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Hydrogen ejectors effectively utilize excess unreacted hydrogen. However, traditional ejectors have limited operational ranges and are only suitable for specific power ranges. This paper presents a dual-nozzle ejector aimed at broadening its operational range through different working modes. A simulation model, validated by experiments, is employed based on the working principles of ejectors. Spiral blades are installed at drive end to improve the recirculation efficiency. Flow characteristics, including streamwise and spanwise vortices at high power, are also investigated in different modes. Results indicate that optimal operating range for Ring-nozzle(RN) mode is between 31.1 and 124.4A. When output current exceeds 124.4A, it is necessary to switch to Central-nozzle (CN) or Double-nozzles (DNs) mode. Analysis of streamlines and velocity vectors reveals that installing spiral blades can modify directions of primary flow. Blades with appropriate distortion should be selected. In CN mode, when stack reaches 60% of rated power, installing 20° spiral blades has a minimal impact on stability of radial flow. Installing 30° or 40° spiral blades is suitable for full-load condition. In DNs mode, installing 30° spiral blades enhances acceleration of gas flow with lower influence from radial influx. This ejector provides assurance for upgrade of fuel cell stacks by incorporating recirculation technology. • A dual-nozzle ejector with spiral guide blades is designed, verified and evaluated. • Installed spiral blades change direction and gather kinetic energy of primary flow. • Advantageous working conditions of different modes with blades are discussed. • Flow mechanism of different blades under high power in two modes is analyzed. • Flow acceleration and losses should be balanced by changing the twist of blades. [ABSTRACT FROM AUTHOR]

In this paper, the effects of regular waves propagating against the current on the wake of a wide bathymetric obstacle are experimentally investigated. For that purpose, a square cylinder of height representing 1 / 8 t h of the water column, mounted perpendicular to the flow, is used. Both Reynolds and Froude numbers are close to the ones encountered at sea and swell properties (amplitude, frequency and wavelength) which are typical conditions in the English Channel. To study this complex hydrodynamic interaction, PIV measurements are used to capture the velocity field around and in the wake of the cylinder. The wake of this wide cylinder has been extensively studied and is characterised by its direction towards the water surface and by energetic and wide vortices arising from the sheared layer at a Strouhal number of approximatively 0.06, after a merging of smaller eddies. In this work, the focus is on how surface gravity waves, propagating against the current, modify the generation of these turbulent structures. The results show that the cylinder wake is strongly modified in the presence of waves, even if, on average, the main wake structure is globally conserved. The waves act both on the vortex generation and the wake development with modifications in terms of turbulent energy distribution and level, wake recovery and ascent rate. The vortex shedding becomes highly energetic and well-organised from the cylinder position. Its return period is extremely regular and corresponds to half of the wave frequency which is really close to the natural vortex shedding frequency. That results in a doubling of the statistical length scale of the vortices. • Swell induced-motion effects on high and isolated bathymetry roughness. • Bottom mounted cylinder wake characterisation in presence of waves opposing current. • Vortex generation and dynamic study. • Energy transfer between wave and vortex shedding frequency in the near-wake. [ABSTRACT FROM AUTHOR]

With the increasing application of bidirectional axial pump, it is necessary to study the flow and cavitation characteristics of bidirectional pump. In this paper, the cavitation and flow characteristics of the tip clearance under different clearances and different cavitation numbers of the S-hydrofoil bidirectional axial flow pump were studied by combining experimental and numerical simulation methods under reverse operation conditions. The section of the bidirectional pump was experimentally studied by high-speed photography and pressure pulsation. Based on the method of RANS, the SST-CC turbulence model and ZGB cavitation model were used to simulate the clearance cavitation of the bidirectional pump, and the cavitation morphology, velocity distribution, clearance vortex structure and energy distribution of blade tip clearance were investigated. The study reveals the flow characteristics of the clearance flow field of the bidirectional pump, and provides reference for the design and operation of it. • Deep cavitation bidirectional pump gap tail will form reverse shear cavitation. • Special distribution of S-shaped hydrofoil gap velocity will form a variety of vortices. • More disorderly flow field at the tail of the bidirectional pump gap. [ABSTRACT FROM AUTHOR]

Using particle agglomeration to help separate fine particles receives increasing attention in air purification processes. Recent studies suggested that enhanced turbulence using bluff bodies can significantly improve the agglomeration of fine particles. However, since micron-sized particles are difficult to measure, the current exploration of flow fields and particles is mainly based on simulations and lacks experimental validation. This paper innovatively used a phase doppler particle analyzer to measure the flow field in terms of velocity, streamline and turbulence intensity, as well as particle size and number concentration in an agglomeration chamber with a bluff body located at the center. Different shaped bluff bodies were also considered. It was found that low-velocity recirculation region with different shapes appeared in the wake, and vortex flow with special structures were observed. From the perspective of particles, the development of the flow field generated by a single bluff body effectively caused an increase in particle size and a decrease in particle number, which is because that the flow field promotes collisional agglomeration. More specifically, for V-structure bluff body, the particle size improved from 6 μm to a maximum of 13 μm and the particle number concentration decreased by 8 times. The agglomeration efficiency for PM 2.5 and PM 10 reached 89.06% and 87.38% respectively. Furthermore, by analyzing the velocity of different sized particles, it was reasonable to assume that different sized particles appeared to have velocity differences due to recirculating vortex flow, while the overlapping trajectories of positive and negative moving particles was mainly responsible for the collision probability enhancement. This paper presents new insights into particle agglomeration separation for this environment-friendly and low-cost technology. [Display omitted] • Flow field and particles were observed simultaneously by PDPA experiments. • The addition of the bluff body considerably enhances the particle size. • V-structure has the most considerable effect in enhancing the particle size. • Different sized particles show significant velocity differences in the wake. • Particle-Particle collision probability is improved by recirculating vortices. [ABSTRACT FROM AUTHOR]

• A novel ANN-CFD based modelling methodology reported for simulating flow in a batch vortex mixer. • Two-phase flow problem requiring interface tracking simulations reduced to a single-phase flow problem. • ANN model trained with a large number of experimental data is used to predict vortex shape. • Validation of ANN-CFD model with the PIV data reported in literature. • Velocity profiles obtained from ANN-CFD model show good match with the profiles obtained from VOF simulations. • Validated ANN-CFD model used to have detailed insights into the flow and turbulence structures. The study presents a novel ANN-CFD model for simulating flow in a vortex mixer (an unbaffled vessel stirred by a magnetic stirrer). Large eddy simulations (LES) are performed to simulate flow and turbulence considering presence of both air and liquid phases. The flow fields in air and liquid phases are coupled by applying suitable boundary conditions at the air–liquid interface represented by the vortex. The shape of the vortex is built-in in the computational domain and obtained by using an artificial neural network (ANN) model trained and validated with the data obtained from experiments carried out to capture the vortex shape under different parametric conditions. ANN-CFD model is validated with PIV data reported in literature. The ANN-CFD model reduces the computational time significantly by obviating the need of performing computationally very intensive interface tracking simulations. The model is used to understand hydrodynamics and turbulence characteristics of flow in the vortex mixer. [ABSTRACT FROM AUTHOR]

[Display omitted] • Interaction of a bubble-particle aggregate with a confined vortex investigated. • An interface resolved 3D CFD model developed to quantify the interaction dynamics. • Highspeed shadowgraphy and PIV experiments performed to validate the CFD model. • Correlation of vorticity and local energy dissipation rate (ε) determined from CFD model. • A force-balance model used to predict particle detachment using ε. In flotation, interactions of bubble-particle aggregates with turbulent flow structures in the liquid medium result in particle detachment. This study aims to simulate this phenomenon involving a bubble-particle aggregate (bubble diameter ∼ 3 mm and particle diameter ∼ 314 µm) interacting with a turbulent flow structure manifested as a confined vortex in a square cavity connected to a square cross-section channel. An interface resolved three dimensional (3D) computational fluid dynamics (CFD) model was developed to quantify the bubble-vortex interaction dynamics over a range of channel Reynolds numbers. The CFD model produced a good agreement with the experimentally measured vorticity magnitude, local energy dissipation rate, and bubble motion. It was shown that a bubble-particle aggregate could be captured within the vortex by suitably varying the channel Reynolds number, eventually leading to particle detachment. A separate force balance analysis was performed to determine a criterion for particle detachment utilising the CFD model predicted vorticity and local energy dissipation rate. It was shown that a critical local energy dissipation rate ∼ 1.59 m2/s3 was required for particle detachment to occur, which was also verified experimentally. [ABSTRACT FROM AUTHOR]

The internal turbulence formed within artificial reefs (ARs) is extremely important. Laboratory experiments were conducted using Particle Image Velocimetry (PIV) to observe the influence of reef configuration on the current and turbulence features of perforated box-type ARs via arranging from one to three blocks, corresponding to the aspect ratio of 1.05, 2.09 and 3.14 respectively. A numerical model with same specifications of the PIV experiment was built and the calculated results were verified using experimental data. Several vortices with different scales and strong turbulence intensity are formed inside and behind the reef. The internal turbulence has been fully developed under the reef configuration with aspect ratio of 2.09. The internal turbulent flow structure was dominated by the horizontal vortex structure, which displayed certain symmetry in the transverse direction because of symmetrical reef structure. The vorticity pattern inside the reefs was dipole-type, regardless of aspect ratio, consisting of two pairs of high-vorticity but opposite rotation direction clouds formed behind the openings of reef stoss face, and symmetrical distribution in both vertical and horizontal direction. This study aims to reveal the reef-induced internal flow structure and fills the gap of existing research. • 3D vortex structure within reef was observed in various reef aspect ratios using PIV. • Horizontal vortices dominate the inner turbulence and increase with reef lengthening. • Dipole-type vorticity pattern appears behind the openings of stoss face inside reefs. • A strong turbulence U-shaped region was formed around reefs due to vx fluctuation. • The reef openings play a key role to internal flow structure and water exchange. [ABSTRACT FROM AUTHOR]

• The capabilities of different drag models under high turbulence /vortex flow conditions are shown. • Impacts of turbulence effects on drag modelling are presented. • A hybrid drag model is proposed for high turbulence flow conditions. • Two-phase flow hydrodynamics under complex flow conditions is analyzed. • Gas velocity and void fraction predictions are compared with experimental data. The accuracy of the modelling of gas–liquid flows depends strongly on a suitable modelling of the interfacial forces. Among these, drag is dominant. Most drag models reported in the literature have been derived and validated only for laminar or low-turbulent flow conditions. In this study, we numerically evaluated several drag models from the literature for high-turbulent gas–liquid flow around an obstacle in a pipe that creates a distinct vortex region. We performed Computational Fluid Dynamics (CFD) simulations and compared the void fraction and gas velocity profiles with experimental data obtained by ultrafast X-ray computed tomography. We found that all models, except Schiller&Naumann and Feng, predicted the void fraction well compared to experimental data upstream of the obstacle, i.e., for a developed two-phase pipe flow with axial symmetry. However, the void fraction downstream is greatly overestimated by all models except those that appropriately consider the turbulence effects. Based on the results, a hybrid drag model is proposed that significantly improves the prediction of the void fraction. [ABSTRACT FROM AUTHOR]