Reiyu Chein

EFFECTS OF VORTEX PAIRING ON PARTICLE DISPERSION IN TURBULENT SHEAR FLOWS

Abstract Particle dispersion in large-scale dominated turbulent shear flow is investigated numerically with special emphasis on the effects of the vortex-pairing phenomenon. The particle dispersion is visualized numerically by following the particle trajectories in a flow consisting of large vortices which are undergoing pairing interaction. The flow field is generated by a discrete vortex method. Important global and local fiow quantities from the numerical simulation compare reasonably well with experimental measurements. For both cases of point sources with continuous particle release and an initially distributed line source, the particle dispersion results demonstrate that the extent of particle dispersion depends strongly on the Stokes number, the ratio of the particle aerodynamic response time to the characteristic time of the vortex-pairing flow field. Particles with relatively small Stokes numbers disperse laterally at approximately the saine rate as that of the fluid particles and particles with large Stokes numbers disperse much less than the fluid particles. Particles with intermediate Stokes numbers (0.5-5) may be dispersed laterally farther than the fiuid particles and may actually be flung out of the vortex structures. Due to the strong particie entrainment power, the flow during the vortex-pairing process seems to produce higher particle lateral dispersion than the pre-pairing and post-pairing flows.

Discrete-vortex simulation of flow over inclined and normal plates

Abstract The wake flow behind a normal or an inclined plate is predicted by a discrete-vortex method. The vortex shedding phenomenon at the leading and the trailing edges is simulated by a vorticity creation technique according to the Kutta condition. This approach required that the strengths and locations of discrete vortices just shed from the edges be determined such that their presence in the flow field will offset the potential flow singularities at the edges of the plate. For gross features in the flow, the current model closely reproduced the measured values of previous experiments. As to the microscopic features in the wake flow, the predictions by the current model compare favorably with the experiments and with the previous solutions also by discrete-vortex simulation. For similar or better solutions, this approach requires about half of the computing time reported by other discrete-vortex approaches.

Microfluidic flow switching design using volume of fluid model.

In this study, a volume of fluid (VOF) model was employed for microfluidic switch design. The VOF model validity in predicting the interface between fluid streams with different viscosities co-flowing in a microchannel was first verified by experimental observation. It was then extended to microfluidic flow switch design. Two specific flow switches, one with a guided fluid to one of five desired outlet ports, and another with a guided fluid flows into one, two, or three outlet ports equally distributed along the outlet channel of a Y-shaped channel. The flow switching was achieved by controlling the flow rate ratios between tested and buffer fluids. The numerical results showed that the VOF model could successfully predict the flow switching phenomena in these flow switches. The numerical results also showed that the flow rate ratio required for flow switching depends on the viscosity ratio between the tested and buffer fluids. The numerical simulation was verified by experimental study and the agreement was good.

Analysis of effect of electrolyte types on electrokinetic energy conversion in nanoscale capillaries

An analytical study on the effect of electrolyte types on the electrokinetic energy conversion is presented using nanoscale cylindrical capillary, which is either positively or negatively charged. The sign of surface charge determines the role and concentration magnitude of ions in the capillary and the energy conversion performance. Our study shows that the electrokinetic energy conversion performance (maximum efficiency, pressure rise and streaming potential) are approximately identical for 1:1 (KCl), 2:1 (CaCl2) and 3:1 (LaCl3) electrolytes when capillary is positively charged. For negatively charged capillary, energy conversion performance degrades significantly with the increase of counter‐ion valence. For both positively and negatively charged capillaries, higher maximum efficiency can be resulted in low bulk concentration and surface charge density regimes. However, high maximum pressure rise generation for the pumping is found in the low bulk concentration and high surface charge density regimes. For the electric power generation, higher maximum streaming potential is found when both bulk concentration and surface charge density are low.

Particle dynamics in a gas-particle flow over normal and inclined plates

Abstract The discrete-vortex simulation is applied to study the gas-particle flows over normal and inclined plates. This study places emphasis on the phenomena of particle impingement on a normal- or inclined-plate surface. It relates the aerodynamic effects of particle motion to the erosion process and also deals with the particle dynamics in the wake region. A line source of particles placed in the upstream of the plate and a point source of particles located at the center of the downstream side of the plate are used in the study. The range of particle size in terms of the Stokes number is from 0.5 to 100. The results of the current vortex simulations are compared with previously published results obtained through potential-flow theory. For particles released from the upstream of the plate, the current approach predicts results that agree closely with those by potential-flow theory except that, instead of symmetrical particle trajectories, a slight asymmetry with respect to the center of plate in particle trajectories was predicted by the current discrete-vortex approach. This is presumably due to the alternating vortex shedding in the wake, which can not be predicted by the potential-flow theory. In the surface erosion study, the current analysis predicts close agreements with the experimental results. For those particles escaping impact with the plate, their dispersion by the wake flow is rather limited. For particles injected into the wake from the center of the downstream side of the plate, little mixing is predicted in the close vicinity of the plate because of the initial inertia of the particles. Relatively far from the plate, the extent of particle mixing and dispersion in the wake is inversely proportional to both the Stokes number and the particle initial velocities. The effects of the Stokes number and the particle initial velocity on the particle mixing and dispersion seem to be superimposable.

Thermodynamic Reversibility Analysis of Electrokinetic Energy Conversion in Nanofluidic Channels

The thermodynamic efficiencies of electrokinetic pumping and electrical power generation are investigated numerically using a two-dimensional axisymmetrical model containing a finite-length nanoscale cylindrical capillary and reservoirs connecting at the capillary ends. The Navier-Stokes, Laplace, Poisson, and Nernst-Planck equations are solved simultaneously to obtain the fluid and electric current flows. The main goal of this study is to justify the reversibility of electrokinetic energy conversion resulting from one-dimensional analysis. Based on our numerical results, it is found that the reversible electrokinetic energy conversion between the pump and electric power generation is valid only when the dimensionless Debye length is greater than 2. Because of the electric double layer (EDL) overlap and current due to the electrostatic potential gradient, significant deviation in maximum efficiencies between the numerical and one-dimensional analysis results are found when the dimensionless Debye length is less than 2.

Numerical study of ionic current rectification through non-uniformly charged micro/nanochannel systems

Ionic transport through cylindrical nanochannels with linearly varied surface charge density was numerically investigated. The ends of the nanochannel were connected to microchannels regarded as reservoirs. The walls at the micro/nanochannel junction were referred to as sidewalls that can be electrically neutral or charged. The results showed that the charged sidewalls could enhance the concentration polarization compared to neutral sidewalls. For neutral sidewall, a limiting current similar to charged permselective membranes and a maximum current rectification ratio at certain bulk concentration similar to charged conical nanopores can be found. For the charged sidewall case, no limiting current regime can be observed and the current varied linearly with the applied voltage with a larger slope compared to the Ohmic relation regime. Moreover, no maximum current rectification ratio can be found and the current rectification ratio increased with the decrease in bulk concentration and increases in surface charge density and sidewall length.

Modeling of Particle Removal using Non-contact Brush Scrubbing in Post-CMP Cleaning Processes

Particle removal using non-contact brush scrubbing for post-CMP (Chemical Mechanical Planarization) cleaning is investigated analytically. The removal of Si O 2 and A l 2 O 3 particles adhered onto Si O 2 film coated on the wafer surface are considered. The cleaning fluid (H 2 O/N H 4 OH = 1:25 and 1:200) flowing between the brush and wafer surface is treated as a thin-film fluid flow. The flow field details and its effect on the drag force acting on the adhered particles are discussed. In addition to the drag force, the electrical double layer (EDL) and thermophoretic force effects on particle removal are also considered. It was found that the dominant force in achieving particle removal using a rolling mechanism is the drag force. The EDL and thermophoretic forces have an insignificant effect on particle removal. Based on the results from this study, particles of submicron size can be removed from a wafer surface using higher brush rotation speed and pure deionized (DI) water as the cleaning fluid.

Numerical modelling of unsteady backward‐facing step flow

Abstract Numerical predictions of flow over a two‐dimensional backward‐facing step are made in the laminar flow regime. Solutions are carried out by solving the Navier‐Stokes equations in Hemholtz formulation. The wall vorticity is formulated through the use of the circulation theorem. The unsteady features of the flow development and the effects of Reynolds number on the flow structures are discussed in detail from the variations of streamline patterns, velocities, vorticities, and reattachment and detachment lengths. The current predictions are compared with the available numerical and experimental results. It is found that the comparisons are in reasonable agreement and provide detailed information about the unsteady separation phenomenon for this particular flow field.

Energy conversion from electrolyte concentration gradient using charged nano-pores

ABSTRACT Power generation based on the reversed electro-dialysis (RED) cell is studied both numerically and experimentally in this work. The membrane that separates the concentrated and dilute electrolytes is treated as a charged nano-pore array. Both numerical and experimental results show that the RED cell performance is similar to the typical electrochemical cell having a linearly varied current–voltage relation. The open circuit voltage and short-circuit current depend on the ion selectivity of the nano-pore membrane, which is related to the concentration ratio, pore surface charge density, and pore size. The highest energy conversion efficiencies are approximately 48% and 24% from numerical predictions and experimental measurements, respectively. The reason for this discrepancy is attributed to the inhomogenous pore size and surface charge density distributions of the Al2O3 membrane used in these experiments.