Dynamic structures of bubble-driven liquid flows in a cylindrical tank

The spatial and temporal structures of turbulent water flows driven by air bubbles in a cylindrical tank were investigated. The time-resolved particle image velocimetry technique was adopted for quantitative visualization. Flow rates of compressed air were changed from 1 to 5?L/min at 0.5?MPa, and the corresponding range of bubble-based Reynolds number (Re) ranged from 8,300 to 21,100. The dynamics of flow structures was further investigated by the time-resolved proper orthogonal decomposition analysis technique. With increasing Re, mean velocity fields driven by the rising bubbles are almost same, but turbulence is dramatically enhanced. Both spatial and temporal modes were quite different with respect to the air flow rates. Three most dominant spatial structures are recirculating flow, bubble-induced motion, and sloshing of free surface, the bigger the latter the higher Re. We found the frequency of sloshing motion from flow visualization and the FFT analysis of temporal modes.     

Development of a robust image processing technique for bubbly flow measurement in a narrow rectangular channel

This paper presents a robust image processing technique for bubbly flow measurement over a wide range of void fractions. The proposed algorithm combines geometrical, optical and topological information recorded with high speed cameras to separate and reconstruct the overlapping bubbles. The common difficulties such as overlapping, irregular bubble shape, surface deformation and large clustering in digital image processing are solved by combining different information based on a preset decision table and flow chart. Test with synthetic bubble images is performed to evaluate the reliability of the algorithm and quantify the uncertainty of the data. The result shows that the proposed algorithm can accurately measure bubbly flows with void fraction up to 18% for large bubbles. Four runs of bubbly flow images in a 30 mm  ×  10 mm rectangular channel are then recorded by three high speed cameras. The area-averaged void fraction of these test runs range from 2.4% to 9.1%. The axial and lateral distributions of bubble number density are obtained by the present algorithm for studying the characteristics of these flows.     

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     

Flow structure and heat transfer of impingement jet

This paper presents the characteristics of flow behavior and thermal fields of both free and impingement jets issued from circular orifice nozzle at Re = 9,700. The flow behavior of a single round jet and impingement jet were observed by smoke flow visualization recorded by a high speed video camera with 5,000 frames per second. Heat transfer coefficient on the impingement surface was calculated varying the Reynolds number and the separation distance between nozzle exit and plate. Time-series analysis was applied to the visualization image to get the information of time variation of flow behavior. Probability distribution of vortex scale induced by the jet at discrete positions was investigated. Experimental results show that the potential core is not a continuous phenomenon with time and the frequency of vortex ring formation have similar features regardless of whether the impingement plate was set on or not, furthermore the time-series analysis with flow visualization images makes clear the detailed flow behavior.     

Quantitative flow visualization of fluidized-bed under normal- and down-flow-mode operations by neutron radiography

Heat transfer characteristics of tube-banks immersed in a fluidized-bed is dominated by the time-averaged as well as statistical characteristics of bed-material movement, especially, in the neighboring region of heat transfer tube. The neutron radiography and image processing technique have been successfully applied to the visualization of flow field and quantitative measurement of void fraction in the bed. This quantitative visualization technique is verified as a useful means in understanding the flow behavior and thus the heat transfer mechanisms. Received: 4 October 1998 / Accepted: 7 June 1999     

A quantitative laser sheet image processing method for the study of the coherent structure of a circular jet flow

A quantitative two-dimensional digital image processing technique is successfully developed to enhance qualitative flow visualization and to obtain quantitative results. The technique is applied to study some less known properties of the coherent structural interaction and evolution mechanism of a low Reynolds number circular jet flow under high level acoustic excitation. Before processing the quantitative data, many inherent errors and uncertainties of the instruments and the system are first discussed and corrected. In this research, the uniformity and the traceability of the flow marker are carefully tested, and the distortion of the imaging system and the fan-shape of the laser sheet are calibrated. Through the image processing technique, the spreading of the jet, the trajectory and the convective velocity of the vortex can be analyzed rapidly and simultaneously. By analyzing the constant jet fluid concentration contour, the mechanism of vortex roll-up and entrainment, which has been ambiguous by traditional pointwise measurements, are more solidly confirmed. Also, the detailed tearing process of the vortex and the evolution mechanism of partial pairing, which can not be clearly detected in the conventional flow visualization pictures, are made clearly visible and carefully delineated.A version of this paper was presented at the 11th Symp. on Turbulence, University of Missouri-Rolla, 17–19 Oct. 1988     

New experimental methods for turbulent spots and turbulent spirals in the Taylor-Couette flow

 The flow between concentrically counter-rotating circular cylinders is investigated experimentally with respect to the appearance of turbulent spots, combining two new methods, based on digital image processing. The simultaneous visualization of the transition to turbulence in the whole flow field in the gap between the two cylinders leads to a qualitative understanding of the phenomenon of turbulent spots. Quantitative results about spiral turbulence are obtained from measurements based on a time-resolved technique introducing a special method of image processing for long sequences of video frames. Variations of the gap width between the cylinders and the investigation of different boundary conditions at the end plates of the rotating cylinders allow conclusions concerning the importance of locally defined parameters of the flow field. Received: 4 December 1995/Accepted: 15 May 1997     

Development of fiberscope PIV system by controlling diode laser illumination

Because of the inherent small size of optical fiberscopes, they provide access and relative handling ease in given closed vessels, which are hardly equipped with extra windows for conventional flow visualization. The use of an optical fiberscope in conjunction with a conventional particle image velocimetry/particle tracking velocimetry (PIV/PTV) system without optimization can lead to degraded transmission of images. The present study proposes a processing technique to filter background noise contained within the coarse bundle image by subtracting the original image of the bundle as reference image. Additionally, efforts were made to increase the reliability of vector processing using particle streak images via judicious pulse interval and duration adjustments. As an applications test we measured classic jet flow using the developed system and using established conventional measurement techniques. Our tests confirmed that our fiberscope PTV system provides vector fields with sufficient accuracy.     

Visualization and image processing of an acoustically excited jet flow

Recent work investigating excitation effects on an acoustically excited jet flow is reported. Emphasis is placed on integrated visualization and high-resolution and digital image processing, sequential planar gas concentrations and some characteristics of the large-scale vortical structures of acoustically excited jet flow, such as multiple-eddy merging, vortex-braids formation, eddy evolution and entrainment processes, jet-boundary expansion, etc. The digital image processing technique employed here extends our previous work (Chao et al. 1990) to optimal threshold filtering of the background noise and in situ calibration of camera and illumination distortions. A compact, low-cost, automatic frame acquisition and analysis system is developed for mean and r.m.s. planar gas concentrations and automatic jet-boundary identification. Also, stroboscopic images are created for detailed sequential study of a quick-changing process using a low framing-rate camera by suitably controlling the acoustic excitation frequency and the jet flow.A version of this paper will be presented at the second symposium on experimental and numerical flow visualization, 1991 ASME Winter Annual Meeting, Atlanta, GA, Dec. 1–6, 1991     

Time-resolved 3D visualization of air injection in a liquid-saturated refractive-index-matched porous medium

The main goal of this work is to implement and validate a visualization method with a given temporal/spatial resolution to obtain the dynamic three-dimensional (3D) structure of an air plume injected into a deformable liquid-saturated porous medium. The air plume develops via continuous air injection through an orifice at the bottom of a loose packing of crushed silica grains. The packing is saturated by a glycerin-water solution having the same refractive index and placed in a rectangular glass container. By using high-speed image acquisition through laser scanning, the dynamic air plume is recorded by sequential tomographic imaging. Due to the overlap between adjacent laser sheets and the light reflection, air bubbles are multiply exposed in the imaging along the scanning direction. Four image processing methods are presented for the removal of these redundant pixels arising from multiple exposure. The respective results are discussed by comparing the reconstructed air plume volume with the injected one and by evaluating the morphological consistency of the obtained air plume. After processing, a 3D dynamic air flow pattern can be obtained, allowing a quantitative analysis of the air flow dynamics on pore-scale. In the present experimental configuration, the temporal resolution is 0.1 s and the spatial resolution is 0.17 mm in plane and about 1 mm out of plane of the laser sheet.     

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     

Visualization and measurement of helicopter rotor flow with swept back tip shapes at hover flight using the “flow visualization gun” time line technique

The Flow Visualization Gun (FVG), a novel time line visualization technique, has been used to investigate the flow field of a helicopter rotor with swept back tip shape in hover flight condition. After introducing the FVG-technique, the paper presents some visualization photographs of the rotor blade tip vortices and the rotor downwash. Using orthogonal sets of flow photographs and digital image analysis, the 3-dimensional time line displacement within the flow and the tip vortex structure are determined. The data of 16 time line experiments are interpolated in space and time to obtain velocity data on an evenly spaced 3-dimensional grid. Vorticity contour plots of the flow field show the complex arrangement of the tip vortices of the blade itself and of the preceding blade and some additional vorticity in the direct wake which may form a secondary vortex. Understanding and control of this vorticity distribution is important for the design of new efficient tip shapes. While the FVG technique has been used so far for qualitative investigation of complex flow patterns at local velocities of up to 20 m/s, a good comparison to laser velocimetry data validates the technique as an interesting tool for both qualitative and quantitative investigation. This research was supported by a “Poste Rouge” grant by the Centre National de la Recherche Scientifique (C.N.R.S.). The authors would like to thank all members of the IRPHE for their help concerning the mechanical and electronical set up of the experiments.     

A displaced-line velocity diagnostic and its application in a visualization engine

A technique for measuring three-dimensional velocity by imaging the displacement of a marked fluid line is described, together with its use in an automotive visualization engine. In a flow seeded with 2–3 μ phosphorescing particles, a line is excited by a UV laser beam, deformed by the local velocity field, and detected by stereo low-light-level video cameras. The derivation of velocity from digitized images is discussed and capabilities of the diagnostic are assessed. Some image data taken in the engine are shown and quantitative two-component velocity plots along the line are presented.     

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.     

Fast quantitative processing of particle image velocimetry photographs by a whole-field filtering technique

A fast quantitative processing of particle image velocimetry photographs by a whole-field spatial filtering technique is described. Photographs are observed through a conventional filtering setup. This produces fringe patterned images with each fringe corresponding to a fixed value of one velocity component. These images are acquired with a CCD camera and digitally processed to retrieve the fringe centerline positions. The interpolation of these data provides the velocity value on a grid of regularly spaced points.Photographs taken from a Rayleigh-Bénard convective flow have been processed with this technique and with a previously reported point-by-point technique. Results from both techniques compare well.This work was supported by Diputación General de Aragón under Grant No. PCB6-90     

Three-dimensional flow pattern visualization and bubble size distributions in stationary and transient upward flashing flow

For the first time, an experimental three-dimensional reconstruction and visualization of stationary and transient flashing flow in a vertical pipe (47 mm diameter) is presented. The measurements have been performed by means of wire-mesh sensors. This type of sensor delivers two-dimensional void-fraction distributions in the pipe cross-section where it is mounted with a maximum sampling rate of 10,000 frames per second. A sampling rate of 1200 frames per second has been used in this work. Steam bubbles have been identified from the wire-mesh data and their complete three-dimensional reconstruction has been performed by taking into account the steam bubble velocity. For the estimation of the bubble velocity, two wire-mesh sensors positioned at a small axial distance from each other have been used. The velocity has been determined by cross-correlation of the two wire-mesh signals, by direct identification of the traveling time of the steam bubbles between the two sensors and by means of a drift-flux model. A comparison between the three methods of bubbles velocity measurement is reported. Stationary and time-dependent bubble size distributions have been derived. The stationary radial void-fraction profiles have been decomposed according to bubble size classes and compared with the results obtained with an equilibrium model.     

Study of cavitation phenomena based on a technique for visualizing bubbles in a liquid pressurized chamber

In this paper, the influence of nozzle geometry on cavitation and near-nozzle spray behavior under liquid pressurized ambient is studied. For this purpose, eight steel drilled plates, with different diameters and degrees of conicity of their holes, are analyzed. A special near-nozzle field visualization technique, using a test rig pressurized with fuel, is used. Due to the difference in refractive index between liquid and vapor phase, bubbles present at the outlet of the orifice are visualized. The pressure conditions at which bubbles start appearing at the orifice outlet are compared with those at which choked flow appears. The results showed that pressure conditions for inception of cavitation obtained in the visualization tests differs from those seen for choked flow (5–8% in terms of cavitation number). In addition to this, the images taken are analyzed to get the angle of the jet formed by fuel bubbles, showing that it increases significantly for those conditions more prone to cavitate. Furthermore, comparison of bubbles generation when increasing or decreasing backpressure indicates the presence of hysteresis in cavitation inception phenomena.     

Quantitative limits of thermal and fluid phenomena measurements using the neutron attenuation characteristics of materials

Temporal and spatial resolution of the neutron radiographic technique were investigated in order to apply this technique to the visualization and measurement of thermal and fluid phenomena. The temporal resolution of three imaging methods of temporally resolved neutron radiography-static neutron radiography with a pulsed neutron beam and high frame rate neutron radiography with either a pulsed or steady neutron beam-was studied. It was determined that the temporal resolution was determined by the sensitivity and light decay time of the image detector and statistical variation of neutrons, and the resolution limits of static and dynamic imaging methods were estimated to be a few microseconds and a few hundred microseconds, respectively. An image processing method was proposed to measure flow characteristics such as void fraction. By performing an error analysis to calculate the limit value of liquid film thickness that can be measured by neutron radiography, it was determined that the limit value of a rectangular channel gap or round tube diameter should be smaller than 3.25 or 4.00 mm, respectively, for measuring the void fraction of air-water flow within an error of 10%. The void fraction measuring method was experimentally confirmed by comparing the void fraction values in a rectangular duct with a 2.4-mm gap obtained by neutron radiography with those obtained by optical and conductance probe methods. It was shown quantitatively that the measurement error decreased when consecutive frames were temporally integrated.     

3D scanning particle tracking velocimetry

In this article, we present an experimental setup and data processing schemes for 3D scanning particle tracking velocimetry (SPTV), which expands on the classical 3D particle tracking velocimetry (PTV) through changes in the illumination, image acquisition and analysis. 3D PTV is a flexible flow measurement technique based on the processing of stereoscopic images of flow tracer particles. The technique allows obtaining Lagrangian flow information directly from measured 3D trajectories of individual particles. While for a classical PTV the entire region of interest is simultaneously illuminated and recorded, in SPTV the flow field is recorded by sequential tomographic high-speed imaging of the region of interest. The advantage of the presented method is a considerable increase in maximum feasible seeding density. Results are shown for an experiment in homogenous turbulence and compared with PTV. SPTV yielded an average 3,500 tracked particles per time step, which implies a significant enhancement of the spatial resolution for Lagrangian flow measurements.     

Experimental study of liquid–vapor phase distribution in a single tube capillary evaporator

An experimental setup was developed to study the vaporization of a liquid inducing capillary pumping. The study was performed on a single square cross-section micro-channel of 500 μm internal edge, heated only on the upper and lower walls to allow visualization through the 2 other faces. The images were obtained by a shadowgraph technique using a digital camera. A program for processing the images was developed to allow the determination of several quantities characterizing the flow, supplemented by information provided by the rest of the instrumentation equipping the experimental device. The results highlight the unstable nature of the phase distribution. Despite these instabilities, films of liquid spread in an averaged manner over a length substantially equal to the length of the zone where the heat flux was applied, independently of the intensity of the heat flux.