The modulation of turbulence by particles has been rigorously investigated in the literature yielding either a reduction or an enhancement of the turbulent kinetic energy at different spatial length ...scales. However, a general description of the turbulence modulation in multiphase flows due to the presence of an interphase force has attracted less attention. In this paper, we investigate the turbulent modulation for interfacial and fluid-particle flows analytically and numerically, where surface tension and drag define the interphase coupling, respectively. It is shown that surface tension and drag appear as additional production/dissipation terms in the transport equations for the turbulent kinetic energies (TKE), which is of particular importance for the turbulence modelling of multiphase flows. Furthermore, we study the modulation of turbulence in decaying homogenous isotropic turbulence (HIT) for both types of multiphase flow. The results clearly unveil that in both cases the energy is reduced at large scales, while the small-scale energy is enhanced compared to single-phase flows. Particularly, at large scales surface tension works against the turbulent eddies and hinders the ejection of droplet from the corrugated interface. In contrast, at the small scales, the surface tension force and the velocity fluctuations are aligned leading to an enhancement of the energy. In the case of fluid-particle flows, particles retain their energy longer than the surrounding fluid increasing the energy at the small scales, while at the large scales the particles do not follow exactly the surrounding fluid reducing its energy. For the latter effect, a considerable dependence on the particle Stokes number is found.
A comprehensive frictional-kinetic model for collisional and frictional gas–particle flows is presented. The model treats gas and particles as a continuum. The kinetic-collisional stresses are closed ...using kinetic theory of granular flows (KTGF). The frictional stresses are based on inertial number dependent rheology and dilation laws. From these laws the frictional normal and shear stresses are derived. These individual contributions to the solids stress tensor are treated additively, which requires a modification of the radial distribution function in the frictional regime. The presented model is validated for both, i.e. frictional and collisional dominated, flow regimes: (1) the collisional-frictional gas–particle flow in multiple-spout fluidized beds is studied and (2) the friction dominated discharge of particles from a rectangular bin is considered. In case of the multiple-spout fluidized beds the numerical simulations show excellent agreement with the experimental data of van Buijtenen et al. (2011). The numerical results demonstrate that the presented model is a substantial improvement compared to the coupled CFD-DEM simulations of van Buijtenen et al. (2011) and to the Princeton model (Srivastava and Sundaresan, 2003). In case of the discharge of particles the model predicts height-independent mass flow rates and stagnant shoulders in the corners of the bin. For the computed discharge rates excellent correlation with measurements (relative error e<2.5%) for three different particle diameters is obtained. In contrast, the Princeton model yields relative errors up to e=41.5%. Finally, the computed solids velocities near the exit orifice show good agreement with experimental particle tracking (PT) results as well.
► The paper describes a comprehensive frictional-kinetic multi-fluid model. ► A μ(I)-rheology is used to close the frictional shear stresses. ► The model also accounts for dilation during frictional contacts under shear. ► The model shows excellent agreement with measurements in both regimes. ► The impact of the form of the solids wall shear stresses on the results is shown.
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•Optimum geometry of flow cells was derived from CFD simulations.•3D printing replaces classical manufacturing techniques.•Perfect coupling of electrochemical and downstream analytics ...possible.•Uniform and reproducible electrode area probing achieved.•High aspect ratio thin film analytics is demonstrated on Cu and Zn as examples.
3D printing was applied for the first time to produce highly customised flow type scanning droplet cell microscope heads which combine electrochemical measurements with downstream analytics of the electrolyte. The main advantages are the optimised fluid dynamics, the homogeneous and laminar mass transport along with the simplicity of the production at low costs. An improved design is presented that is hard to be machined in a classical way. This flow-type scanning droplet cell microscope (FT-SDCM) combines features from older versions of the techniques, the classical theta capillary based version and V-shaped microscopes. Different versions are compared and fluid dynamic simulations were performed to reveal their particularities in terms of electrolyte flow and surface wetting. Fabricating of the complex design of the flow cell was realised using a rapid prototyping approach. The newly proposed prototype is tested under various experimental conditions for assessing its stability, wetting and sealing performances. Both chemical and electrochemical dissolution experiments have shown a perfect electrolyte confinement within the cell and a complete wetting of the addressed area together with high throughput experimentation capabilities due to the robust design and ease of use in combination with a gantry robot.
•We compared wall boundary conditions (BC) for kinetic theory based two-fluid models.•Li and Benyahia BCs overestimate the granular temperature in the all sliding regime.•Jenkin’s model reveals poor ...predictions of transition between sliding and no-sliding.•Johnson and Jackson’s theory appears inappropriate in the all sliding regime.•The most complete theory appears to be the BCs of Schneiderbauer et al. (2012a).
Sediment deposition in reservoirs is an important research topic in engineering practice. Reservoir sedimentation has the potential to affect flood levels, drainage for agricultural land, pump ...station and hydropower operation as well as navigation. This paper describes the development of a coupled fully three-dimensional (3D) numerical model for the prediction of the local sediment flushing scour upstream of the bottom outlet. The presented numerical model solves the Navier-Stokes equations in conjunction with the k- ɛ turbulence model which includes both sediment transport and hydrodynamic parameters. The proposed coupled fully 3D numerical model is used to simulate experimental tests based on non-cohesive sediment. The geometric features of the scour hole (temporal and spatial hole development) upstream of the bottom outlet were reasonably well predicted compared to the experimental data. Furthermore, the velocity field upstream of the bottom outlet was in good agreement with measurements. The proposed numerical model for bottom outlet flushing was, therefore, validated because of its ability to accurately predict the scour hole development during the flushing process. The proposed numerical model can be considered reliable provided that the model is correctly calibrated and set up to reflect the conditions of a particular case study.
Two different methods for closure modeling of the unresolved terms appearing in the filtered two‐fluid model are discussed and compared. The spatially averaged two‐fluid model is based on ...generalizing the concepts of large eddy simulation to gas‐particle flows. In the approximate deconvolution method‐two‐fluid model approach, the unresolved terms are modeled by an approximate deconvolution method, where an approximation of the unfiltered solution is obtained by repeated filtering. Finally, these models are applied to a lab‐scale and a pilot‐scale fluidized bed. Both approaches yield fairly good agreement with a highly resolved reference simulation as well as with experimental data. Additionally, both methods deliver reasonable grid‐independent solutions up to a grid resolution of 2 cm in the case of Geldart type A particles.
The problem of coarse‐grid simulations of gas‐solid fluidized beds is investigated. Two different approaches are discussed, compared and applied to large‐scale gas‐solid flows. The spatially averaged two‐fluid model is based on generalizing the concepts of large eddy simulation to gas‐particle flows, whereas the unresolved terms were modeled by an approximate deconvolution method.
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