Water electrolysis using a proton exchange membrane (PEM) holds substantial promise to produce green hydrogen with zero carbon discharge. Although various techniques are available to produce hydrogen gas, the water electrolysis process tends to be more cost-effective with greater advantages for energy storage devices. However, one of the challenges associated with PEM water electrolysis is the accumulation of gas bubbles, which can impair cell performance and result in lower hydrogen output. Achieving an in-depth knowledge of bubble dynamics during electrolysis is essential for optimal cell performance. This review paper discusses bubble behaviors, measuring techniques, and other aspects of bubble dynamics in PEM water electrolysis. It also examines bubble behavior under different operating conditions, as well as the system geometry. The current review paper will further improve the understanding of bubble dynamics in PEM water electrolysis, facilitating more competent, inexpensive, and feasible green hydrogen production.

Aortic valve disease is a significant health issue. Research on the effects of aortic sinus geometry and aortic valve motion during a heartbeat is imperative, as numerous valvular diseases are associated with blood flow near the heart valve. Changes in the internal space of the sinus and the formation of blood clots have been reported after the implantation of a transcatheter aortic valve, an artificial heart valve. Although the sinus shape and leaflet motion significantly affect the hemodynamic characteristics and platelet aggregation, the blood flow behaviors near the heart valve associated with clot formation have not been fully elucidated. This study conducted in vitro experiments to investigate the flow behavior near an aortic sinus model based on the sinus shape. Additionally, hemodynamic changes associated with variations in the aortic sinus geometry and leaflet length within an aortic sinus model under pulsatile-flow conditions were elucidated. In a curved sinus model, a large single vortex formed near the center of the sinus region. Meanwhile, the center of the vortex in the sinus region shifted during a cycle in an expanded sinus model. Furthermore, a small vortex remained where the leaflet was initiated, thus increasing the likelihood of thrombus formation. Thus, the curved sinus model is advantageous for preventing material accumulation by maintaining a large vortex structure. The experimental results confirmed that the aortic sinus shape and leaflet length affect the likelihood of thrombus formation inside the aortic sinus.

Nanofluids, which are liquids containing nanoparticles, are used to modify heat transfer performance in various systems. To explain the mechanism of heat transfer modification with nanofluids, many theories have been suggested based on numerical simulations without experimental validation because there is no suitable experimental method for measuring the velocity fields of nanofluid flows. In this study, the measurement accuracy of micro-particle image velocimetry (μ-PIV) is systemically quantified with Al₂O₃ and multi-walled carbon nanotube (MWCNT) nanofluids. Image quality, cross-correlation signal-to-noise ratio, displacement difference, and spurious vector ratio are investigated with static images obtained at various focal plane positions along the beam pathway. Applicable depth is enough to investigate micro-scale flows when the concentrations of Al₂O₃ and MWCNT nanofluids are lower than 0.01% and 0.005%, respectively. The velocity fields of Hagen–Poiseuille flow are measured and compared with theoretical velocity profiles. The measured velocity profiles present good agreement with the theoretical profiles throughout. This study provides the criteria for μ-PIV application and demonstrates that μ-PIV is a promising technique for measuring the velocity field information of nanofluids.

This study emphasizes the potential risk posed by microplastics, particularly in tap water. Numerous studies have reported the removal of microplastics, but the limitations in addressing this issue remain challenging. To tackle this problem, a new method is introduced using tandem flexible structures (FSs) for microplastic removal. The present study focused on understanding the hydrodynamic characteristics between FSs to utilize microplastic removal. This comprehension of fluid flow and FSs offers valuable insights for improving the efficiency of microplastic removal methods. Therefore, the optimal conditions for removing microplastics were experimentally investigated inside the FSs gap region. Based on the gap distance and height, the flow structures between FSs were investigated. A small secondary vortex structure that could trap particles from upstream was continuously maintained behind the upstream FSs under certain geometric conditions. It is shown that this vortex structure has an effective way of confining the particles from upstream. The persistency of a small secondary vortex was also evaluated. This study may be helpful to researchers working on microplastic removal and FSs with a tandem arrangement.