Design and numerical investigation of a circular microchannel for particle/cell separation using dielectrophoresis

Precisely separating particles/cells with different sizes and physical properties has been an interest for point-of-care diagnostics and personalized treatment. Dielectrophoresis (DEP) is widely known as a powerful and non-invasive technique to separate particles and cells. This paper presents a comprehensive numerical investigation of particle/cell separation in circular microchannels using DEP. First, the geometrical parameters of the circular microchannel affecting DEP force are determined by performing an analytical solution. Then, by developing a solver in OpenFOAM, the effect of these parameters on particles deflection is investigated. According to the results, two different circular microchannels are presented to investigate the continuous separation of bio-particles (based on their physical properties) and polystyrene particles (based on their size). The results showed that a minimum voltage of 7, 9, and 12 V is required to achieve 100 % purity and separation efficiency for separating red blood cells from MDA-MB-231 cancer cells at the flow rate of 0.5, 1.0, and 1.5 µl/min, respectively. Also, the efficient separation of 5 and 10 µm polystyrene particles at the flow rate of 0.1 µl/min is possible only at the voltage of 9 V. The results of this numerical study can be useful for the fabrication of an optimal microdevice for the continuous DEP separation of particles and cells.     

Image processing of computed tomography scanned poly-dispersed beds for computational fluid dynamic studies

Achieving our emission reduction goals requires the bulk production of carbon-neutral fuels and chemicals, which are catalytically produced through heterogeneous fixed bed chemical reactors. To optimise and scale-up these reactors, accurate and validated Computational Fluid Dynamics (CFD) models are crucial. Of especial importance to CFD simulations is the accurate depiction of the 3D bed structure used during the experimental setup. A direct one-to-one coupling between experiments and simulations can be achieved by scanning the experimental bed using computed tomography and reconstructing the scanned images as a 3D geometry for CFD simulations. However, processing of the scanned images is necessary to minimise highly coarse features that could impact the overall mesh size. A highly poly-dispersed lab-scale fixed bed reactor, previously scanned and analysed, is processed using various image-processing operations. Depending on the number and the crudeness of the processing operations, the bed is progressively deformed, which impacts both its porosity and its interparticle pore connectivity. The impact of image-processing becomes more evident when the hydrodynamic behaviour, i.e., X-, Y-, and Z-velocity and static pressure, of the beds is explored. CFD simulations revealed highly heterogeneous flow profiles, with the maximum velocity reached being 16-times higher than the average superficial velocity within the bed. Moreover, small modifications in local topological features introduce significant changes to the flow profiles, while the 3D pore interconnectivity was seen to play an equally important role as the interparticle porosity. A particle size study revealed that large particles form less interconnected networks with higher pore volumes, which significantly reduce the flow velocity and the pressure drop experienced by the flow. The generated results yield key insights towards a deeper understanding of the behaviour of fixed bed chemical reactors, highly valuable for catalyst and reactor engineering.     

Numerical and experimental study of molten pool behaviour and defect formation in multi-material and functionally graded materials laser powder bed fusion

Laser powder bed fusion (LPBF) of multi-material and functionally graded materials (FGM) has attracted significant research interest due to its ability to fabricate components with superior performance compared with those manufactured with single powder material. However, the forming mechanisms of various defects remain unknown. In this paper, a DEM-CFD model was first established to obtain an in-depth understanding of this process. It was discovered that the defects including partially melted and un-melted Invar36 powder were embedded in the lower level of the powder layer; this was attributed to the low laser absorptivity, low melting point and high thermal conductivity of the Cu10Sn powder. Inter-layer defects were more likely to occur with an increased powder layer thickness. In addition, the scanned track width was found related to an equilibrium achieved among the thermal properties of the powder mixture. Process parameters were optimised to obtain FGM structures without defects in both horizontal and vertical directions. Invar36/Cu10Sn samples were fabricated with a multi-material LPBF system using different mixed powder contents and laser volumetric energy densities (VEDs). By increasing the VED, fewer defects were observed between the interface of two processed powder layers, which had a good agreement with the modelling results.     

CFD modeling of erosion wear in pipe bend for the flow of bottom ash suspension

In the present study, erosion wear behavior of slurry pipeline due to solid–liquid suspension in the pipeline has been investigated using commercial computational fluid dynamics (CFD) code FLUENT. A multiphase Euler–Lagrange model was adopted to predict the solid particle erosion wear in a 90° pipe bend for the flow of bottom ash–water suspension. A standard k–ε turbulence modeling scheme was used to simulate the flow through the pipeline. Water and bottom ash were taken as liquid and as a dispersed phase of solid–liquid mixture, respectively. A simulation study for erosion wear in a pipe bend was carried out to investigate the influence of various parameters including velocity, solid concentration, and particle size. The velocity of the bottom ash–water suspension varied from 0.5 to 2.5?m/s for solid concentrations with a range of 2.5 to 10.0% (by volume). The particle diameters of the bottom ash were 162 and 300?µm. The simulation results agree with the results of previous studies.     

Fragility analysis of curtain walls based on wind-borne debris considering wind environment

Increasing attention has been attached to the risk assessment and fragility analysis of envelopes of high-rise buildings subjected to the impact of wind-borne debris in hurricanes or typhoons. A probabilistic model of wind-borne debris is proposed in the present paper for risk assessment and fragility analysis of high-building curtain walls based on the numerical solution of three-dimensional flight trajectories of debris and the computational fluid dynamics simulation of local wind environment in a residential area. The influence of sources of randomness in the generation of debris, such as generated location, size, and initial attack angle, as well as local wind environment, on the location of debris impacting on the building curtain wall and the impact damage effect, are considered in the proposed model. The evolution of the probability characteristics of debris is driven by its physical flying behavior in the local wind environment, and the failure probability of each piece of glass of building curtain walls impacted by debris can be determined. Furthermore, a numerical algorithm is given for the fragility analysis under different incoming wind speeds, which can be used to determine the vulnerability and the fragility curve. Finally, two numerical examples for application to residential areas are provided. Results show that the proposed model can provide evaluation and prediction for wind disaster risk and fragility of high-rise buildings in urban areas.     

分裂导线微风振动数值仿真

基于流体动力学软件,采用二维圆柱绕流的计算方法,对分裂导线的微风振动进行了仿真分析。首先对于固定双圆柱绕流进行了仿真,与经典文献对比验证了本文模型以及计算方法的正确性,然后基于自编程序实现了分裂导线振动的流固耦合分析,研究了不同支撑,不同分裂导线间距以及不同分裂倾角下双分裂导线的振动特性,得到了各工况下的子导线竖向振幅随缩减风速的变化曲线,对分裂导线微风振动特性的研究具有重要意义。     

Immersed-boundary/soft-sphere method for particle–particle-fluid interaction in a viscous flow: An OpenFOAM solver

We developed a stable OpenFOAM solver for Immersed Boundary Method based on direct forcing and regularized delta function. The soft-sphere model and a lubrication model were implemented to consider particle–particle collision in a viscous flow. We proposed a fluid–structure interaction (FSI) coupling method to accurately calculate the fluid forcing term and particle velocity. Our solver was validated for fixed and moving bodies, including rotation. The accuracy of various FSI schemes was evaluated in predicting the solid and fluid flow behavior in a viscous flow. It was demonstrated that neglecting or simplifying the fluid momentum change affects the accuracy of the solid velocity and fluid flow dynamic; for higher solid-to-fluid density ratios, a larger deviation was predicted. Furthermore, the FSI schemes highly influenced the behavior of the formed vortices.The solver was validated to predict the effective restitution coefficient of particles in a viscous flow as a function of the Stokes number. We also thoroughly analyzed the dynamic flow behavior of colliding particles through the pressure and velocity field and fluid force. This analysis helped us accurately determine the rebound velocity of particles in case of high Stokes numbers when the effect of viscous force is significant.