The overall performance of PEMFC (proton exchange membrane fuel cell) is affected by the flow field structure, especially the cathode flow field design can effectively solve the uneven distribution ...of gas concentration in the traditional flow channel and the cathode flooding phenomenon. In order to solve the above problems, a PEMFC single cell model with waveform staggered flow field of cooling flow field and small cathode channel was established in this study. Three-dimensional (3D) multi-phase CFD (computational fluid dynamics) simulation method is used to compare with gas concentration, liquid water distribution, pressure drop, and net power density of three different cases, and the influence of different cooling velocity on the temperature of cooling flow field is considered. The results show that the overall performance of the proposed flow field is the best, in which the maximum current density is 1.391 A⋅cm−2 and increases by 14.9%. The cathode and anode waveform staggered flow field makes the proton exchange membrane (PEM) water distribution more uniform, at the same time, the small size of the cathode flow channel facilitates the discharge of heat, and the convective heat transfer effect is enhanced. The electrochemical reaction rate is fast, which accelerates the temperature reduction in the fuel cell under the action of the cooling flow field, and the temperature uniformity of the cooling flow field is better. In addition, net power density is improved by 39.7%, and the output performance is significantly improved.
•Waveform staggered flow field with small cathode channel is established.•The overall performance of the proposed flow field increases by 14.9%.•Case 4 has large pressure drop, strong water removal and heat dissipation ability.•The temperature distribution is optimal with the coolant flow rate is 0.05 m/s.•The net power density is improved by 39.7% in the wavy staggered flow field.
•The quantitative evolution law of flow field structures is investigated.•The hydrodynamic mechanisms of L-L-L three-phase flow is revealed.•A correlation is established to predict the average ...velocity of compound droplets.
Numerical simulations have been performed to reveal the hydrodynamic characteristics of liquid–liquid–liquid three-phase flow in a confined microchannel via a VOF-CSF model. Further the effects of outer phase flowrate, dimensions, the viscosities of all three phases as well as the interfacial tensions of both inner and outer interfaces are systematically investigated. It is found that the shear stress and pressure gradient together determine the formation and disappearance of vortices inside the compound droplets. The internal fluid velocity of compound droplets is significantly altered by the flowrate and viscosity, while the effect of interfacial tension can be ignored. As for the vorticity distribution inside the compound droplets, it exhibits a positive correlation with the capillary number of outer phase. Besides, a prediction correlation for the average velocity of compound droplets with several dimensionless numbers is also established with good precision. The results obtained in this study could provide meaningful theoretical guidance for rational design and regulation of transport behaviors with liquid–liquid–liquid three-phase flow in a confined microchannel.
•Interfaces deformation behaviors of droplets in a helical microchannel were studied.•The correlation equations were established to predict the flow characteristics.•The flow field structures in a ...helical microchannel were quantitatively identified.
Hydrodynamics of liquid–liquid (L-L) two-phase flow in a confined helical microchannel are investigated via a VOF-CSF model, and further the influences of continuous phase flowrate, interfacial tension, viscosity ratio, helical screw pitch, helical radius as well as the diameter ratio of droplet to channel were systematically ascertained. To achieve the precise control over the interface deformation in a confined helical microchannel, the correlation equations with several key dimensionless numbers are established to predict the droplet aspect ratio, eccentricity and the minimum liquid film thickness. Especially, the minimum film thickness can be used to distinguish the flow field structures, and when the dimensionless outer minimum liquid film thickness (δ*out) is greater than 0.16, there is only one complete vortex system structure inside the droplet. Otherwise, two vortices are observed. Besides, the Q-criterion is used to quantitatively confirm the disappearance of vortices inside the droplet. Finally, the force field characteristics are also discussed to reveal the corresponding droplet interface deformation mechanisms, and it is found that, the droplet is driven forward by the differential pressure force, and deformed by the shear force along its flow direction. Both of the Dean force and Coriolis force usually control the interface deformation of droplet in the radial direction. The results in our study offer valuable guidance for designing the L-L two-phase flow microsystems to realize the goal of process intensification.
Rotating detonation engine has been widely studied in recent years because of its high theoretical efficiency and heat release rate. In many numerical simulations, the combustible mixture is injected ...and fully filled at the head of the combustor. In this paper, annular injection slits are proposed and three representative injection patterns are simulated by changing the injection directions. Stable single-wave modes are formed in all three patterns and two kinds of combustible mixture layer structures are found, namely “L-shape” and “T-shape” structures. Following the combustible mixture layer, the detonation wave is not fully filled in the radial direction, thus radial and circumferential shock waves are induced from the detonation wave, forming more complex wave structures. After the radial shock wave, velocity vortex and significant deflagration are found and propagate with the shock wave, thus maintaining a higher pressure and temperature there.
•Two kinds of combustible mixture layer are found in the RDE combustor.•Following the detonation wave, the radial and circumferential shock waves are seen.•Velocity vortex is formed after the radial shock wave and propagates with it.•Deflagration is found in the vortex with high temperature and pressure there.
•MTAM1: first non-zonal time-averaged geomagnetic field model for the Miocene era.•Reversed flux patch under the South Atlantic region in all versions of the models.•Similar features to the last 5 ...million years, suggesting long-term mangle control.
Reconstructions of long-term time-averaged geomagnetic field structures are important to understand geomagnetic field evolution and to identify the longevity and scale of non-dipolar field morphology. This study presents MTAM1, a non-zonal time-averaged field model for the Miocene era (5.3–23 million years ago), or indeed any time period prior to 5 million years ago. The time averaged field model for the Miocene is an inverse model based on a directional data compilation comprising 38 different localities, each with a minimum of 10 sites, called PSVM (Engbers et al., 2022a). The data were separated into normal (PSVMN) and normalised reversed (PSVMR) datasets, yielding two corresponding models MTAM1N and MTAM1R. Allowing for the opposite sign, no substantial differences were found between these two models, suggesting symmetry between the morphology of the normal and reversed fields and no evidence for non-reversing features in the geomagnetic field. Under this assumption of symmetry, the normal and reversed data can be modelled together, enhancing the data distribution and thus the robustness of the complete time-averaged field model for the Miocene. The broad structure of the models resembles previous time-averaged field models for the past 5 Myr but there are some clear differences, particularly under the South Atlantic, where all Miocene models include a reversed flux patch (RFP). To investigate whether this difference is well defined, or could result from differences in modelling methodology or data distribution, the data of the last 5 Myr were inverted with our normal model for the Miocene as a prior constraint. We find no evidence that the Miocene model is inconsistent with the field structures required by data from the past 5 Myr, suggesting an overall stability in the averaged geomagnetic field morphology for the past 23 Myr. This is consistent with long-term mantle control on geomagnetic field morphology leading to consistent deviations from the geocentric axial dipole on a multi-million-year timescale on the Core-Mantle boundary (CMB).
Recent observations by the Van Allen Probes spacecraft have demonstrated that a variety of electric field structures and nonlinear waves frequently occur in the inner terrestrial magnetosphere, ...including phase space holes, kinetic field line resonances, nonlinear whistler mode waves, and several types of double layer. However, it is unclear whether such structures and waves have a significant impact on the dynamics of the inner magnetosphere, including the radiation belts and ring current. To make progress toward quantifying their importance, this study statistically evaluates the correlation of such structures and waves with plasma boundaries. A strong correlation is found. These statistical results, combined with observations of electric field activity at propagating plasma boundaries, are consistent with the scenario that the sources of the free energy for the structures and waves of interest are localized near and comove with these boundaries. Therefore, the ability of these structures and waves to influence plasma in the inner magnetosphere is governed in part by the spatial extent and dynamics of macroscopic plasma boundaries in that region.
Key Points
Electric field structures and waves preferentially appear at boundaries
Regions of electric field structure and wave growth comove with boundaries
Data support an Alfvénic wave free energy source rather than boundary gradients
Cluster data have been examined for quasi‐stationary electric field structures and field‐aligned currents (FACs) in the vicinity of the dayside cusp region. We have related the measurements to the ...Region 1/Region 2 (R1/R2) current system and the cusp current system. It has been theoretically proposed that the dayside R1 current may be located on open field lines, and experimental evidence has been shown for R1 currents partially on open field lines. We document that R1 currents may flow entirely on open field lines. The electric field structures are found to occur at plasma density gradients in the cusp. They are associated with strong FACs with current directions that are consistent with the cusp currents. This indicates that the electric field structures are closely coupled to the cusp current system. The electric equipotential structures linking the perpendicular electric fields seen at Cluster altitudes to field‐aligned electric fields at lower altitudes fall into one of two categories: S shape or U shape. Both types are found at both the equatorward edge of the cusp ion dispersion and at the equatorward edge of injection events within the cusp. Previous studies in the nightside auroral region attributed the S‐shaped potential structures to the boundary transition between the low‐density polar cap and the high‐density plasma sheet, concluding that the shape of the electric potential structure depends on whether the plasma populations on each side of the structure can support intense currents. This explanation is not applicable for the S‐shaped structures observed in the dayside cusp region.
We re‐examine the basic premises of a single‐spacecraft data analysis method, developed by Sonnerup and Hasegawa (2005), for determining the axis orientation and proper frame velocity of quasi ...two‐dimensional, quasi‐steady structures of magnetic field and plasma. The method, which is based on Faraday's law, makes use of magnetic and electric field data measured by a single spacecraft traversing the structure, although in many circumstances the convection electric field, − v × B, can serve as a proxy for E. It has been used with success for flux ropes observed at the magnetopause but has usually failed to provide acceptable results when applied to real space data from reconnection events as well as to virtual data from numerical MHD simulations of such events. In the present paper, the reasons for these shortcomings are identified, analyzed, and discussed in detail. Certain basic properties of the method are presented in the form of five theorems, the last of which makes use of singular value decomposition to treat the special case where the magnetic variance matrix is non‐invertible. These theorems are illustrated using data from analytical models of flux ropes and also from MHD simulations as well as a 2‐D kinetic simulation of reconnection. The results make clear that the method requires the presence of a significant, non‐removable electric field distribution in the plane transverse to the invariant direction and that it is sensitive to deviations from strict two‐dimensionality and strict time stationarity.
Key PointsMotion and orientation of 2D field structures from Faraday's lawUses electric and magnetic field data from a single spacecraftDifficulties in application to reconnection events
The theoretical explanation of electric field structures associated with density depletion in the Earth's upper ionosphere is presented. Using the quasi-neutrality hypothesis, the effect of excess ...energetic electron species is studied on the evolution of nonlinear low-frequency ion-cyclotron and ion-acoustic waves in a magnetized auroral plasma. The dynamics of the cold ion beam is governed by the fluid equations and the electron is treated as energetic species with non-thermal density distribution. Numerical computations appear in a series of periodic oscillations, such as spiky, sawtooth and sinusoidal waveforms. The present model can generate up to 18 mV/m electric field amplitude, which is in the range of the FREJA satellite measurements in the auroral acceleration region.
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