Full-field bubbly flow velocity measurements using a multiframe particle tracking technique

The study is an examination of two-phase dispersed air bubble flow about a cylindrical conductor emitting a constant heat flux. The technique of Particle Image Velocimetry is utilized in order to obtain a full-field non-invasive measurement of the resulting bubbly flow velocity field. The employed approach utilizes a flow visualization technique in which the instantaneous velocity profile of a given flow field is determined by digitally recording particle or bubble images within the flow over multiple successive video frames and then conducting a completely computational analysis of the data. The use of particle tracking algorithms which perform a point-by-point matching of seed images from one frame to the next allows construction of particle or bubble pathlines and instantaneous velocity field. Results were initially obtained for a synthetically created flow field and a single phase liquid convective field seeded with flow-following tracer particles. The method was additionally extended to measurements within a gas/liquid system in which bubble rise velocities over a substantial two-dimensional flow area were determined in order to demonstrate the effectiveness of the developed digital data acquisition and analysis methodology.A version of this paper was presented at the 12th Symposium on Turbulence, University of Missouri-Rolla, 24–26 September, 1990     

Local liquid velocity measurements in air-water bubbly flow

Local measurements of axial liquid velocity were performed for vertical upward air-water bubbly flow in a 101.6-mm inner-diameter round pipe by using a laser Doppler anemometer (LDA) and a hot-film anemometer (HFA). The data reduction approaches for both the LDA and HFA are discussed in detail. A threshold scheme with the information of local void fraction and velocity distribution in single-phase flow was applied to the LDA to approximately discriminate liquid velocity signals from those of the bubble interface velocity. Furthermore, a formulation was given to account for the effect of the bubble relative velocity on the liquid in the front and wake regions of the bubbles. For the HFA, an amplitude threshold scheme and a slope criterion were used to extract liquid velocity information. To reduce the measurement uncertainty, the experiments were performed in flow conditions where the area-averaged void fraction was less than 20%. The experimental results showed satisfactory agreement between the liquid volumetric flow rates calculated by area integration of the local liquid velocity and void fraction measurements, and the measured value by a magnetic flow meter. Also, the area-averaged relative velocity between the gas and liquid phases obtained from the current measurements agreed well with previous research.     

A light attenuation technique for void fraction measurement of microbubbles

 A non-intrusive technique to measure the two-dimensional distribution of line averaged void fraction in a two-phase flow is discussed. A CCD camera is used to measure the attenuation of light as it passes through a bubbly flow, and this attenuation is related to the bubble concentration. The technique is appropriate for microbubbly flows where the bubble size is much smaller than the area imaged by a single pixel and where there are many bubbles attenuating light within each pixel. The measurement system is calibrated by using a two-dimensional line source microbubble plume as a reference. Revised: 30 March 2000/Accepted: 14 April 2000     

The interrelation between void fraction fluctuations and flow patterns in two-phase flow

A fast response, linearized X-ray void measurement system has been used to obtain statistical measurements in normally fluctuating air-water flow in a rectangular channel. It is demonstrated that the probability density function (PDF) of the fluctuations in void fraction may be used as an objective and quantitative flow pattern discriminator for the three dominant patterns of bubbly, slug, and annular flow. This concept is applied to data over the range of 0.0 to 37 m/sec mixture velocities to show that slug flow is simply a transitional, periodic time combination of bubbly and annular flows. Film thicknesses calculated from the PDF data are similar in magnitude in both slug and annular flows. Calculation of slug length and residence time ratios along with bubble lengths in slug flow are also readily obtainable from the statistical measurements. Spectral density measurements showed bubbly flow to be stochastic while slug and annular flows showed periodicities correlatable in terms of the liquid volume flux.     

A pressure–velocity coupling approach for high void fraction free surface bubbly flows in overset curvilinear grids

A methodology for improved robustness in the simulation of high void fraction free surface polydisperse bubbly flows in curvilinear overset grids is presented. The method is fully two‐way coupled in the sense that the bubbly field affects the continuous fluid and vice versa. A hybrid projection approach is used in which staggered contravariant velocities at cell faces are computed for transport and pressure–velocity coupling while the momentum equation is solved on a collocated grid arrangement. Conservation of mass is formulated such that a strong coupling between void fraction, pressure, and velocity is achieved within a partitioned approach, solving each field separately. A pressure–velocity projection solver is iterated together with a predictor stage for the void fraction to achieve a robust coupling. The implementation is described for general curvilinear grids detailing particulars in the neighborhood to overset interfaces or a free surface. A balanced forced method to avoid the generation of spurious currents is extended for curvilinear grids. The overall methodology allows simulation of high void fraction flows and is stable even when strong packing forces accounting for bubble collisions are included. Convergence and stability in one‐dimensional (1D) and two‐dimensional (2D) configurations is evaluated. Finally, a full‐scale simulation of the bubbly flow around a flat‐bottom boat is performed demonstrating the applicability of the methodology to complex problems of engineering interest. Copyright © 2015 John Wiley & Sons, Ltd.     

Optical fibre probe measurements of bubbly flow in hydraulic jumps

This paper describes measurements of void fractions, bubble frequencies and bubble sizes in hydraulic jumps with Froude numbers 2.0, 2.4, 3.7 and 4.8. In each case data were obtained with a dual-tip optical fibre probe at a large number of points throughout the jump. Across the lower part of the flow, dominated by air entrainment into a region of turbulent shear, void fractions follow a Gaussian distribution. In the upper region, dominated by interactions with the free surface, the void fraction follows the form of an error function. The intersection between these two profiles provides a well-defined boundary between the two regions. Comparisons are made with measurements at higher Froude numbers [by Chanson, H., Brattberg, T., 2000. Experimental study of the air–water shear flow in a hydraulic jump. International Journal of Multiphase Flow 26, 583–607] revealing a very large measure of compatibility between the two sets of data.     

Turbulence measurements in the bubbly flow region of hydraulic jumps

A hydraulic jump is characterized by a highly turbulent flow with macro-scale vortices, some kinetic energy dissipation and a bubbly two-phase flow structure. New air–water flow measurements were performed in a large-size facility using two types of phase-detection intrusive probes: i.e. single-tip and double-tip conductivity probes. These were complemented by some measurements of free-surface fluctuations using ultrasonic displacement meters. The void fraction measurements showed the presence of an advective diffusion shear layer in which the void fractions profiles matched closely an analytical solution of the advective diffusion equation for air bubbles. The free-surface fluctuations measurements showed large turbulent fluctuations that reflected the dynamic, unsteady structure of the hydraulic jumps. The measurements of interfacial velocity and turbulence level distributions provided new information on the turbulent velocity field in the highly-aerated shear region. The velocity profiles tended to follow a wall jet flow pattern. The air–water turbulent integral time and length scales were deduced from some auto- and cross-correlation analyses based upon the method of Chanson [H. Chanson, Bubbly flow structure in hydraulic jump, Eur. J. Mech. B/Fluids 26 (3) (2007) 367–384], providing the turbulent scales of the eddy structures advecting the air bubbles in the developing shear layer. The length scale L_xz is an integral air–water turbulence length scale which characterized the transverse size of the large vortical structures advecting the air bubbles. The experimental data showed that the dimensionless integral turbulent length scale L_xz/d₁ was closely related to the inflow depth: i.e. L_xz/d₁ = 0.2–0.8, with L_xz increasing towards the free-surface.     

Wall shear stress and void fraction in Poiseuille bubbly flows: Part I: simple analytic predictions

Upward, co-current bubbly flows in a vertical rectangular duct are investigated at low liquid Reynolds numbers. The conditions considered are such that the pseudo-turbulent stresses remain negligible compared to the viscous stresses. The void fraction transverse distribution is idealised as step-functions and is then inserted in the conservation equations supplemented by appropriate closure laws. Analytical expressions are then obtained for the axial velocity profiles, for the lineic gas fraction and for the wall friction. The sensitivity of these quantities to the void distribution, characterised by the void fraction and the width of the three layers introduced, is discussed. It is shown that differential buoyancy effects govern the modification of the liquid velocity profiles. Notably, void peaking near walls is able to induce a wall shear stress many times higher than its single-phase flow counterpart at the same liquid flow rate. Also, the presence of a near wall region free of gas favours the onset of downward directed secondary flows. All these features correspond to experimental observations, and a few quantitative comparisons are also presented which support the validity of the model even in case of void coring. A companion paper (part II) will be devoted to systematic comparisons between predictions and experiments in the case of axisymmetric Poiseuille bubbly flows.     

Two-phase flow pattern identification in a tube bundle based on void fraction and pressure measurements,with emphasis on churn flow

Two-phase flow are frequently encountered in the industry. In particular, in steam generators of nuclear plants, water is heated so that at the top of the generator an important fraction of water flows as vapor. In this upper part, a rising co-current two phase flow transverse to the tube bundle takes place. Fluids exert significant forces on the tubes in this area which highly depend on the two-phase flow pattern. Thus, as a prerequisite, it is essential to gather information on the flow conditions associated with the different two-phase flow patterns, which can be bubbly, intermittent, or annular. Then we must analyze the potentially dangerous flow patterns. This paper presents an experimental campaign aimed at characterizing those flow patterns for a rising co-current transverse flow in a tube bundle representative of the geometry in a steam generator. A new methodology based on the understanding of key contributions to vertical two-phase flow pattern maps in tube bundles is proposed that leads to a more complete flow pattern map. Finally, the paper focuses on the churn flow, which is the flow pattern for which significant pressure fluctuations occur. For this pattern, important damages could be expected on the tubes of a steam generator. Different kinds of pressure fluctuations are observed at different frequencies depending on the flow rates and the location in the test section.     

Estimating void fraction in a hydraulic jump by measurements of pixel intensity

A hydraulic jump is a sudden transition from supercritical to subcritical flow. It is characterized by a highly turbulent roller region with a bubbly two-phase flow structure. The present study aims to estimate the void fraction in a hydraulic jump using a flow visualization technique. The assumption that the void fraction in a hydraulic jump could be estimated based on images’ pixel intensity was first proposed by Mossa and Tolve (J Fluids Eng 120:160–165, 1998). While Mossa and Tolve (J Fluids Eng 120:160–165, 1998) obtained vertically averaged air concentration values along the hydraulic jump, herein we propose a new visualization technique that provides air concentration values in a vertical 2-D matrix covering the whole area of the jump roller. The results obtained are found to be consistent with new measurements using a dual-tip conductivity probe and show that the image processing procedure (IPP) can be a powerful tool to complement intrusive probe measurements. Advantages of the new IPP include the ability to determine instantaneous and average void fractions simultaneously at different locations along the hydraulic jump without perturbing the flow, although it is acknowledged that the results are likely to be more representative in the vicinity of sidewall than at the center of the flume.     

Void fraction and pressure fluctuations of bubbly flow in a vertical annular channel

In this investigation some hydrodynamic characteristics of two phase, two component, air water bubbly flow in a vertical annulus were studied. In particular, the void fraction profiles, and the pressure fluctuations were measured by the electrical resistivity probe and a capacitive type differential transducer respectively. These measurements were assessed under various system parameters, viz the air and water flux, the perforation ratio (Area of holes/channel cross sectional area) and the dimensionless axial distance. In addition, the pressure drop calculated from the void fraction measurements was in very good agreement with the corresponding one measured by the pressure transducers.List of symbols D _eq equivalent diameter of the annular channel (m) - j flux (discharge/channel cross sectional area) (m/s) - m mass flow rate (kg/s) - P pressure (Pa) - AP static pressure difference along the test section (Pa) - P pressure fluctuations (Pa) - P ^* dimensionless pressure (P _m/P _S.P. ) - P dimensionless pressure fluctuations (P _max /P _T.P. ) - r radius (m) - z axial distance (m) Greek symbols void fraction - dimensionless axial distance (Z/Dimeq) - perforation ratio (area of holes/channel cross sectional area) - density (kg/m3) - time (s) - dimensionless radial distance (r–r _i )/(r _o-r _i ) Suffix g gas - i inner - L liquid - m mean - Max Maximum - O outer - S.P. single-phase - T.P. two-phase     

Non-invasive measurement of void fraction and liquid temperature in microchannel flow boiling

Past thermometry research for two-phase microfluidic systems made much progress regarding wall temperature distributions, yet the direct measurement of fluid temperature has received little attention. This paper uses a non-invasive two-dye/two-color fluorescent technique to capture fluid temperature along with local liquid fraction in a two-phase microflow generated by injecting air into a heated microchannel. The fluorescent emission of Rhodamine 110 and Rhodamine B, measured with photodiodes, is used to obtain local liquid temperature (±3°C) and void fraction (±2% full-scale) over a temperature range from 45 to 100°C. Arrays of these sensors can significantly expand the set of measurable flow parameters to include bubble/slug frequency, size, velocity, and growth rates in addition to mapping the local liquid temperature and void fraction.     

Full-field bubbly flow velocity measurements by digital image pulsed laser velocimetry

Digital Pulsed Laser Velocimetry (DPLV) is a full-field, two dimensional, noninvasive, quantitative flow visualization technique. The technique described here includes the novel use of direct digitization of two-phase bubbly flow images using a high resolution imaging system. The image data is stored for further analysis by new image processing and analysis software developed for flow experiments.In the technique, ten consecutive frames of data separated by a time increment of 150 ms, are recorded. Each of these ten frames contains the images of bubbles at that one instant of time. A program smooths the instantaneous image and calculates bubble parameters. Another program matches the bubbles from each of the frames into tracks of bubbles through time. This program uses a statistical technique to determine the best possible path of the bubbles.The ability of pulsed laser velocimetry to capture simultaneous and quantitative rather than qualitative information along with these image processing techniques gives the experimentalist a powerful tool to perform flow visualization and analysis.     

The bubble-slug flow pattern transition and instabilities of void fraction waves

The propagation of spontaneous void fraction disturbances in a nitrogen-water flow has been studied through the statistical analysis of conductivity probe signals, for void fractions ranging from 0.1 to 0.5 and including the bubble-slug transition. The power spectral density function and the standard deviation of the void fraction have been computed for each probe, as well as the system phase factor (related to the wave velocity), the coherence function and the system gain factor between each pair of consecutive probes as functions of frequency. For bubble flow, the results are compatible with the results obtained by other authors. The transition from bubble to slug flow is associated with void fraction wave instabilities. Two kinds of instabilities seem to occur simultaneously: amplitude increase (system gain factor > 1) and wave-breaking.     

Comparison of void fraction correlations for different flow patterns in horizontal and upward inclined pipes

A comparison of the performance of 68 void fraction correlations based on unbiased data set (2845 data points) covering wide range of parameters than previous assessments was made. A comprehensive literature search was undertaken for the available void fraction correlations and experimental void fraction data. After systematically refining the data, the performance of the correlations in correctly predicting the diverse data sets was evaluated. Comparisons between the correlations were made and appropriate recommendations drawn. The analysis showed that most of the correlations developed are very restricted in terms of handling a wide variety of data sets. Based on the observations made, an improved void fraction correlation which could acceptably handle all data sets regardless of flow patterns and inclination angles was suggested. It was shown that this correlation has the best predictive capability than all the correlations considered in this study.     

Wall shear stress and void fraction in Poiseuille bubbly flows: Part II: experiments and validity of analytical predictions

In a companion paper, a simple analytical formulation has been established which provides the wall shear stress in laminar bubbly flows for idealised transverse void fraction distributions. In the present paper, this approach is applied to Poiseuille bubbly flows in circular ducts. New measurements of the void fraction profiles and wall friction angular distribution in a pipe are presented for a wide range of flow parameters. Approximating the void profiles by step-functions allows us to evaluate the wall friction with the above mentioned model. Results are shown to agree satisfactorily with measurements. Notably, negative wall shear stress and wall shear stress much higher than their single-phase flow counterpart at the same liquid flow rate are recovered. Therefore, the principal mechanisms responsible for friction modification are captured with this simple model.     

Fiber-optic probe measurements of void fraction and bubble size distributions beneath breaking waves

Experiments were performed to determine the accuracy of single-tip fiber-optic probes for making simultaneous measurements of the void fraction and bubble size distributions under breaking waves. Tests in a vertical bubble column showed that the normalized RMS error in the void fraction measurements was ∼10%. The relationship between bubble rise time and bubble velocity was investigated in a unidirectional flow cell. Similar to previous studies the rise time and bubble velocity were found to be related by a power law equation. The probes can provide mean bubble velocities accurate to ±10% when a minimum of ∼15 individual bubble velocities are averaged. The fiber-optic probes were deployed beneath a plunging breaking wave in a laboratory wave channel. The slope and shape of the bubble cord length size distribution measured with the probes was found to agree closely with the size distribution measured from digital video recordings. The probes were then positioned in the splash-up zone of a plunging breaker and the resulting cord length distribution had a shape and slope that was in agreement with previous measurements. These results demonstrate that single-tip fiber optic probes can provide accurate simultaneous measurements of the void fraction and bubble sizes inside the dense bubble clouds entrained by breaking waves.