Flow structure of microbubble-laden turbulent channel flow measured by PIV combined with the shadow image technique (original) (raw)
Related papers
Two-phase Turbulence Structure in a Microbubble Channel Flow
2000
The turbulence structure of flow field including microbubbles in a horizontal channel is experimentally investigated using combined particle image velocimetry in order to clarify the mechanism for drag reduction caused by microbubbles. Firstly, a simultaneous measurement system for liquid phase and dispersed bubbles is proposed, which is based on the combination of Particle Tracking Velocimetry, Laser Induced Fluorescence and Shadow
Turbulence structures of microbubble flow measured by PIV/PTV and LIF techniques
2000
In this research, in order to clarify the mechanism for the drag reduction caused by microbubbles, the turbulence structure of the microbubble flows in a vertical pipe and a horizontal channel is experimentally investigated using PIV/PTV and LIF techniques. Furthermore, a bubble projection technique is used to obtain the position and shape of individual bubbles. Particularly, we consider the local
Experimental study of micro-bubble drag reduction using particle image velocimetry
Drag reduction was studied when micro-bubbles with low void fractions were injected in the boundary layer of a turbulent channel flow. The particle tracking velocimetry (PTV) flow measurement technique was used to measure velocity fields. Data sets of flow images were acquired to obtain Reynolds-averaged quantities and monitor the flow dynamics. Micro-bubbles, with average diameter of 30 µm, were generated via an electrolysis process using a 76 µm platinum wire with high voltage. Drag reductions were realized with small void fractions. Similarities with results obtained from drag reduction due to addition of surfactants and polymers as the thickening of the buffer zone, and upward shifts of the logarithmic region were observed. The present results support the theory of an interaction of micro-bubbles with turbulence in the buffer zone, as a mechanism leading to the drag reduction.
PIV Methods for Turbulent Bubbly Flow Measurements
Analysis of particle properties in dispersed multiphase flow and simultaneous determination of velocity fields for both phases is main concern in the study of many industrial problems. Particle Image Velocimetry (PIV) is a powerful tool to study the structure of multiphase, three-dimensional, transient fluid flows . The aim of this study is to find the most suitable PIV method to study turbulent bubbly flows and to measure the properties of bubbles in a mixing vessel. The method should be accurate, inexpensive, fast and easy to use in varying measurement conditions. Back lighting is used to detect bubble outlines and the laser light sheet is produced to illuminate the tracer particles. Images of bubble shadows and particles are recorded with the same camera. Two experiments with different measurement methods are performed. The first set uses silver-coated hollow glass spheres as tracer particles and the second fluorescent particles. The optical filter blocks the scattered light from bubbles in the second experiment. In the first experiment the scattered light from bubbles is enhanced by digital image processing methods and by geometrical alignment of the camera. Also a stereo-PIV setup using two cameras at Brewster angles is tested for turbulent bubbly flow. In order to study the performance of the proposed methods, velocity fields and relative velocities of the continuous phase and bubbles are measured. The sizes and shapes of the bubbles are also measured and the quality of the results is analyzed. The accuracy of three dispersed particle velocity measurement methods is analyzed with simulated particle images.
Laser Techniques for Fluid Mechanics, 2002
Defocusing digital particle image velocimetry (DDPIV) is the natural extension of digital particle image velocimetry (DPIV), planar or quasi three-dimensional, to a true and unique three-dimensional PIV technique. This work presents the defocusing optical concept by which the depth information can be retrieved, thus overriding the limitation to inplane measurements of actual PIV techniques, either standard or stereo-based. The concept is implemented into a threedimensional imaging system specifically designed for the purpose of mapping two-phase bubbly flows. Digital images of the bubble field are recorded and analysed to provide information both on the physical location of every single particle/bubble and on its respective size, which is estimated from the scattered light intensity. The calculation of the true three-component velocity field is done by local spatial cross-correlation between two consecutive sets of particle/bubble locations. The spatial resolution and uncertainty limits are established based on a simplified model of the defocusing optical system. Accuracy measurements show that the average error on the displacements is about 0.02 pixels. The methodology used to measure the size is laid out by application of the Mie scattering theory. A DDPIV prototype instrument was fabricated on specific requirements. The instrument records high resolution images of the bubble field and is capable of providing bubble size and bubble location within a cubic foot volume. The technique is applied to the study of the dynamics of sub-millimeter air bubbles in a three-dimensional vortical flow generated by a propeller. Velocity, bubble size distribution and void fraction for these flows are discussed.
Engineering reports, 2020
Particle image velocimetry (PIV) measurement technique provides an excellent opportunity for investigating instantaneous spatial structures which are not always possible with point measurements techniques like laser Doppler velocimetry. In this review, it was shown that PIV technique provides an effective means of visualizing important structures of Newtonian and drag-reducing fluid flows. Such structures include large-scale-events that constitute an important portion of the Reynolds stress tensor; shear layers of drag-reducing flows, which have been suggested to constitute the mechanism of drag reduction (DR); and near wall vortices/low speed streaks which constitute the mechanism of turbulence production. PIV investigations of turbulence statistics in Newtonian and drag-reducing fluid flows were reviewed with the view of providing explanation to DR by additives. Results of turbulence statistics, for Newtonian fluid flow, showed that streamwise velocity fluctuations and turbulence intensity had peak values close to the wall, in a region of high mean velocity gradient, while radial fluctuating velocity and Reynolds stress tensor had peaks further from the wall and at approximately the same detachment from the wall. In single-and two-phase flows in horizontal channels, the velocity profile of polymer solution, in the turbulent regime, show asymmetric behavior. This review highlighted important interfacial characteristics in gas-liquid flows such as S-shaped velocity profile as well as the turbulences statistics in each phase and across the interface region. Drag-reducing agents (DRAs)-imposed changes on turbulence statistics and flow structures were also examined. PIV studies of drag-reducing flows showed that DRAs act to dampen wall-normal-, streamwise fluctuating velocity, and Reynolds stress tensor. The reduction in Reynolds stress tensor is higher than the reduction of both wall-normal and streamwise velocity fluctuations and this discrepancy has been associated with the decorrelation of the component of fluctuating velocity. Furthermore, the addition of DRA produces a shift of the peak of wall-normal velocity fluctuations further from the wall due to increased buffer layer thickness. DRAs do not only act to reduce drag but also to modify the flow structure. The major influence of DRAs on flow structures is seen in the This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.
Engineering Reports, 2020
Particle image velocimetry (PIV) measurement technique provides an excellent opportunity for investigating instantaneous spatial structures which are not always possible with point measurements techniques like laser Doppler velocimetry. In this review, it was shown that PIV technique provides an effective means of visualizing important structures of Newtonian and drag-reducing fluid flows. Such structures include large-scale-events that constitute an important portion of the Reynolds stress tensor; shear layers of drag-reducing flows, which have been suggested to constitute the mechanism of drag reduction (DR); and near wall vortices/low speed streaks which constitute the mechanism of turbulence production. PIV investigations of turbulence statistics in Newtonian and drag-reducing fluid flows were reviewed with the view of providing explanation to DR by additives. Results of turbulence statistics, for Newtonian fluid flow, showed that streamwise velocity fluctuations and turbulence intensity had peak values close to the wall, in a region of high mean velocity gradient, while radial fluctuating velocity and Reynolds stress tensor had peaks further from the wall and at approximately the same detachment from the wall. In single-and two-phase flows in horizontal channels, the velocity profile of polymer solution, in the turbulent regime, show asymmetric behavior. This review highlighted important interfacial characteristics in gas-liquid flows such as S-shaped velocity profile as well as the turbulences statistics in each phase and across the interface region. Drag-reducing agents (DRAs)-imposed changes on turbulence statistics and flow structures were also examined. PIV studies of drag-reducing flows showed that DRAs act to dampen wall-normal-, streamwise fluctuating velocity, and Reynolds stress tensor. The reduction in Reynolds stress tensor is higher than the reduction of both wall-normal and streamwise velocity fluctuations and this discrepancy has been associated with the decorrelation of the component of fluctuating velocity. Furthermore, the addition of DRA produces a shift of the peak of wall-normal velocity fluctuations further from the wall due to increased buffer layer thickness. DRAs do not only act to reduce drag but also to modify the flow structure. The major influence of DRAs on flow structures is seen in the This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.