Transport of Viruses Through Saturated and Unsaturated Columns Packed with Sand

Laboratory-scale virus transport experiments were conducted in columns packed with sand under saturated and unsaturated conditions. The viruses employed were the male-specific RNA coliphage, MS2, and the Salmonella typhimurium phage, PRD1. The mathematical model developed by Sim and Chrysikopoulos (Water Resour Res 36:173–179, 2000) that accounts for processes responsible for removal of viruses during vertical transport in one-dimensional, unsaturated porous media was used to fit the data collected from the laboratory experiments. The liquid to liquid–solid and liquid to air–liquid interface mass transfer rate coefficients were shown to increase for both bacteriophage as saturation levels were reduced. The experimental results indicate that even for unfavorable attachment conditions within a sand column (e.g., phosphate-buffered saline solution; pH = 7.5; ionic strength = 2 mM), saturation levels can affect virus transport through porous media.     

Natural convection in composite systems with phase change materials

In this study, we carried out a numerical simulation of transient heat transfer in a composite passive system consisting of air–phase change material–air, arranged as a rectangular enclosure. The vertical boundaries of the enclosure are isothermal and the horizontal ones adiabatic. The enthalpy formulation with a fixed grid is used to study the process of phase change with liquid–solid interface zone controlled by natural convection. The flow in this zone is simulated by a model based on the Darcy porous medium. The numerical solution of the mathematical model is done using finite difference–control volume algorithm. The influence of the geometrical and thermal parameters is studied. It is found that subcooling coefficient is the most important parameter influencing heat transfer, and for a given subcooling, there is an optimum phase change partition thickness.     

Liquid film characterization in horizontal,annular, two-phase,gas–liquid flow using time-resolved laser-induced fluorescence

A non-intrusive optical technique was developed to provide time-resolved longitudinal and cross-sectional images of the liquid film in horizontal annular pipe flow of air and water, revealing the interfacial wave behavior. Quantitative information on the liquid film dynamics was extracted from the time-resolved images. The planar laser-induced fluorescence technique was utilized to allow for optical separation of the light emitted by the film from that scattered by the air–water interface. The visualization test section was fabricated from a tube presenting nearly the same refractive index as water, which allowed the visualization of the liquid film at regions very close to the pipe wall. Longitudinal images of the liquid film were captured using a high-frame-rate digital video camera synchronized with a high-repetition-rate laser. An image processing algorithm was developed to automatically detect the position of the air–water interface in each image frame. The thickness of the liquid film was measured at two axial stations in each processed image frame, providing time history records of the film thickness at two different positions. Wave frequency information was obtained by analyzing the time-dependent signals of film thickness for each of the two axial positions recorded. Wave velocities were measured by cross-correlating the amplitude signals from the two axial positions. For the film cross-section observations, two high-speed digital video cameras were used in a stereoscopic arrangement. Comparisons with results from different techniques available in literature indicate that the technique developed presents equivalent accuracy in measuring the liquid film properties. Time-resolved images of longitudinal and cross-section views of the film were recorded, which constitute valuable information provided by the technique implemented.     

Experimental study of the gas flow in an air-blasted liquid sheet

An experimental study has been performed to improve the understanding of the initial air–liquid interaction in the near field of an air-blasted breaking water sheet. For the first time, planar laser-induced fluorescence (PLIF) has been used to visualize the air-flow field, seeding the air streams with acetone vapor. Mie scattering from the liquid sheet, together with the acetone fluorescence signal has enabled simultaneous determination of the instantaneous water sheet location and the air-flow structures. The two-phase flow visualization has revealed detachment of the air boundary layer over the air–water interface behind the zones of strong curvature. The pressure field induced by these vortices has been identified as a cause of the enhanced sheet flapping and the instability growth. Received: 30 October 2000/Accepted: 29 March 2001     

A backlighted imaging technique for particle size measurements in two-phase flows

 An imaging technique that uses backlighting has been developed to measure drop sizes in annular two-phase flows with small concentrations of drops in the gas phase. Advantages over conventional photography are realized in that data collection and analysis times are shortened considerably, and consistent unbiased results can be expected. A magnification of 1.9 was used to measure drops above 50 μm. A drop size distribution was obtained for an air–water system as a superficial gas velocity of 30 m/s and a liquid flow of 20 g/s. The data are used to substantiate a theory for the rate of deposition. Received: 6 February 1997/Accepted: 3 February 1998     

Development of a local continuous sampling probe for the equivalence air–fuel ratio measurement. Application to spark ignition engine

This paper is a contribution to the development of an original technique for measuring the in-cylinder equivalence air–fuel ratio. The main objective was to construct an instrument able to furnish instantaneous values of hydrocarbon concentration for many consecutive cycles at a definite location, especially at the spark plug location. The probe is based on a hot-wire-like apparatus, but involves catalytic oxidation on the wire surface in order to be sensitive to the hydrocarbon concentration. In this paper, we present the different steps needed to develop and validate the probe. The first step focuses on the geometric configuration to simplify as much as possible the mass transfer phenomena on the wire. The second step is a parametric study to evaluate the sensitivity, confidence and lifetime of the wire. By physical analysis, we propose a relationship between the electrical signal and the air–fuel equivalence ratio of the sampled gases. The third step is the application of the probe to in-cylinder motored engine measurements, which confirms the ability of the technique to characterise, quantitatively, the homogeneity of the air–fuel mixture, especially during the compression stroke. This work points out that the global sensitivity is estimated at 4 V per unit of equivalence air–fuel ratio and the response time is estimated at about 400 μs. The equivalence air–fuel ratio range is from pure air to 1.2. Experiments show that it is necessary to calibrate the system before use because of the existence of multiple catalysis states. The probe presents advantages associated with its simplicity, its low cost and its direct engine application without any modifications. Received: 1 November 2000 / Accepted: 30 May 2001     

Detection of liquid–vapor–solid triple contact line in two-phase heat transfer phenomena using high-speed infrared thermometry

Heat transfer in complex physical situations such as nucleate boiling, quenching and dropwise condensation is strongly affected by the presence of a liquid–vapor–solid triple contact line, where intense energy transfer and phase change occur. A novel experimental technique for the detection of the liquid–vapor–solid line in these situations is presented. The technique is based on high-speed infrared (IR) thermometry through an IR-transparent silicon wafer heater; hence the name DEPIcT, or DEtection of Phase by Infrared Thermometry. Where the heater surface is wet, the IR camera measures the temperature of the hot water in contact with the heater. On the other hand, where vapor (whose IR absorptivity is very low) is in contact with the heater, the IR light comes from the cooler water beyond the vapor. The resulting IR image appears dark (cold) in dry spots and bright (hot) in wetted area. Using the contrast between the dark and bright areas, we can visualize the distribution of the liquid and gas phases in contact with the heater surface, and thus identify the liquid–vapor–solid contact line. In other words, we measure temperature beyond the surface to detect phases on the surface. It was shown that even small temperature differences (∼1 °C) can yield a sharp identification of the contact line, within about 100 μm resolution. DEPIcT was also shown to be able to detect thin liquid layers, through the analysis of interference patterns.     

Hydrodynamic and mass transfer characteristic of a novel grid-structured plastic packing

The hydraulic and mass transfer characteristic of a novel grid-structured plastic packing is presented. The geometry of this structured packings differ substantially from conventional corrugated structured packings resulting in an open structure which enables a free exchange of vapour and liquid also in horizontal direction. The hydraulic performance has been measured by air–water experiments in a 440 mm diameter column, the mass transfer characteristic has been determined with the absorption system ammonia–air–water. Is is shown that the grid-structured plastic packing is highly efficient, particularly in terms of the hydraulic capacity compared with random plastic packings. Beside this, the main advantage of the packing is the easy handling and installation as well as the low investment costs compared to the well-known conventional corrugated structured packings.     

Measurement of multiple gas-bubble velocities in gas–liquid flows using hot-film anemometry

A dual-probe hot-film anemometry technique has been developed to measure multiple gas-bubble velocities corresponding to different gas-bubble size groups in air–water flows. A data reduction scheme using wavelet analysis combined with a phase-detection technique is used to discriminate the hot-film anemometer output signals into signals corresponding to different bubble size groups. The phase and bubble size discrimination is based on the magnitude and time derivative of the signal, and the streamwise length of the gas bubbles. A cross-correlation between the discriminated signals from the two probes yields an average time difference of arrival of the gas bubbles at the two sensor locations. The velocities are estimated from the distance between the sensors and the time difference of arrival. The mean bubble size is estimated from the chord length distribution. Measurements performed in vertical-up air–water slug flow show the technique to be a viable method for obtaining bubble velocity and size information. The velocity measurements from the hot-film anemometry are corroborated using a high-speed quantitative flow visualization system. Received: 22 December 1999/Accepted: 8 May 2001     

Turbulent time and length scale measurements in high-velocity open channel flows

In high-velocity open channel flows, the measurements of air–water flow properties are complicated by the strong interactions between the flow turbulence and the entrained air. In the present study, an advanced signal processing of traditional single- and dual-tip conductivity probe signals is developed to provide further details on the air–water turbulent level, time and length scales. The technique is applied to turbulent open channel flows on a stepped chute conducted in a large-size facility with flow Reynolds numbers ranging from 3.8E+5 to 7.1E+5. The air water flow properties presented some basic characteristics that were qualitatively and quantitatively similar to previous skimming flow studies. Some self-similar relationships were observed systematically at both macroscopic and microscopic levels. These included the distributions of void fraction, bubble count rate, interfacial velocity and turbulence level at a macroscopic scale, and the auto- and cross-correlation functions at the microscopic level. New correlation analyses yielded a characterisation of the large eddies advecting the bubbles. Basic results included the integral turbulent length and time scales. The turbulent length scales characterised some measure of the size of large vortical structures advecting air bubbles in the skimming flows, and the data were closely related to the characteristic air–water depth Y ₉₀. In the spray region, present results highlighted the existence of an upper spray region for C > 0.95–0.97 in which the distributions of droplet chord sizes and integral advection scales presented some marked differences with the rest of the flow.     

Turbulence structure in the immediate vicinity of the shear-free air–water interface induced by a deeply submerged jet

 A PIV based technique is developed to perform flow measurements in the vicinity of the air–water interface of a submerged confined jet. Both the interface movement and the velocity field immediately beneath it are measured simultaneously. A detailed turbulence structure in the surface influence region is thus obtained. Flow parameters evaluated without and w.r.t. the interface are quantified and compared against previous works obtained using the conventional Eulerian-based instrumentation which do not account for the interface fluctuation, and checked against analytical model characterising the turbulence close to a assumed flat air–water interface. Received: 15 March 1998/Accepted: 19 October 1998     

Visualizing near-surface concentration fluctuations using laser-induced fluorescence

A method for observing near-surface fluctuations in pH caused by a water–air flux of carbon dioxide under conditions of ambient atmospheric carbon dioxide levels is developed and tested. Peaks in fluorescence intensity measured as a function of pH and turbulence are shown to be consistent with predictions from a chemical kinetics model of CO₂ exchange. The square root of the frequency of the pH fluctuations scale linearly with independently measured bulk air–water gas transfer velocities in agreement with surface divergence models for air–water gas transfer. These data indicate that the method proposed here is tracking changes in near-surface CO₂ concentrations. This laser-induced fluorescence method can be used to study the air–water exchange of CO₂ in wind-wave tunnels without the need for elevated CO₂ concentrations in the gas phase.     

Molecular dynamics study of the liquid-vapor interface of lithium bromide aqueous solutions

The molecular dynamics simulations of the liquid–vapor interface of LiBr aqueous solutions were carried out to investigate the structural and thermophysical properties. As concerns the structural properties, the results of molecular dynamics simulation show that the ions exist in the liquid apart from the surface and this tendency becomes strong as the solute concentration is lowered. This phenomenon is due to the desorption of ion. The calculated values such as density or surface tension agree with experimental ones. As concerns thermophysical properties, the number of water molecules in the bulk gas decreases with an increase of the solute concentration. This result represents the depression of vapor pressure. In addition, in order to investigate the dynamic process of water vapor absorption into LiBr aqueous solution, the molecular dynamics simulation under non-equilibrium condition was carried out. The results show that when the solute concentration is low and the temperature is also low, almost all incident water molecules become trapped at the solution surface and then easily diffuse into the bulk liquid, and when the solute concentration is high and temperature is also high, most incident water molecules become trapped at the solution surface, and the sequent processes are very complicated. Received on 28 September 1998     

Convective and diffusive surfactant transfer in multiphase liquid systems

Laser interferometry was used to investigate diffusive and convective mass transfer in a multicomponent fluid mixture with a liquid–liquid or liquid–gas interface. For this purpose, an immobile gas bubble or insoluble fluid droplet, having the shape of a short cylinder with a free lateral surface, was inserted into a thin liquid layer. In the case of non-uniform distribution of the dissolved surfactant component, the Marangoni convection near the drop/bubble was initiated by the surface tension inhomogeneities, depending on the surfactant concentration. The applied experimental techniques allowed us to study the structure and evolution of the convective flows and concentration fields in a liquid layer, which due to its small thickness were nearly two-dimensional. Making use of both the vertical and horizontal orientation of the liquid layer, we investigated the mass transfer process at different levels of the interaction between gravity and capillary forces. During the experiments, we detected new solutocapillary phenomena, which were found to be caused by oscillatory regimes of solutal convection occurring around air bubbles and chlorobenzene drops in heterogeneous aqueous solutions of alcohol with a vertical surfactant concentration gradient. The role of the oscillatory instability in the processes of drop saturation by the surfactant from its water solution and an inverse process of surfactant extraction from the drop into the surrounding homogeneous fluid (water) was determined. A reasonable explanation for the driving mechanisms of the discovered effects has been proposed.     

Dynamic film thickness between bubbles and wall in a narrow channel

The present paper describes a novel technique to characterize the behavior of the liquid film between gas bubbles and the wall in a narrow channel. The method is based on the electrical conductance. Two liquid film sensors are installed on both opposite walls in a narrow rectangular channel. The liquid film thickness underneath the gas bubbles is recorded by the first sensor, while the void fraction information is obtained by measuring the conductance between the pair of opposite sensors. Both measurements are taken on a large two-dimensional domain and with a high speed. This makes it possible to obtain the two-dimensional distribution of the dynamic liquid film between the bubbles and the wall. In this study, this method was applied to an air–water flow ranging from bubbly to churn regimes in the narrow channel with a gap width of 1.5 mm.     

Flow patterns and interfacial velocities near a moving contact line

Experimental data on velocity fields and flow patterns near a moving contact line is shown to be at variance with existing hydrodynamic theories. The discrepancy points to a new hydrodynamic paradox and suggests that the hydrodynamic approach may be incomplete and further parameters or forces affecting the surfaces may have to be included. A contact line is the line of intersection of three phases: (1) a solid, (2) a liquid, and (3) a fluid (liquid or gas) phase. A moving contact line develops when the contact line moves along the solid surface. A flat plate moved up and down, inside and out of a liquid pool defines a simple, reliable experimental model to characterize dynamic contact lines. Highlighted are three important conclusions from the experimental results that should be prominent in the development of new theoretical models for this flow. First, the velocity along the streamline configuring the liquid–fluid interface is remarkably constant within a distance of a couple of millimeters from the contact line. Second, the relative velocity of the liquid–fluid interface, defined as the ratio of the velocity along the interface to the velocity of the solid surface, is independent of the solid surface velocity. Third, the relative interface velocity is a function of the dynamic contact angle.     

Fourier transform profilometry for water waves: how to achieve clean water attenuation with diffusive reflection at the water surface?

We present a study of the damping of capillary-gravity waves in water containing pigments. The practical interest comes from a recent profilometry technique (FTP for Fourier Transform Profilometry) using fringe projection onto the liquid-free surface. This experimental technique requires diffusive reflection of light on the liquid surface, which is usually achieved by adding white pigments. It is shown that the use of most paint pigments causes a large enhancement of the damping of the waves. Indeed, these paints contain surfactants which are easily adsorbed at the air–water interface. The resulting surface film changes the attenuation properties because of the resonance-type damping between capillary-gravity waves and Marangoni waves. We study the physicochemical properties of coloring pigments, showing that particles of the anatase (TiO₂) pigment make the water surface light diffusive while avoiding any surface film effects. The use of the chosen particles allows to perform space-time resolved FTP measurements on capillary-gravity waves, in a liquid with the damping properties of pure water.     

Detection of liquid–metal, free-surface flow using the DLP measurement technique

Novel accelerator applications favor free-surface liquid–metal flows, in which the liquid acts both as a target producing secondary particles but also to remove efficiently the heat deposited. A crucial aspect for the operation is the continuous monitoring of both shape and position of the liquid’s surface. This demands, in a nuclear environment, a non-intrusive measurement technique with high temporal and spatial resolution. In this context, the double-layer projection (DLP) technique based on geometric optics has been developed, allowing one to detect either point-wise or area-wise the shape and position of the nearly totally reflecting liquid–metal surface. The DLP technique employs a laser beam projected through a coplanar glass plate to the surface from which it is reflected to the plate again. Beam locations captured by means of a camera permit the position and shape of the surface to be reconstructed. The parameters affecting the resolution and performance of the DLP technique are discussed. Additionally, validation studies using static and moving objects of pre-defined shape are conducted, exhibiting spatial and temporal resolutions of 300 μm and 100 Hz, respectively. Finally, the DLP system is applied to perform measurements of a circular hydraulic jump (CHJ) in a liquid metal. The DLP system has proved the capability to measure the jump both qualitatively and quantitatively. Additionally, the experiments identified, at high Reynolds numbers, the existence of a two-step jump. The analysis of spectral data of the DLP surface measurements shows clearly that, at the outer radius, gravity waves occur. Also, contributions from the pump oscillations were found, demonstrating the high performance of the DLP system.     

Measurement of gas transfer across wind-forced wavy air–water interfaces using laser-induced fluorescence

Optical distortions have previously prevented non-intrusive measurements of dissolved oxygen concentration profiles by Laser induced fluorescence (LIF) to within 200 μm of the air–water interface. It is shown that by careful experimental design, reliable measurements can be obtained within 28 μm of moving air–water interfaces. Consideration of previously unidentified optical distortions in LIF imagery due to non-linear effects is presented that is critical for robust LIF data processing and experimental design. Phase resolved gas flux measurements have now been accomplished along wind forced microscale waves and indicate that the highest mean gas fluxes are located in the wave troughs. The local mean oxygen fluxes as determined by LIF techniques can be reconciled to within 40% of those obtained by bulk measurement in the water. These data provide a new perspective on wind-wave enhancement of low solubility gas transfer across the air–water interface.