Begell House Inc.
Multiphase Science and Technology
MST
0276-1459
15
1-4
2003
Exact Conditions for Virtual Boundaries in Initial Boundary-Value Problems of the Wave Scattering Theory
21
Andrei Olegovich
Perov
A.Ya. Usikov Institute for Radiophysics and Electronics of the National Academy of Sciences of Ukraine 12, Academician Proskura St., Kharkiv 61085, Ukraine
Yuriy
Sirenko
IRE NAN Ukraine
The paper is devoted to limiting correctly the calculation space in the Finite-Difference Time-Domain method, a technique applied for the analysis of transient wave processes in infinite domains with compact resonant irregularities. The computational procedure developed within the framework of this approach involves the exact absorbing boundary conditions, which do not distort the simulated physical processes.
WORKSHOP ON SCIENTIFIC ISSUES IN MULTIPHASE FLOW SUMMARY OF CONTRIBUTIONS
1-19
10.1615/MultScienTechn.v15.i1-4.10
Geoffrey F.
Hewitt
Department of Chemical Engineering & Chemical Technology, Imperial College of Science, Technology & Medicine, Prince Consort Road, London SW7 2B Y, England, UK
This Summary introduces and discusses papers presented in this issue of Multiphase Science and Technology which had their origin at the Workshop on Scientific Issues in Multiphase Flow held at the University of Illinois in May 2002. The areas covered were: Flow Regimes in Multi-fluid Flow, Disperse Flow, Computational Physics and Micro Physics.
FULLY DEVELOPED GAS-LIQUID FLOWS
21-31
10.1615/MultScienTechn.v15.i1-4.20
Thomas J.
Hanratty
Department of Chemical and Biomolecular Engineering, University of Illinois at Urbana-Champaign Urbana, Illinois 61801, USA
The prediction of how phases distribute in a flow field is of first order importance in developing a scientific approach to multiphase flow. This involves the specification of the type of pattern and a quantitative description of where the phases are located for a given pattern. A striking feature of the physics is that macroscopic behavior is governed by small-scale interactions. A first step is to identify the critical microphysics by careful scientific (rather than empirical) analyses of results from studies in long pipelines. The "fully-developed" flows, that evolve far enough downstream, offer a simple system to test physical understanding. Furthermore, such studies provide results that can find direct use.
Striking examples of the influence of small scale behavior in multiphase systems are the sensitivity of the flow pattern in a gas-solid fluidized bed to the characteristics of the solid particles and the influence of polymers on the behavior of a gas-liquid flow. An annular flow pattern changes to a stratified pattern because polymers destroy disturbance waves on the wall film (Al-Sarkhi and Hanratty, 2001a, 2001b). This eliminates atonmation mid reduces the ability of the film to climb up the wall against gravity. Polymers have also been observed to delay the transition to slug flow by decreasing the stability of slugs (Soleimani et al, 2002). The shedding of liquid from a slug can be related to the velocity of the gas bubble behind it. Polymers cause an increase in the velocity of this bubble by damping turbulence and, thereby, changing the flow pattern in the slug and the bubble velocity.
This paper illustrates the approach outlined above by considering gas-liquid flows in horizontal or near-horizontal pipes. Stratified, slug and annular configurations are considered.
The following recommendations are made: (1) An understanding of the physics governing the transition from one regime to another should be a top priority. (2) Critical issues in annular flow are the prediction of the fraction of the liquid which is entrained as drops and the development of a physical understanding of how the drops and the wall film distribute asymmerically under the influence of gravity. (3) Several scientific issues that arise in slug flow need more attention. These include the mechanisms by which slugs are formed and the frequency of slugging (particularly, when the time intervals are stochastic). A better model for the flow pattern in a slug is needed to identify how aeration is occurring and to show how the velocity' of the bubble behind a slug depends on slug length (Bernicot and Drouffe. 1991; Fabre and Line, 1992; Barnea and Taitel, 1993). (4) The understanding of interfacial drag is a central problem in stratified, as well as annular flow. (5) Facilities are needed to carry out these integrative experiments.
OUTLINE OF RESEARCH FOR INTERFACIAL AREA TRANSPORT PHENOMENA IN TWO-PHASE FLOW
33-43
10.1615/MultScienTechn.v15.i1-4.30
Mamoru
Ishii
Therma-Hydraulics and Reactor Safety Laboratory, School of Nuclear Engineering, Purdue University, 400 Central Drive, West Lafayette, IN 47907, USA
The use of interfacial area transport equation method represents a practical approach to formulation of an equation set for multiphase flows. It incorporates sufficient physics to allow the capture of key phenomena in a manner not accessible to the more widely used methods. This contribution introduces interfacial area transport formulations and also describes the experimental data base requirements needed for closure of this model.
CRITICAL REMARKS ABOUT FLOW CHARTING
45-51
10.1615/MultScienTechn.v15.i1-4.40
Daniel D.
Joseph
University of Minnesota, AEM, 107 Akerman Hall, 110 Union Street, Minneapolis, MN 55455, USA
In a recent paper (Mata, Pereyra, Trallero and Joseph 2002) we computed stability limits for Kelvin-Helmholtz instability of superposed gas-liquid flow, comparing theories of Jeffreys (1925, 1926), Taitel and Dukler (1976), Lin and Hanratty (1986), Barnea and Taitel (1993), and Fimada and Joseph (2001). The theories we compared with literature data on air-water flow and with new data on heavy oil. A problem encountered in the experiments is that the experimental data is presented in a plane of superficial gas and liquid velocities which do not uniquely determine the flow type. A critical view of this method of presenting results is the subject of the remarks to follow...
BUBBLY-TO-SLUG TRANSITION IN VERTICAL FLOWS
53-56
10.1615/MultScienTechn.v15.i1-4.50
A.
Prosperetti
Department of Mechanical Engineering, The John Hopkins University, Baltimore, MD 21218, USA; and Department of Applied Physics, IMPACT, University of Twente, AE 7500 Enschede, The Netherlands
Alternative mechanisms for the bubble-to-slug transition in vertical flows are discussed. Linking the transition to void wave formation has tended to replace earlier hypotheses of bubble coalescence but neither is consistent with all the observations. Thus, several fundamental questions remain.
ON THE PREDICTION OF FLOW PATTERNS AS A PRINCIPAL SCIENTIFIC ISSUE IN MULTIFLUID FLOW
57-76
10.1615/MultScienTechn.v15.i1-4.60
Theo G.
Theofanous
Center for Risk Studies and Safety, University of California, Santa Barbara, 6740 Cortona Drive, Goleta, CA-93117, California, USA
True-Nam
Dinh
Center for Risk Studies and Safety, University of California, Santa Barbara, 6740 Cortona Drive, Goleta, CA-93117, California, USA; and Royal Institute of Technology, Stockholm S-10044, Sweden USA
This article discusses multiphase flow as one of a general class of "complex" systems which occur in nature. A system is defined as complex when its behaviour cannot be explained (and predicted) based on an understanding of its component parts. The concept is illustrated by considering examples including boiling, droplet breakup and mixing processes between a hot molten material and a hot fluid. Methods of addressing such complex systems using multi-scale computational models are discussed.
TWO QUESTIONS CONCERNING "FULLY DEVELOPED GAS-LIQUID FLOWS"
77-78
10.1615/MultScienTechn.v15.i1-4.70
A.
Prosperetti
Department of Mechanical Engineering, The John Hopkins University, Baltimore, MD 21218, USA; and Department of Applied Physics, IMPACT, University of Twente, AE 7500 Enschede, The Netherlands
This short note raises two questions, namely whether adequate use has been made of the literature on wave formation on water bodies (e.g. the ocean) and, secondly, whether more use could be made of controlled perturbations in the study of flow regimes.
SOME REMARKS ON MECHANISMS OF PHASE DISTRIBUTION IN AN ADIABATIC BUBBLY PIPE FLOW
79-98
10.1615/MultScienTechn.v15.i1-4.80
Akimi
Serizawa
Department of Nuclear Engineering, Kyoto University, Yoshida-Honmachi, Kyoto 606-8501, Japan
This contribution discusses the behaviour of single bubbles and bubble structures in adiabatic bubbly pipe flow. The liquid velocity fields induced by single small bubbles, by small Taylor bubbles, by bubble swarms and by bubble clusters are discussed and illustrated by computational simulations. Next, the forces on single bubbles are reviewed; it is shown that, within certain ranges of bubble size, the force on the bubble (and hence its terminal rise velocity) may be strongly affected by small initial distortions in the bubble. Finally, questions of bubble clustering and coalescence are discussed.
ON THE MULTIDIMENSIONAL ANALYSIS OF TWO-PHASE FLOWS
99-129
10.1615/MultScienTechn.v15.i1-4.90
This paper presents a research road map for the development of a multidimensional, four field, two-fluid model for the analysis of two-phase flows. It is shown that accurate mechanistically-based computational fluid dynamic (CFD) predictions are already possible for a wide variety of adiabatic and diabatic bubbly flows using this computational model. Moreover, it appears that similar models can be developed for other flow regimes, and a research plan is proposed.
FLOW REGIMES: TRANSITIONS AND FLOW BEHAVIOUR
131-143
10.1615/MultScienTechn.v15.i1-4.100
Geoffrey F.
Hewitt
Department of Chemical Engineering & Chemical Technology, Imperial College of Science, Technology & Medicine, Prince Consort Road, London SW7 2B Y, England, UK
The limitations of current methods for predicting flow patterns are discussed and a plea is made for two specific flow patterns (namely churn flow and wispy annular flow) to be recognised and more widely studied. Issues arising in annular, slug, stratified and bubbly flows are reviewed.
RESEARCH NEEDS IN MULTIPHASE FLOW: DILUTE PARTICLE-LADEN GAS FLOWS
145-149
10.1615/MultScienTechn.v15.i1-4.110
John K.
Eaton
Dept. of Mechanical Engineering Stanford University 488 Panama Mall Stanford, CA 94305 USA
This contribution addresses open research questions related to modeling of dilute particle-laden flows. Most current modeling approaches either directly or implicitly assume that the particles are much smaller than the Kolmogorov scale and have small Reynolds numbers. These assumptions are invalid in many real systems, and they lead to erroneous predictions of particle dispersion and turbulence modification. Large eddy simulations are used increasingly to predict particle-laden flows, but there are many open issues including a lack of understanding of the two-way interaction of particles with sub-grid scale turbulence. Fully resolved simulations and experiments including significant particle parameter variation in fixed geometry are needed to advance the field.
SCIENTIFIC ISSUES IN THE FLOW OF GASES WITH DISPERSED SOLIDS
151-156
10.1615/MultScienTechn.v15.i1-4.120
Michel
Louge
Sibley School of Mechanical and Aerospace Engineering, Cornell University, 192 Rhodes Hall, Ithaca, NY 14853, USA
This brief document contains suggestions for scientific issues to be studied in flows of gases and dispersed solids. It begins by enunciating principles that the US Department of Energy may consider for its funding of fundamental research in this area. It then provides a partial list of general topics that continue to challenge our basic understanding.
PARAMETERIZATION OF FORCE ON A PARTICLE/BUBBLE/DROPLET
157-171
10.1615/MultScienTechn.v15.i1-4.130
S.
Balachandar
Department of Theoretical and Applied Mechanics, University of Illinois, Urbana, IL 61801, USA
This contribution explores the nature and prediction of the forces on dispersed phase elements (particles, bubbles, droplets) in dispersed two-phase flows. The difficulties in parameterization of the force on a sphere embedded in a shear or straining flow at finite Reynolds number are illustrated by considering several examples (contaminated bubble in a vortex, particle moving at an angle through a straining ambient flow, particle moving freely in a shear flow). The challenges in prediction of the respective forces are reviewed and the significance of temporal acceleration and spatial variation in the flow explored.
OPPORTUNITIES FOR EXTRACTING CORRELATIONS FROM NUMERICAL AND REAL EXPERIMENTS USING DIGITAL TECHNOLOGY
173-176
10.1615/MultScienTechn.v15.i1-4.140
Daniel D.
Joseph
University of Minnesota, AEM, 107 Akerman Hall, 110 Union Street, Minneapolis, MN 55455, USA
I wrote a book called, Interrogation of Direct Numerical Simulation of Solid-Liquid Flows, which is available on www.efluids.com. The Epilogue to the book sets out the case for putting new life into the tried and true engineering approach to correlations. Excerpts from the Epilogue updated for discussion at our workshop appears below.
A COMPUTATIONAL PLATFORM FOR MULTIPHASE FLOW AND HEAT TRANSFER
177-180
10.1615/MultScienTechn.v15.i1-4.150
Theo G.
Theofanous
Center for Risk Studies and Safety, University of California, Santa Barbara, 6740 Cortona Drive, Goleta, CA-93117, California, USA
A new computational platform for the prediction of multiphase flows is described which is centered on an Effective Field treatment supported by a hierarchy of tools ranging from Molecular Dynamics (MD) through Discrete Particle Methods (DPM) to Direct Numerical Simulation (DNS).
TWO-FLUID MODELLING AND AVERAGED EQUATIONS
181-192
10.1615/MultScienTechn.v15.i1-4.160
A.
Prosperetti
Department of Mechanical Engineering, The John Hopkins University, Baltimore, MD 21218, USA; and Department of Applied Physics, IMPACT, University of Twente, AE 7500 Enschede, The Netherlands
Averaged-equations models are and will remain a practical necessity for many years to come. In spite of their importance and the large body of literature devoted to their derivation and numerical solution, this is an area where very serious difficulties still exist. The root of the problems is that, in order to formulate a closed system of equations, part of the information lost upon averaging must be reintroduced in the equations. There is no consensus on the proper way to effect this step and most of the approaches tried to date have failed to produce a well-behaved mathematical model. In their turn, the mathematical difficulties of the models generate difficulties for the numerical methods used for their solution. The formulation of satisfactory averaged equations emerges therefore as one of the foremost problems in multiphase flow. It is suggested that a better approach to the derivation of averaged equations might be reliance on the results of direct numerical simulations, in addition to experiment.
DIRECT NUMERICAL SIMULATIONS (DNS) OF FLUID-SOLID SYSTEMS
193-240
10.1615/MultScienTechn.v15.i1-4.170
Howard H.
Hu
Department of Mechanical Engineering and Applied Mechanics, University of Pennsylvania, Philadelphia, PA 19104-6315, USA
This contribution reviews the application of Direct Numerical Simulations (DNS) to the prediction of fluid-solid systems. First, the governing equations are presented and the numerical methods (fully explicit, Lagranian-Eulerian and explicit-implicit) discussed. The temporal and spatial discretization methods are described. Methods for moving the particles in the computational domain are presented and the influence of collisions is discussed. Finally, examples are given of the application of the code, including sedimentation of a single sphere in a tube, migration of neutrally buoyant sphere in a Poiseuille flow, interaction of particles in Newtonian and viscoelastic fluids and lubrication in pressure-driven particulate flows.
SIMULATION AND MODELING OF DISPERSED MICROSTRUCTURES IN TURBULENT FLOWS
241-254
10.1615/MultScienTechn.v15.i1-4.180
Lance R.
Collins
Sibley School of Mechanical and Aerospace Engineering, Cornell University, USA
This report focuses on the turbulent transport of three microstructures: (i) small aerosol particles; (ii) deformable drops (including breakup); and (iii) flexible polymer molecules. Rather than limit our discussion to direct numerical simulation (DNS) alone, we advocate a three-pronged approach, in which simulation is combined with experiment and theory to study problems that are amenable to all three approaches. In each of the three examples I will discuss, there has been some progress along all three prongs. It may appear that simulation and experiment are really providing the same information; however, simulation typically provides more complete information compared to experiment, but only for a limited range of parameters (especially Reynolds number) and so cannot stand alone. Moreover, there is the obvious need to validate the simulation to be sure one is not chasing an artifact.
To maximize the effectiveness the effectiveness of the three-pronged approach, it is essential that all three activities be tightly coupled (e.g., selecting a common flow with overlapping parameters, hopefully a flow that is simple enough for theoretical developments, etc.). Under this circumstance, it seems reasonable to expect that as DNS and experimental observations improve, theory is likely to advance as well (albeit with some time lag).
DIRECT NUMERICAL SIMULATIONS OF MULTIPHASE FLOW
255-265
10.1615/MultScienTechn.v15.i1-4.190
Gretar
Tryggvason
Department of Mechanical Engineering, Johns Hopkins University, Baltimore, MD, 21218, USA
During the last decade, direct numerical simulations of multiphase flow have emerged as a major research tool. It is now possible, for example, to simulate the motion of several hundred bubbles and particles in simple flows and to obtain meaningful average quantities that can be compared with experimental results. While the generation of new numerical algorithms that are more accurate, faster, and more robust is important and will continue, the state-of-the-art in direct numerical simulations will be defined by two issues: The use of direct numerical simulations to achieve breakthrough understand of real physical systems and the development of numerical methods to deal with complex multiphysics systems found in engineering applications. Although progress can be made by ad-hoc funding of individual projects, it is argued that funding focused on project that are on the critical path will accelerate progress.
PARTICLE MOTION IN DISPERSED TWO-PHASE FLOW
267-274
10.1615/MultScienTechn.v15.i1-4.200
Martin R.
Maxey
Division of Applied Mathematics, Brown University, Providence, RI 02912, USA
The alternative methods for numerical simulation of dispersed flows are briefly reviewed before going on to a more detailed discussion of the Force Coupling Method (FCM). In this method, the fluid is simulated numerically in a Eulerian framework with a fixed mesh. The particles are represented by a set of body forces applied locally to the fluid that correspond to the force each particle exerts on the flow. The application of the FCM to gas-solids flows is discussed.
ON THE MULTISCALE TREATMENT OF MULTIFLUID FLOW
275-288
10.1615/MultScienTechn.v15.i1-4.210
True-Nam
Dinh
Center for Risk Studies and Safety, University of California, Santa Barbara, 6740 Cortona Drive, Goleta, CA-93117, California, USA; and Royal Institute of Technology, Stockholm S-10044, Sweden USA
R. R.
Nourgaliev
Center for Risk Studies and Safety, University of California, Santa Barbara, 6740 Cortona Drive, Goleta, CA-93117, California, USA
Theo G.
Theofanous
Center for Risk Studies and Safety, University of California, Santa Barbara, 6740 Cortona Drive, Goleta, CA-93117, California, USA
This contribution surveys the various scales at which multi-phase systems can be modelled (effective field modelling, capturing large scale discontinuities at the largest scale, down to molecular dynamics modelling at the smallest scale) and shows how a multiscale treatment is necessary to capture the important flow features. The multiscale approach is illustrated with several examples.
PREDICTION OF MULTIPHASE FLOWS: A PERSONAL VIEW
289-292
10.1615/MultScienTechn.v15.i1-4.220
Geoffrey F.
Hewitt
Department of Chemical Engineering & Chemical Technology, Imperial College of Science, Technology & Medicine, Prince Consort Road, London SW7 2B Y, England, UK
A short overview is given of the historical development and future prospects for predicting multiphase systems. This starts with a discussion of empirical correlations and goes on to review the roles of phenomenological models, multifluid models and direct numerical simulation (DNS).
MICROPHYSICS OF ANNULAR FLOWS
293-305
10.1615/MultScienTechn.v15.i1-4.230
Thomas J.
Hanratty
Department of Chemical and Biomolecular Engineering, University of Illinois at Urbana-Champaign Urbana, Illinois 61801, USA
This paper reviews a number of microphysics issues in annular flows. These include the atomisation (drop formation) processes, the formation and behaviour of interfacial waves, the deposition of particles (droplets) and the effects of drag reducing agents.
THE PHYSICS OF THE CONTACT LINE REGION OF NON-ISOTHERMAL SYSTEMS
307-313
10.1615/MultScienTechn.v15.i1-4.240
Peter C.
Wayner, Jr.
The Isermann Department of Chemical Engineering, Rensselaer Polytechnic Institute, Troy, New York 12180-3590
This paper presents a brief survey of the phenomena associated with the three phase contact region where a liquid-vapour interface intersects a solid substrate. The equilibrium and non-equilibrium cases are considered and the areas that need to be addressed are listed. Some significant roadblocks to progress are identified.
NUCLEATION SITE DENSITY
315-321
10.1615/MultScienTechn.v15.i1-4.250
Dean Vijay K.
Dhir
Henry Samueli School of Engineering and Applied Science, Mechanical and Aerospace Engineering Department, University of California, Los Angeles, Los Angeles, California 90095, USA
Nucleation is a fundamental process in boiling heat transfer. This paper briefly surveys the characteristics of homogeneous and heterogeneous nucleation and discusses the density of nucleation sites on a boiling surface and the role of these sites in governing boiling heat transfer. Key areas needing further investigation are listed,
GAS-LIQUID TWO-PHASE FLOW AND BOILING IN MINI AND MICROCHANNELS
323-334
10.1615/MultScienTechn.v15.i1-4.260
S. Mostafa
Ghiaasiaan
Department of Mechanical Engineering, Georgia Institute of Technology,
Atlanta, GA, USA
This paper reviews work on adiabatic two-phase flow and forced flow boiling in microchannels with hydraulic diameters of the order 0.1 to 1 mm. In contrast to normal channels, stratification does not occur, the flow regimes are insensitive to channel orientation, and plug and slug/bubble train regimes are dominant. Pressure drop and void fraction are reasonably well predicted by models for macrochannels, except in annular or froth flows. In flow boiling, the onset of nucleate boiling (ONB) and onset of significant void (OSV) are more complex than in macrochannels. Critical heat flux in flow boiling has been extensive studied; for low flows it is dominated by film dryout but models developed for film dryout in macrochannels are unlikely to be applicable to microchaimels since many of their constituent relationships (e.g. droplet entrainment and deposition) would be expected to break down for this case.
THE IMPORTANCE OF MICRO AND MACRO SCALES IN BREAKUP AND COALESCENCE
335-342
10.1615/MultScienTechn.v15.i1-4.270
Ellen K.
Longmire
Department of Aerospace Engineering and Mechanics, University of Minnesota, 110 Union Street SE, Minneapolis, MN 55455, USA
This paper briefly reviews the status of knowledge of breakup and coalescence of fluid volumes, topological transitions that occur frequently in liquid/liquid and liquid/gas systems. Although many studies have been performed on both breakup and coalescence, significant questions remain unanswered concerning the dynamics of each process. A key difficulty is that, in most applications, the driving flow scales are macroscopic but the breakup or coalescence transition scales are microscopic such that non-continuum or molecular effects can play a role. A list of basic issues related to breakup and coalescence is given. Then, suggestions are made for experimental and computational research strategies that can address these issues.
ATOMIZATION AND DROPLET BREAKUP, COLLISION/COALESCENCE AND WALL IMPINGEMENT
343-348
10.1615/MultScienTechn.v15.i1-4.280
Rolf D.
Reitz
Engine Research Center, University of Wisconsin-Madison, Rm 1018A, 1500 Engineering Drive, Madison, Wisconsin 53706, USA
This paper discusses the phenomena associated with the creation and subsequent behaviour of liquid droplets. Droplets are created via primary (liquid jets or filaments) and secondary processes (turbulence, cavitation) and undergo subsequent collisions between themselves and between the containing surfaces. These processes are reviewed and areas where future research should be focussed are discussed.
NUCLEATION PHENOMENA IN BOILING
349-363
10.1615/MultScienTechn.v15.i1-4.290
True-Nam
Dinh
Center for Risk Studies and Safety, University of California, Santa Barbara, 6740 Cortona Drive, Goleta, CA-93117, California, USA; and Royal Institute of Technology, Stockholm S-10044, Sweden USA
Theo G.
Theofanous
Center for Risk Studies and Safety, University of California, Santa Barbara, 6740 Cortona Drive, Goleta, CA-93117, California, USA
The mechanisms of phase nucleation are reviewed, starting with the classical theories of homogeneous and heterogenous nucleation. The principle issues in nucleation are addressed (including nucleation on smooth cavity-free surfaces and the role of nano-scale phenomena). It is concluded that nucleation is not well understood and the importance of strict protocols in experiments is stressed.
THE HYDRODYNAMICS OF TWO-PHASE FLOWS WITH SURFACTANTS
365-371
10.1615/MultScienTechn.v15.i1-4.300
J. B.
McLaughlin
Clarkson University, Potsdam, NY 13699-5705, USA
This contribution reviews the influence of surface-active molecules in two-phase flows. Surfactants at their interfaces can exert a strong influence on the motion, the coalescence of and the mass transfer from bubbles and drops. The need for more realistic models and careful experiments (particularly in turbulent systems) is stressed.
SOME ISSUES RELATED TO THE MICROPHYSICS OF FORCED CONVECTIVE BOILING
373-375
10.1615/MultScienTechn.v15.i1-4.310
Jean-Marc
Delhaye
Department of Mechanical Engineering, Clemson University, Clemson, SC, USA
The microphysics of forced convective boiling is discussed in the context of boiling in nuclear fuel assemblies. The prediction of wall heat transfer and local area concentration are unsolved problems. It is suggested that these problems be addressed using micro-visualization and direct numerical simulation.
COMMENTS ON THE BREAK UP OF AN IMMISCIBLE FLUID PARTICLE (DROP OR BUBBLE) IMMERSED IN A TURBULENT FLOW
377-379
10.1615/MultScienTechn.v15.i1-4.320
Juan C.
Lasheras
Department of Mechanical & Aerospace Engineering, University of California, San Diego, La Jolla, CA 92093-0411, USA
The breakup of fluid lumps (drops or bubbles) immersed in a turbulent carrier fluid has been widely studied and there is an extensive bibliography evolving from the original models of Kolmogorov (1949), Baranev et al. (1949) and Hinze (1955). However, it is very doubtful if these simplistic models can apply to realistic engineering systems. Future pathways for experimental studies of this problem are reviewed, including the possible uses of Taylor-Conette flow systems for studying droplet/turbulence interactions.
TWO-PHASE FLOW VISUALIZATION OF R134A IN A MULTI-PORT MICROCHANNEL TUBE
381-393
10.1615/MultScienTechn.v15.i1-4.330
V.
Nino
Department of Mechanical and Industrial Engineering, University of Illinois at Urbana-Champaign, 1206 W. Green St., Urbana, IL 61801, USA
P.
Hrnjak
Department of Mechanical and Industrial Engineering, University of Illinois at Urbana-Champaign, 1206 W. Green St., Urbana, IL 61801, USA
T.
Newell
Department of Mechanical and Industrial Engineering, University of Illinois at Urbana-Champaign, 1206 W. Green St., Urbana, IL 61801, USA
Results are presented from a flow visualization investigation of a multi-port microchannel tube using R134a, a medium pressure refrigerant. The study covers mass fluxes from 50 to 300 kg/s.m2 and qualities ranging from 10% to 90% with a 6-port microchannel tube with a hydraulic diameter of 1.5mm under adiabatic conditions.
The results from the flow visualization studies indicate that several flow configurations may exist in multi-port microchannel tubes at the same time while constant mass flux and quality flow conditions are maintained. Based on these observations, development of a typical flow regime map does not appear to be an appropriate manner for describing the flow field.
Flow mapping of the fluid regimes in this multi-port microchannel is accomplished by developing functions that describe the fraction of time or the probability that the fluid exists in an observed flow configuration. Pictures from flow visualization results over the mass flux and quality range investigated are included.