A multiphase,micro-scale PIV measurement technique for liquid film velocity measurements in annular two-phase flow

Prediction methods for two-phase annular flow require accurate knowledge of the velocity profile within the liquid film flowing at its perimeter as the gradients within this film influence to a large extent the overall transport processes within the entire channel. This film, however, is quite thin and variable and traditional velocimetry methods have met with only very limited success in providing velocity data. The present work describes the application of Particle Image Velocimetry (PIV) to the measurement of velocity fields in the annular liquid flow. Because the liquid is constrained to distances on the order of a millimeter or less, the technique employed here borrows strategies from micro-PIV, but micro-PIV studies do not typically encounter the challenges presented by annular flow, including very large velocity gradients, a free surface that varies in position from moment to moment, the presence of droplet impacts and the passage of waves that can be 10 times the average thickness of the base film. This technique combines the seeding and imaging typical to micro-PIV with a unique lighting and image processing approach to deal with the challenges of a continuously varying liquid film thickness and interface. Mean velocity data are presented for air–water in two-phase co-current upward flow in a rectangular duct, which are the first detailed velocity profiles obtained within the liquid film of upward vertical annular flow to the authors’ knowledge. The velocity data presented here do not distinguish between data from waves and data from the base film. The resulting velocity profiles are compared with the classical Law of the Wall turbulent boundary layer model and found to require a decreased turbulent diffusivity for the model to predict well. These results agree with hypotheses previously presented in the literature.     

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     

Simultaneous velocity field measurements in two-phase flows for turbulent mixing of sprays by means of two-phase PIV

This paper describes a novel derivative of the PIV method for measuring the velocity fields of droplets and gas phases simultaneously using fluorescence images rather than Mie scattering images. Two-phase PIV allows the simultaneous and independent velocity field measurement of the liquid phase droplets and ambient gas in the case of two-phase flow mixing. For phase discrimination, each phase is labelled by a different fluorescent dye: the gas phase is seeded with small liquid droplets, tagged by an efficient fluorescent dye while the droplets of the liquid phases are tagged by a different fluorescent dye. For each phase, the wavelength shift of fluorescence is used to separate fluorescence from Mie scattering and to distinguish between the fluorescence of each phase. With the use of two cross-correlation PIV cameras and adequate optical filters, we obtain two double frame images, one for each phase. Thus standard PIV or PTV algorithms are used to obtain the simultaneous and independent velocity fields of the two phases. Because the two-phase PIV technique relies on the ability to produce two simultaneous and independent images of the two phases, the choice of the labelling dyes and of the associated optical filter sets is relevant for the image acquisition. Thus a spectroscopic study has been carried out to choose the optimal fluorescent dyes and the associated optical filters. The method has been evaluated in a simple two-phase flow: droplets of 30–40 μm diameter, produced by an ultrasonic nozzle are injected into a gas coflow seeded with small particles. Some initial results have been obtained which demonstrate the potential of the method.     

Limits and accuracy of the Stereo-LFC PIV technique and its application to flows of industrial interest

Particle image velocimetry with local field correction (LFC PIV) has been tested in the past to obtain two components of velocity in a two dimensional domain (2D2C). When compared to conventional correlation based algorithms, this advanced technique has shown improvements in three important aspects: robustness, resolution and ability to cope with large displacements gradients. A further step in the development of PIV algorithms consists in the combination of LFC with the stereo technique, which is able to obtain three components of velocity in a plane (2D3C PIV). In this work this combination is implemented and its performance is evaluated carrying out the following two different tasks:

Existence and properties of flow structure waves in two-phase bubbly flows

Structure waves occur in two-phase flows because one phase drifts with respect to the other, the drift flux being primarily a function of the flow structure. The wave properties provide information on the closure laws required in engineering models. Experiments made with an air-water bubbly mixture flowing in a vertical annular test section are reported. Void fluctuations involving structure disturbances were detected by capacitance measurements, the effect of individual bubbles being always negligible. Only low frequency disturbances were present, high frequency disturbances being strongly damped. Within the low frequency range, the wave velocity is independent of the frequency, and the damping is small. The wave velocity is always comprised between the average liquid velocity and the average gas velocity.

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Résumé Des ondes de structure apparaissent dans les écoulements diphasiques parce que les phases n'ont pas la même vitesse moyenne, le flux de glissement correspondant étant essentiellement fonction de la structure de l'écoulement. Les propriétés des ondes apportent des informations sur les lois de fermetures requises par les modèles pratiques. On présente les résultats d'expériences effectuées avec un écoulement eau-air à bulles dans une section annulaire verticale. Les fluctuations de taux de vide dues aux perturbations de structure sont détectées par mesure de capacité (l'effect d'une bulle unique est toujours négligeable). Seules les perturbations de basse fréquence peuvent subsister, les perturbations de haute fréquence étant fortement amorties. Dans la gamme basse fréquence, la vitesse des ondes est indépendante de la fréquence et l'amortissement est faible. La vitesse des ondes est toujours comprise entre la vitesse moyenne du liquide et la vitesse moyenne du gaz.

     

Turbulent cascade modeling of single and bubbly two-phase turbulent flows

The analysis of turbulent two-phase flows requires closure models in order to perform reliable computational multiphase fluid dynamics (CMFD) analyses. A spectral turbulence cascade-transport model, which tracks the evolution of the turbulent kinetic energy from large to small liquid eddies, has been developed for the analysis of the homogeneous decay of isotropic single and bubbly two-phase turbulence. This model has been validated for the decay of homogeneous, isotropic single and two-phase bubbly flow turbulence for data having a 5 mm mean bubble diameter. The Reynolds number of the data based on bubble diameter and relative velocity is approximately 1400.     

A laser-fluorescence technique for turbulent two-phase flow measurements

A non-intrusive measurement technique has been developed for accurate determination of gas and particle velocities in a turbulent two-phase flow field. The principle of the technique is based on the discrimination between the scattered light from particles and the fluorescence emission from particles coated with a fluorescent dye. A high-powered, argon-ion based, single-channel, on-axis backscatter laser-Doppler velocimetry system was used. The fluorescent dye was Rhodamine 6G. A study of the gas-solid two-phase flow behaviour in the freeboard of a cold gas-fluidized bed was undertaken. The solid phase contained two particle groups: bed material (sand) and fuel particles (wood). Measurements of the axial velocity and turbulence intensity distributions of the gas phase and both particle groups within the solid phase were made along the column centre and across the freeboard. Excellent discrimination of velocities from the two phases and from the two particle groups within the solid phase was achieved.     

Advances in studies on two-phase turbulence in dispersed multiphase flows

Particle/droplet/bubble fluctuation and dispersion are important to mixing, heat and mass transfer, combustion and pollutant formation in dispersed multiphase flows, but are insufficiently studied before the 90 years of the last century. In this paper, the present author reports his systematic studies within nearly 20 years on two-phase turbulence in dispersed multiphase flows, including particle fluctuation in dilute gas-particle and bubble-liquid flows, particle-wall collision effect, coexistence of particle turbulence and inter-particle collisions, fluid turbulence modulation due to the particle wake effect and validation of the two-fluid RANS modeling using large-eddy simulation.     

Light extinction technique for void fraction measurements in bubbly flow

 The analysis of the scattering induced by a dispersion of gas bubbles in a liquid medium on a collimated, monochromatic light beam, traversing the two-phase flow, allows for the direct measurement of the 2-D distribution of the line-average of the interfacial area density. The 2-D distribution of the line-average of the void fraction is deduced from that of the interfacial area density through an image processing algorithm. To demonstrate the technique, experiments are performed in a pool of water injected with air and illuminated with a CW argon ion laser. Since the bubble diameters range from a fraction of a millimeter to a few millimeters, the scattering processes are entirely in the Mie range. The limits of applicability of the technique and the measurement uncertainty are discussed. The results compare favorably with level-swell based measurements used as a reference. Received: 14 February 1997/Accepted: 4 February 1998     

On image pre-processing for PIV of single- and two-phase flows over reflecting objects

A novel image pre-processing scheme for PIV of single- and two-phase flows over reflecting objects which does not require the use of additional hardware is discussed. The approach for single-phase flow consists of image normalization and intensity stretching followed by background subtraction. For two-phase flow, an additional masking step is added after the background subtraction. The effectiveness of the pre-processing scheme is shown for two examples: PIV of single-phase flow in spacer-filled channels and two-phase flow in these channels. The pre-processing scheme increased the displacement peak detectability significantly and produced high quality vector fields, without the use of additional hardware.     

Phase discrimination method for simultaneous two-phase separation in time-resolved stereo PIV measurements

A phase discrimination method for two-phase PIV is presented that is capable of simultaneously separating the two phases from time-resolved stereoscopic PIV images taken in a particle-laden jet. The technique developed expands on previous work done by Khalitov and Longmire (Exp Fluids 32:252–268, 2002), where by means of image processing techniques, a raw two-phase PIV image can be separated into two single-phase images according to particle size and intensity distributions. The technique is expanded through the use of three new image processing algorithms to separate particles of similar size (up to an order of magnitude better than published work) for fields of view much larger than previously considered. It also addresses the known problem of noisy background images produced by high-speed CMOS cameras, which makes the particle detection and separation from the noisy background difficult, through the use of a novel fast Fourier transform background filter.     

A turbulence model for inclined, bubbly flows

A new turbulence model for the flow of a two phase (liquid-liquid) flow in an inclined pipe is presented. An eddy viscosity is used to model the effects of shear induced turbulence and bubble induced turbulence. The cross-pipe momentum transport arising from the buoyant rise of bubbles across the axial flow is also modelled. Numerical simulations have been carried out in both one and two dimensions. One and two dimensional numerical simulations are presented.On leave from the University of Leeds, Leeds LS2 9JT, U.K.     

A pulsed-wire technique for velocity and temperature measurements in natural convection flows

A pulsed-wire probe based on the use of one or two parallel wires, capable of measuring the velocity and the temperature in natural convection flows is described. These measurements are based on the analysis of the relaxation response of a pulsing wire, submitted to a very short electrical pulse. The analysis of the temperature variation on an optional second receiver wire, gives information about the velocity direction. The implementation simplicity of this probe, its good spatial precision, the lack of thermal contamination of the flow, as well as the possibility of obtaining simultaneous velocity and temperature measurements, allow the integration of the device in a multi-point measurement network, capable to deliver thermal and dynamic cartographies of unsteady convection flows.     

PIV measurements in multiphase flow with nominally high concentration of the solid phase

Optical PIV measurements were aimed at studying fluid mechanical problems in a coal flotation process. Since the associated solid phase (coal/mineral) develops a high degree of light extinction, a solution had to be found for allowing a high concentration of the solid phase without losing the optical access for the PIV light sheet. The solution consisted in simulating the solid phase with glass spheres whose refractive index was matched by an organic liquid in the two-phase system.     

An incompressible smoothed particle hydrodynamics scheme for Newtonian/non-Newtonian multiphase flows including semi-analytical solutions for two-phase inelastic Poiseuille flows

An incompressible smoothed particle hydrodynamics (ISPH) method is developed for the modeling of multiphase Newtonian and inelastic non-Newtonian flows at low density ratios. This new method is the multiphase extension of Xenakis et al, J. Non-Newtonian Fluid Mech., 218, 1-15, which has been shown to be stable and accurate, with a virtually noise-free pressure field for single-phase non-Newtonian flows. For the validation of the method a semi-analytical solution of a two-phase Newtonian/non-Newtonian (inelastic) Poiseuille flow is derived. The developed method is also compared with the benchmark multiphase case of the Rayleigh Taylor instability and a submarine landslide, thereby demonstrating capability in both Newtonian/Newtonian and Newtonian/non-Newtonian two-phase applications. Comparisons with analytical solutions, experimental and previously published results are conducted and show that the proposed methodology can accurately predict the free-surface and interface profiles of complex incompressible multi-phase flows at low-density ratios relevant, for example, to geophysical environmental applications.     

A multi-scale architecture for multi-scale simulation and its application to gas-solid flows

A multi-scale hardware and software architecture implementing the EMMS （energy-minimization multi-scale） paradigm is proven to be effective in the simulation of a two-dimensional gas-solid suspension. General purpose CPUs are employed for macro-scale control and optimization, and many integrated cores （MlCs） operating in multiple-instruction multiple-data mode are used for a molecular dynamics simulation of the solid particles at the meso-scale. Many cores operating in single-instruction multiple- data mode, such as general purpose graphics processing units （GPGPUs）, are employed for direct numerical simulation of the fluid flow at the micro-scale using the lattice Boltzmann method. This architecture is also expected to be efficient for the multi-scale simulation of other comolex systems.     

An interface-capturing method for incompressible two-phase flows. Validation and application to bubble dynamics

We report on the development and applications of an interface-capturing method aimed at computing three-dimensional incompressible two-phase flows involving high density and viscosity ratios, together with capillary effects. The numerical approach borrows some features to the Volume of Fluid method (since it is essentially based on the transport of the local volume fraction of the liquid) as well as to the Level Set technique (as no explicit reconstruction of the interface is carried out). The transport of the volume fraction is achieved by using a flux-limiting Zalesak scheme and the fronts are prevented from spreading in time by a specific strategy in which the velocity at nodes crossed by the interface is modified to keep the thickness of the transition region constant. As shown on several test cases, this algorithm allows the interface to deform properly while maintaining the numerical thickness of the transition region within three computational cells whatever the structure of the local flow field. The full set of governing equations is then used to investigate some fundamental aspects of bubble dynamics. More precisely we focus on the evolution of shape and rise velocity of a single bubble over a wide range of physical parameters and on head-on and side-by-side interactions between two rising bubbles.