents and children in Southwest China, and to evaluate the effectiveness of myopia control tools., Design: Retrospective cohort study., Setting: Across 95 basic education institutions in Southwest China., Participants: 96 146 children aged 3-17 years from a school-based survey conducted between 2019 and 2021., Primary Outcome Measures: The data of vision assessment and eye disease examination of school students were analysed, including a total of four surveys once per semester. The prevalence of myopia categorised as low (-0.5D to -3.0D), moderate (-3.0D to -6.0D) and high (≥-6.0D), along with the prevalence of significant ocular diseases, was assessed. Stratified analyses were conducted to investigate the impact of correction time on visual acuity (VA) and biological parameters. Subsequently, the subjects across the groups were matched using the nearest neighbour method, followed by multidimensional statistical analysis., Results: The prevalence of myopia among the surveyed students was 38.39%. After controlling for confounding variables, the statistical analysis revealed a 0.1 increase in mean VA within the orthokeratology group and a 0.1 decrease in VA within the spectacle group (p<0.001), with statistically significant differences in corneal radius, corneal curvature and equivalent spherical lens (p<0.05). Multivariate analysis indicated a statistically significant reduction in VA in the ophthalmopathy group compared with the control group (p=0.031). Furthermore, it was demonstrated that the risk of eye disease during vision correction was greater among older students than their younger counterparts (OR>1), and that female students exhibited a higher risk than male students (OR=1.5)., Conclusions: The current high prevalence of myopia and eye diseases among Southwest China's school youths demands public health attention. Minors wearing orthokeratology lenses at night, especially in primary school, exhibit significantly improved naked-eye vision. However, vigilant eye healthcare during the correction period is crucial, especially for girls., Competing Interests: Competing interests: None declared., (© Author(s) (or their employer(s)) 2024. Re-use permitted under CC BY-NC. No commercial re-use. See rights and permissions. Published by BMJ.)