J. M. MacInnes

Stochastic particle dispersion modeling and the tracer‐particle limit

Simulations of spray and particle‐laden flows have commonly relied on random walk models to represent dispersion of liquid or solid particles by turbulent motions in the carrier fluid. Particles respond, through a Lagrangian equation of motion, to the mean fluid velocity, computed simultaneously from an Eulerian solution, and to a random fluctuation velocity. In this paper the performance of available models for the case of tracer particles in dilute concentrations is tested. It is demonstrated that several models in wide use do not preserve the required divergence properties of the imposed mean flow. It is found that the method of sampling the fluctuation velocity in these models leads to a spurious component of mean velocity, causing particles to drift relative to the mean flow. Particles concentrate where the turbulence intensity is minimum (at shear layer edges), and are depleted from regions of high turbulence intensity (near the core of the shear layers). The particle concentration may be up more th...

Density-Viscosity Product of Small-Volume Ionic Liquid Samples Using Quartz Crystal Impedance Analysis

Quartz crystal impedance analysis has been developed as a technique to assess whether room-temperature ionic liquids are Newtonian fluids and as a small-volume method for determining the values of their viscosity-density product, rho eta. Changes in the impedance spectrum of a 5-MHz fundamental frequency quartz crystal induced by a water-miscible room-temperature ionic liquid, 1-butyl-3-methylimiclazolium trifluoromethylsulfonate ([C4mim][OTf]), were measured. From coupled frequency shift and bandwidth changes as the concentration was varied from 0 to 100% ionic liquid, it was determined that this liquid provided a Newtonian response. A second water-immiscible ionic liquid, 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide [C4mim][NTf2], with concentration varied using methanol, was tested and also found to provide a Newtonian response. In both cases, the values of the square root of the viscosity-density product deduced from the small-volume quartz crystal technique were consistent with those measured using a viscometer and density meter. The third harmonic of the crystal was found to provide the closest agreement between the two measurement methods; the pure ionic liquids had the largest difference of approximately 10%. In addition, 18 pure ionic liquids were tested, and for 11 of these, good-quality frequency shift and bandwidth data were obtained; these 12 all had a Newtonian response. The frequency shift of the third harmonic was found to vary linearly with square root of viscosity-density product of the pure ionic liquids up to a value of square root(rho eta) approximately 18 kg m(-2) s(-1/2), but with a slope 10% smaller than that predicted by the Kanazawa and Gordon equation. It is envisaged that the quartz crystal technique could be used in a high-throughput microfluidic system for characterizing ionic liquids.

Prediction of electrokinetic and pressure flow in a microchannel T-junction

It is generally accepted that in simple microchannel flows the electrical double layer at walls is thin enough for “slip velocity” boundary conditions to be used with good approximation. Recent theoretical work by one of the authors has considered the limits of this approach in cases characterized by nonuniform liquid properties and complex channel geometries. In that work, the chemically reacting flow in an arbitrary channel geometry produced by electric potential and pressure differences with heat transfer and electrophoresis is considered. The present work undertakes a broad test of the model approach in a complex channel network geometry, in the case of nonreacting uniform-property liquid. Velocity is measured by particle tracking with correction for electrophoretic motion. Measured and predicted velocities in a three-dimensional experimental T-junction within a network of five channel segments are compared for three cases of steady flow including electrically driven flow, pressure-driven flow, and mi...

Computation of reacting electrokinetic flow in microchannel geometries

Abstract Devices using an electric field to produce flow in microchannel networks have application to precise chemical reaction, analysis and separation. There is a need for accurate computational design tools that can be used with the physically and geometrically complex conditions of practical devices. The equations governing electrokinetic reacting flow are presented together with classical one-dimensional cases that are directly relevant to the flows in electrokinetic devices. This provides the background for an order of magnitude study of the importance of the various terms in each governing equation, both for the conditions of the double-layer flow and for the main flow in the channel outside the double-layer regions. In agreement with previous studies, for channel widths in the range from several microns to hundreds of microns, it is found that representing the double layer using a local one-dimensional solution to produce the boundary conditions at the walls for the main flow is a good approximation. The ‘layer model’ that emerges is consistent with models proposed in previous studies under more restricted conditions than those considered here, where the role of non-uniform ion species concentration is analysed. The model is applied to the example of alternating reacting flow in a tee junction, both for a two-dimensional and a three-dimensional channel section. The case of the same flow driven by pressure instead of electric field is computed for comparison.

Nitrogen Removal from Wastewater by a Catalytic Oxidation Method

The ammonia-containing waste produced in industries is usually characterized by high concentration and high temperature, and is not treatable by biological methods directly. In this study, a hydrophobic Pt/SDB catalyst was first used in a trickle-bed reactor to remove ammonia from wastewater. In the reactor, both stripping and catalytic oxidation occur simultaneously. It was found that higher temperature and higher oxygen partial pressure enhanced the ammonia removal. A reaction pathway, which involves oxidizing ammonia to nitric oxide, which then further reacts with ammonia to produce nitrogen and water, was confirmed. Small amounts of by-products, nitrites and nitrates were also detected in the resultant reaction solution. These compounds came from the absorption of nitrogen oxides. Both the minimum NO2- selectivity and maximum ammonia removal were achieved when the resultant pH of treated water was near 7.5 for a feed of unbuffered ammonia solution.

Micro-PIV simulation and measurement in complex microchannel geometries

Using low numerical aperture lenses to achieve a large field of view when carrying out micron resolution particle image velocimetry (micro-PIV) experiments may result in the out-of-plane resolution being a significant fraction of the overall channel depth. A method to estimate the effect of out-of-plane resolution on micro-PIV velocity measurements is applied to two microchannel flows: a two-dimensional developed flow in a straight channel and a three-dimensional periodic flow in a ribbed channel. The method combines numerical simulation based on computational fluid dynamics (CFD) with an approximation for the contribution to the correlation function arising from partially defocused particles. The flows are then investigated experimentally with measurements obtained on a number of evenly spaced planes. The dominating factor in the comparison between the micro-PIV results and CFD simulations is not the spatial resolution of the experimental data, but instead the precision with which the geometrical parameters can be determined. A methodology is also presented for using micro-PIV results to measure the depth of microfluidic devices. Parabolic fitting of flow profiles allows the top and bottom surfaces of the channel to be located to within 0.2 µm.

Small volume laboratory on a chip measurements incorporating the quartz crystal microbalance to measure the viscosity-density product of room temperature ionic liquids.

A microfluidic glass chip system incorporating a quartz crystal microbalance (QCM) to measure the square root of the viscosity-density product of room temperature ionic liquids (RTILs) is presented. The QCM covers a central recess on a glass chip, with a seal formed by tightly clamping from above outside the sensing region. The change in resonant frequency of the QCM allows for the determination of the square root viscosity-density product of RTILs to a limit of approximately 10 kg m(-2) s(-0.5). This method has reduced the sample size needed for characterization from 1.5 ml to only 30 mul and allows the measurement to be made in an enclosed system.

Dynamics of Electroosmotic Switching of Reacting Microfluidic Flows

This work presents a detailed theoretical and experimental investigation of the dynamics of electroosmotic switching of flows in a microchannel ‘Y’ junction. The work is considered directly relevant to the design of slug flow reactors and to flow stream switching in high throughput microfluidic testing systems. It considers reagents and products with both uniform and non-uniform properties and shows that the normally accepted, simple view of clean switching is rarely achieved in practice. Instead there is contamination of each reagent supply channel with the other reagent. The dependence of this contamination on the properties of the fluids has been addressed in a simple model that serves to illustrate well the observed behaviour of the interfaces formed by switching.

Separate density and viscosity determination of room temperature ionic liquids using dual Quartz Crystal Microbalances

The drive towards cleaner industrial processes has led to the development of room temperature ionic liquids (RTIL) as environmentally friendly solvents. They comprise solely of ions which are liquid at room temperature and with over one million simple RTIL alone it is important to characterize their physical properties using minimal sample volumes. Here we present a dual Quartz Crystal Microbalance (QCM) which allows separate determination of viscosity and density using a total sample volume of only 240µL. Liquid traps were fabricated on the sensing area of one QCM using SU-8 10 polymer with a second QCM having a flat surface. Changes in the resonant frequencies were used to extract separate values for viscosity and density. Measurements of a range of pure RTIL with minimal water content have been made on five different trap designs. The best agreement with measurements from the larger volume techniques was obtained for trap widths of around 50 µm thus opening up the possibility of integration into lab-on-a-chip systems.

Isothermal Modelling of a Combustion System With Swirl: a Computational Study

Isothermal physical scale models, employing an equivalent jet radius, are used to simulate combusting flows in furnaces. Computational fluid dynamics is used to assess the isothermal analogues of combustion systems with and without swirl. The accuracy of representation of the flow in the isothermal analogue depends on the swirl number, S, and the ratio of the equivalent jet radius to chamber radius, re / rc. The representation was good without swirl, S=0, and with swirl for S up to 0.12 with re/ rc >0.16 and for S up to 0.24 with re rc > 0 08. This behaviour was assessed through the normalised recirculation mass flow and the points of separation and reattachment of the main eddy, and should extrapolate to higher swirl numbers as re/rc is reduced. The differences between the combusting and isothermal flows arise from a shifting of the eddies in the enclosure flow.