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The atomization of a water column by a gas jet flow (Reynolds number $\sim O(10^{4}\unicode{x2013}10^{5})$ ) issued from a two-stage annular nozzle is investigated experimentally. Varying the nozzle geometry, the momentum flux ratio of the upper and lower jets, and the water flow rate, we measure the processes of atomization with high-speed imaging, analysed analytically into four regimes. In the bulk atomization regime, the atomization is driven by the lower jet, but it is forced to occur earlier by the stronger upper jet before the water column reaches the lower jet in the droplet atomization regime. Interestingly, the size of the atomized droplets remains unaffected by the momentum flux ratio of upper to lower jets. The atomization process is governed by the Rayleigh–Taylor instability, by which the estimated droplet size agrees well with the measurement. In the backflow regime, a strong reverse flow is induced to force a substantial portion of atomized droplets to be drawn backward to the nozzle; a floating liquid column regime is captured transitionally, i.e. the column stagnates near the lower nozzle when the water flow rate is very low. To understand the mechanisms of each regime, the single-phase jet flow is measured separately using particle image velocimetry, and implemented into the control volume analysis with which we predicted analytically and validated the conditions for the occurrence of each regime. It is found that the acceleration of gas flow (velocity gradient) experienced by the falling water is the key parameter to drive the atomization.

In this study, the dangers surrounding vehicular fires were identified by means of statistical analysis of the national fire information system. The analysis was conducted on the operation time of the sprinkler fire extinguisher, which plays the most essential role during the initial fire extinguishing attempt, in case of a fire outbreak. A simulation was created using the application guidelines pertaining to a sprinkler head located close to a beam as provided in NFSC 103. The operation time of the sprinkler head was analyzed for the case of a parking lot fire where a fan was operated. Fire Dynamics Simulator was used as the simulation program. If the revised guidelines were applied to thesimulation, the sprinkler head located directly above the fire source was activated approximately 1 minin advance compared to the sprinkler head operating under current standards. Therefore, it was found that the revised guidelines were suitable and aided in early fire extinguishing attempts. In the case where a fan was operated along with the detector, during the outbreak of a fire, it was found that the operation of the sprinkler head located directly above the fire was delayed by two or more minutes compared to the simulated case where no fan was operated.

Electroconvection (EC) near an ion-selective membrane has attracted significant attention in concentration polarization platform and been generally accepted as a cause of the overlimiting current. Especially, the heterogeneous structures of the ion-selective membrane have been extensively studied for enhancing perm-selective mass transportations in an EC-dominant regime. However, there is lack of studies considering EC on a multiscale porous membrane, which has been often utilized in desalination and fuel cell platform. In this work, we developed a modeled micro/nanofluidic device consisted of an array of ion-selective patches so that micropores were formed between the patches. Rigorous theoretical and experimental analysis on EC near the membrane demonstrated that asymmetric Coulombic force distribution among the micropores induced electroconvective circulating flows, leading to significant enhancement of ion transport and lowering resistivity of the entire system. Furthermore, we verified that the appearance of the circulating flow critically depended on the configuration of micropores so that one can maximize the ion transportation by tweaking the configuration. Therefore, the new finding of the circulating flows would advance the fundamental understanding of electrokinetics in concentration polarization platforms which have been extensively applied for environmental and energy applications.

In the present study, we perform a series of water-tunnel experiments to investigate the flow around an in-line sphere array at the Reynolds number of 1000, based on the free-stream velocity and sphere diameter, focusing on how the sphere wake changes by the additional spheres located behind while the spacing between them being varied. We try to capture the qualitative picture of the wake behind each sphere and to measure quantitative distribution of flow statistics using dye visualization and particle image velocimetry techniques. When the spacing is less than one sphere diameter, the gap flow is steady and axisymmetric, while the last wake is planar symmetric with a weaker turbulence level. When the spacing is twice larger than the sphere diameter, on the other hand, the flows in the gap and final wake tend to recover the axisymmetric feature, which is similar to that of a single sphere wake at the same Reynolds number. Between these two regimes, there is a transitional flow regime (the spacing is comparable to the sphere diameter) where the flow is still planar symmetric, but the turbulence level is enhanced. Depending on the flow topology, it is found that the characteristic frequencies (i.e., Strouhal number) in the shear layers and the gap are also affected. Finally, based on our analysis, we propose a flow regime map for each wake in the considered sphere arrays and also estimate the pressure distribution from the velocity measurements, by which the forces acting on each sphere can be assessed.