Begell House Inc.
Multiphase Science and Technology
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16
4
2004
DISTRIBUTION OF INERTIAL PARTICLES IN THE WALL REGION OF A TURBULENT BOUNDARY LAYER
281-293
10.1615/MultScienTechn.v16.i4.10
Maurizio
Picciotto
Centro Interdipartimentale di Fluidodinamics e Idraulica, Dipartimento di Energetica e Macchine, Universita degli Studi di Udine, Via delle Scienze 208, 33100 Udine, Italy
Cristian
Marchioli
Centro Interdipartimentale di Fluidodinamics e Idraulica, Dipartimento di Energetica e Macchine, Universita degli Studi di Udine, Via delle Scienze 208, 33100 Udine, Italy
Alfredo
Soldati
Centro Interdipartimentale di Fluidodinamics e Idraulica, Dipartimento di Energetica e Macchine, Universita degli Studi di Udine, Via delle Scienze 208, 33100 Udine, Italy
The problem of particle preferential distribution in turbulent boundary layer is addressed. Several observations confirm that, in this type of flow, particles have a non-uniform distribution in the wall normal direction and it has also been observed that, when in the viscous sub-layer, particle distribution is not uniform in the wall parallel plane so that particles appear segregated along streamwise streaks with strong time persistency. Starting from our previous works [1, 2], in which we examined the mechanisms for particle transfer toward and away from the wall, we aim at characterizing the regions of particle preferential distribution. Particle motion in the wall region is deminated by instantaneous Reynolds stresses — i.e. strong downwash of outer fluid toward the wall, sweeps, and strong upwash of fluid away from the wall, ejections — which are generated by the coherent vortical structures populating the wall region.
Specifically, in this work we correlate particle preferential position with the distribution of the coherent structures in the wall region. Results confirm that particles tend to avoid the strongly coherent vortifcal structures and tend to concentrate in regions neighbouring the wall which are characterized by low shear stress values where the flow is generally directed away from the wall.
CRITICAL ROLES PLAYED BY AN OSCILLATING BUBBLE IN BUBBLE-SURFACE PHENOMENA
295-308
10.1615/MultScienTechn.v16.i4.20
Katsumi
Tsuchiya
Dept. of Chemical Engineering and Materials Science, Doshisha University, Kyotanabe 610-0321, Japan
Hirofumi
Fukuta
Dept. of Chemical Science and Technology, The University of Tokushima, Tokushima 770-8506, Japan
Dynamics of a single bubble rising in oscillation are examined experimentally for N2 bubbles of mainly 2−3-mm diameter in the presence of a surfactant, n-butanol, in distilled water. The bubble oscillating characteristics, including local fluctuations in the rise velocity and shape, are visually traced over a limited span of vertical distance. The bubble lifetime is determined as the net period from its reaching the upper free surface to the liquid-film rupturing. The rise-path mode is found to be always zigzag if the bubble eccentricity is less than 1.5; if on the other hand the eccentricity exceeds this critical value, the rising bubble initially exhibits the spiraling mode. The amplitude of the oscillations is much larger for spiraling bubbles than for zigzagging bubbles and may vary—often being dampened—with time, while the frequency of rise-velocity and shape fluctuations remains essentially invariant with time, especially for spiraling bubbles, on the order of 10 Hz. The extent of bubble life could critically be dominated by the phase of oscillation when the bubble has reached the free surface. For the zigzag mode, three cases of horizontal velocity contribution, i.e. zero, maximum and in-between magnitudes, are identified which result in different extents of lifetime; no classification, however, is possible for the spiral mode.
MODELLING OF TWO-PHASE DIVERSION CROSS-FLOW BETWEEN SUBCHANNELS BASED ON A TWO-FLUID MODEL
309-334
10.1615/MultScienTechn.v16.i4.30
Michio
Sadatomi
Department or Advanced Mechanical System, Graduate School of Science and Technology, Kumamoto University, Kurokami 2-39-1, Chuo-Ku, Kumamoto City, 860-8555, Japan
Akimaro
Kawahara
Advanced Thermal and Fluid Energy System
Division of Industrial Fundamentals
Faculty of Advanced Science and Technology, Graduate School of Science and Technology, Kumamoto University, Chuo-ku,
Kurokami 2-39-1, Kumamoto, Japan
K.
Kano
Dept. of Mechanical Engineering and Materials Science, Kumamoto University, Kumamoto, 860-8555, Japan
S.
Machidori
Dept. of Mechanical Engineering and Materials Science, Kumamoto University, Kumamoto, 860-8555, Japan
Following the previous studies, single- and gas-liquid two-phase diversion cross-flows between subchannels have been studied. In the previous experiment, a vertical channel consisting of two circular subchannels of 16 mm i.d. was used as the test channel, while water and air at atmospheric pressure and room temperature as the test fluids. Experimental data on the axial variations of pressure difference between the subchannels, the ratio of flow rate in one subchannel to the whole channel, the void fraction in each subchannel were obtained for bubble, slug-churn, and annular flows in two-phase case. In addition, correlations of cross-flow resistance coefficient for the liquid phase and interfacial friction coefficient were obtained from an analysis of the cross-flow rate data in our latest experiments. In this study, by incorporating these correlations into a cross-flow momentum equation in our subchannel analysis code, which was based on a two-fluid model, flow redistribution processes due to diversion cross-flow have been calculated for various single- and two-phase hydraulically non-equilibrium flows with pressure difference between the subchannels. From a comparison with the experimental data on the flow redistribution processes, the validity of the analysis has been confirmed in the region where the effects of void drift can be neglected.
MOLECULAR TAGGING VELOCIMETRY BASED ON PHOTOBLEACHING REACTION AND ITS APPLICATION TO FLOWS AROUND SINGLE FLUID PARTICLES
335-353
10.1615/MultScienTechn.v16.i4.40
Akio
Tomiyama
Department of Mechanical Engineering, Graduate School Engineering, Kobe
University, 1-1 Rokkodai, Nada, Kobe, Hyogo, 657-8501 Japan
Shigeo
Hosokawa
Faculty of Societal Safety Science, Kansai University, 7-1 Hakubai, Takatsuki,
Osaka 569-1098, Japan
Most of unintrusive velocimetry techniques utilize small tracer particles to visualize a flow field. The addition of tracer particles in flows induces errors and flow disturbance due to their poor followability and accumulation on gas-liquid interface. A molecular tagging velocimetry based on photobleaching reaction is developed in this study. The most significant features of the developed method are as follows: (a) local instantaneous velocity, strain rate and rotation are measurable, (b) the measurement can be performed at arbitrary points in a flow field, and (c) tracer particles disturbing a flow are not required. The method was applied to a laminar pipe flow and flows around a bubble and a drop. It is confirmed that (1) the method can accurately measure radial distributions of liquid velocity and shear stress in the pipe flow, (2) local instantaneous velocity distributions around single bubbles and a drop are successfully measured, (3) the time evolution of velocity field is measurable by forming multiple tags in a matrix arrangement, and (4) the liquid velocity in the vicinity of gas-liquid interface can be measured, provided that tags are formed there.
CLOSURE RELATIONS FOR THE SHEAR STRESS IN TWO-FLUID MODELS FOR CORE-ANNULAR FLOW
355-387
10.1615/MultScienTechn.v16.i4.50
Amos
Ullmann
School of Mechanical Engineering,
The Iby and Aladar Fleischman Faculty of Engineering, Tel-Aviv University, Ramat Aviv 6139001, Israel
Neima
Brauner
School of Mechanical Engineering, Faculty of Engineering, Tel-Aviv University, Ramat Aviv 6139001, Israel
The exact solution for laminar core-annular flow (CAF) in inclined pipes is used to derive new closure relations for the wall and interfacial shear stresses. These are incorporated in a two-fluid model for CAF. With these closure relations, the two-fluid model yields the exact solution for the hold-up and pressure drop in case of laminar horizontal or inclined CAF. It is shown that multiple solutions can be obtained in upward or downward inclined systems. The multiple solution regions are identified in terms of the controlling dimensionless parameters. The predictions of the two-fluid model for laminar and turbulent CAFs are tested against experimental data available from the literature for oil-water systems. The good comparison suggests the two-fluid model as convenient tool for evaluating the pressure drop and holdup in CAF.