T.P. Figueroa - teodora@sc.up.br
P. Seleghim Jr. - seleghim@sc.usp.br
Núcleo de Engenharia Térmica e Fluidos
Departamento de Engenharia Mecânica
Escola de Engenharia de São Carlos
Universidade de São Paulo
Av. Dr. Carlos Botelho, 1465
CEP 13560-970 São Carlos - SP - Brasil

The basic motivation of this work comes from the fact that most of the numerical reconstruction algorithms were developed for hard sensing fields, i.e. they have the underlying assumption that the sensing field is two-dimensional and parallel. When using soft sensing fields, such as in electrical capacitance tomography of two-phase flows, these conditions are barely satisfied, unless in the situation of a poor contrast between the electrical properties of the flowing phases for which the ill-conditioned nature of the problem will be much more critical anyway. The development of a less restrictive method can be achieved by refining the qualitative images produced by a direct imaging probe, by means of the minimization of a conveniently defined error functional reflecting the difference between experimental and numerical values calculated at the boundary of the sensing volume. However, in order to do this, it is required that these boundary values be sufficiently sensitive to changes in the distribution of the electrical properties of the flow. This paper presents a sensitivity analysis of different excitation strategies and their applicability in a reconstruction algorithm such as described above. More precisely speaking, two classical excitation profiles are compared to a proposed one in what regards the sensitivity of the resulting measured profile at the boundary to changes in the positions of inclusions in the sensing volume. The results confirm that classical strategies suffer from a major lack of sensitivity and that new ones must be developed, possibly based on the optimization of the excitation profiles or on multi-sensing techniques.
 
 

Pires, M. - manolo@mecanica.ufrgs
Ferreira, V. C.S. - vferreira@pro.via-rs.com.br
Gutkoski, L.L. - hukax@ct.ufrgs.br
Universidade Federal do Rio Grande do Sul - Laboratório de Turbo-Máquinas e Medições de Fluxo - LTMF - UFRGS
CEP 900560-170 - Porto Alegre - RS, Brasil

Van der Laan, F.T. - ftdlc@vortex.ufrgs.br
Universidade Federal do Rio Grande do Sul - Departamento de Engenharia Nuclear -
DENUC - UFRGS

The present work investigates experimentally the pressure fluctuations due to discontinuities and internal instabilities in both phases of a two-phase flow. Specifically, is analysed the effects of the dynamic response of a cloud of bubbles in the transient regime. The interactive effects due to the variation of the bubbles volume upon the pressure distribution and on the flow field are very significant. Also, the flow velocity is strongly affected as the concentration of the bubbles increases turning larger the possibility of a global movement of the mixture. The results of this investigation reveal how the presence of the air bubbles affects the nature of the medium throughout the pressure wave, changing the specific gravity of the mixture and the bulk compressibility, therefore, changing the speed of the sound. It is apparent that an increment of the void fraction causes substantial reduction of amplitude response of the bubble.

Keywords: Group of Bubbles, Two-phase Flow, Discontinuities, Attenuation
 
 

Gherhardt Ribatski - ribatski@sc.usp.br
Samuel Freire de Barros - samuel@sc.usp.br
Evandro Fockink da Silva - fockink@usa.net
José M. Saiz Jabardo - mjabardo@sc.usp.br
Laboratório de Refrigeração - Departamento de Engenharia Mecânica
Escola de Engenharia de São Carlos - Universidade de São Paulo
Av. do Trabalhador São-carlense, 500 13560-120 - São Carlos - SP

Nucleate boiling heat transfer coefficient has been experimentally obtained for refrigerants R-11 and R-113. The boiling surface was a horizontal copper tube of 19 mm diameter immersed in a pool of saturated refrigerant. Operating reduced pressure varied in the range. Heat flux was limited to the range where fully developed nucleate boiling occurs, which in present study was limited to a maximum of 100 kW/m 2 . Results indicate a raising trend of the heat transfer coefficient with pressure e some influence of the pressure in the exponent of the heat flux in the heat transfer correlation. Experimental results have been compared with those from several correlations. Of those considered in present study, the correlation proposed by Cooper is the one that fits better the experimental heat transfer coefficient.
 
 

Arlindo de Matos (arlinmat@fem.unicamp.br)
Eugênio Spanó Rosa (erosa@fem.unicamp.br)
Fernando de Almeida França (ffranca@fem.unicamp.br)
FEM/DE UNICAMP CP 6122
13083-970 - Campinas, SP, Brasil.

Experimental results of local and cross section average void fraction in an air water flow in bubbly regime occurring on U bend are presented. The phase segregation is characterized by measuring the void fraction profiles along four sections by means of a conductive electrical probe. This flow is dominated by the buoyancy and centrifugal forces. The first is responsible for an increase on the cross section void fraction along the U bend while the former causes the bubble migration toward the inner radius of the curve. It is also observed that mostly of the bubble migrations movements takes place near the walls.
 
 

Rafaela Frota Reinaldo(*) - rafaela@labsolar.ufsc.br
Vinícius Fernando Kuser(*) - kuser@iname.com
Hein Auracher - auracher@iet.tu-berlin.de
Technical University of Berlin, Germany
Júlio César Passos(*) - jpassos@emc.ufsc.br
(*)Universidade Federal de Santa Catarina, Departamento de Engenharia Mecânica LABSOLAR-NCTS
88010-970 - Florianópolis, SC, Brazil

This work presents experimental results for R113 nucleate pool boiling inside smooth and grooved vertical and horizontal aluminum tubes, at atmospheric pressure and moderate heat flux (<45 kW/m 2 ). Heat flux and wall temperature related to the nucleation phenomenon are obtained. For the nucleate boiling regime, the effects of R113 subcooling, surface effect and tube orientation are investigated. Smooth tube experimental heat transfer coefficients are compared with Stephan and Abdelsalam, Cooper, Rohsenow and Forster and Zuber correlations and with grooved tube experimental values. For all tests, heat transfer coefficient is higher in the grooved tube than in the smooth tube. For horizontal orientation, there is an azimuthal temperature gradient due to the existence of different boiling mechanisms on the inside wall of the tube.

Keywords : Boiling, Enhanced surface, Nucleate boiling , Nucleation
 
 

Edvaldo Angelo - edvaldo.angelo@poli.usp.br
José Roberto Simões Moreira - jrsimoes@usp.br
Escola Politécnica da Universidade de São Paulo
SISEA - Laboratório de Sistemas Energéticos Alternativos
Caixa Postal 61548 - São Paulo, SP, Brasil

This paper presents a numerical analysis of evaporation dynamics of flashing jets. The study deals with a jet issuing from a conical converging nozzle discharging into a low-pressure chamber. The analysis is mostly concerned with the jet regime where neither heterogeneous or homogeneous nucleation takes place inside the nozzle. The sudden phase change occurs abruptly via an evaporation wave downstream the exiting section. The work is divided into two parts: (1) In the first part, the theory of oblique evaporation waves is considered; (2) Next, the expansion region of two-phase mixture is analyzed. A two-dimensional axisymmetric approach is used along with the classical method of finite difference of MacCormack.
 
 

R. K. Sharma sharmar@colorado.edu
R. L. Mahajan mahajan@colorado.edu
Department of Mechanical Engineering
University of Colorado at Boulder, CO-80309-0427, USA

In this paper, we report first ever-experimental data for laminar condensation heat transfer on horizontal and vertical cylinders for fluids with Stefan number greater than unity. The condensation experiments were carried out in saturated vapor of FC5311 . with 1/4" and 1/2" diameter copper cylinders for aspect ratios (L/D) ranging from 1 to 32. In our first set of experiments, lumped capacity method was used to determine the heat transfer coefficients. Benchmarking experiments for spheres matched with those reported in literature. Our results for horizontal cylinder indicate that for large aspect ratios (>16) condensation heat transfer coefficients were close to those predicted by Nusselt correlation for horizontal circular cylinders. With decrease in aspect ratio, the heat transfer rate increased due to contribution from cross flow and the sides. A correlation incorporating the effect of aspect ratio is presented. For vertical cylinders, condensation heat transfer for high aspect ratio (>8) was higher than that predicted by Nusselt correlation. This was attributed to the onset of waviness in the condensate film. At low aspect ratios, however, heat transfer in vertical cylinders was lower than the predicted values due to condensate run-off from the top horizontal surface. These results will contribute towards improved condenser design for applications, such as vapor phase condensation soldering process, deploying such liquids.

Keywords: condensation heat transfer, Stefan number, finite cylinders
 
 

Oscar M. Hernandez Rodriguez - oscarmh@fem.unicamp.br
Antonio C. Bannwart - bannwart@fem.unicamp.br
UNICAMP - FEM - Departamento de Energia
Cx.P. 6122 - Cidade Universitária - B. Geraldo - 13083-970 - Campinas, SP
Fone: (0xx19) 788-3262 / 3264 - Fax: 289-3722

The annular flow pattern formed by two immiscible liquids of very different viscosities (also called 'core annular flow') has found important applications in the transportation of heavy oils in horizontal pipes, through the addition of small quantities of a thinner fluid (usually water). Here the vertical flow is focused, in view of its possible application in heavy oil production. Including the interfacial tension and neglecting inertial terms in the annulus, equations are derived which govern the shape of the liquid-liquid interface. First, it is shown that the interface must be circular in the pipe cross section, as expected. Besides, the condition that pressure is continuous on each side of the interface leads to the conclusion that the interface profile generally presents axially symmetric waves, whose shape is governed by the Laplace-Young equation. Its solution reveals interface shapes which are entirely compatible with the "bamboo waves" observed by Bai, Kelkar & Joseph (1996) for upward flow. A simple model for wavelength prediction is proposed, which is in reasonably good agreement with presently available data. Keywords: Multiphase flow, Liquid-liquid flow, Interface shape, Vertical pipe, Heavy oil production
 
 

Alexandre Mendes
Marcos Sebastião de Paula Gomes (*) - mspgomes@mec.puc-rio.br
Pontifícia Universidade Católica do Rio de Janeiro, Departamento de Engenharia Mecânica
Rua Marquês de São Vicente 225 - Rio de Janeiro, RJ 22453-900
(*) Autor para o envio de correspondência

The work studies the generation of liquid and solid particles by the atomization process. The main objective was to investigate, experimentally, the influence of control parameters such as the atomization pressure and the air and liquid flow rates, in the size of the generated particles for a given atomizer geometry. Among the applications of practical interest we may find the generation of particles for the calibration of atmospheric samplers. An atomizer was designed and built for producing droplets in the size range between 0,5 µm and 10 µm, as a result of the atomization of a salt-water solution. The droplets were then dried by diffusion in dry air, producing solid particles. These particles were collected and analyzed by optical microscopy for the determination of the size distribution. By knowing the shape of the particles and the concentration of the salt solution, the mean equivalent volume diameter and the droplets Sauter mean diameter (SMD) were calculated. Despite some dispersion in the generation process (geometric standard deviation between 1,5 and 2), the method may be applied to the generation of particles in the calibration of atmospheric samplers.

Keywords: Aerosol Instrumentation, Calibration, Atomization
 
 

Luiz G. C. Duarte - duarte@nrva.ufsc.br
Centro Federal de Educação Tecnológica da Bahia, Departamento de Tecnologia Mecânica e Materiais, Rua Emídio dos Santos s/n, 40300-010, Salvador, BA

Álvaro T. Prata - prata@nrva.ufsc.br
Universidade Federal de Santa Catarina, Departamento de Engenharia Mecânica, NRVA, 88040-910, Florianópolis, SC

The main purpose of this work is to study the project of capacitive sensors for determining void fraction of oil-refrigerant mixture flowing in a small diameter glass tube. To develop the void fraction measurements, several experimental techniques were reviewed and the capacitance method was chosen to do it. This technique suggested best results in applications with very small tube diameter. Several parameters of capacitive sensors project are discussed and the best design are proposed to get high sensitivity. Looking for a uniform electric field between electrodes, a numerical simulation of several electrodes geometry and an evaluation of the effects of guard electrodes were done. The mixture electrical properties are unfavorable but it is possible to do void fraction measurements with capacitance technique getting goods results. It is necessary to be careful with sensors calibration because there are high influence of flow regimes and mixture used. Preliminary results of some measurements are shown in a situation where the void fraction value was maximum and minimum.

Key words: Void fraction, Capacitive sensor, Multiphase flow, Gas-liquid solubility, Lubrication
 
 

Norberto Mangiavacchi - norberto@icmc.sc.usp.br
Antonio Castelo Filho - castelo@icmc.sc.usp.br
Murilo F. Tom´ e - murilo@lcad.icmc.sc.usp.br
Jos´ e A. Cuminato - jacumina@icmc.sc.usp.br
Armando O. Fortuna - fortuna@icmc.sc.usp.br
Valdemir G. Ferreira - valdemir@lcad.icmc.sc.usp.br
Luis G. Nonato - nonato@lcad.icmc.sc.usp.br
Universidade de S˜ ao Paulo, Departamento de Ciˆ encias de Computac¸˜ ao e Estat´ýstica
Cx.P. 668 - 13560-161 - S˜ ao Carlos, SP, Brasil

Sean McKee - caas29@maths.strath.ac.uk
University of Strathclyde, Department of Mathematics, Glasgow, Scotland

This work presents a method for simulating axisymmetric free surface flows dominated by surface tension forces. The surface tension effects are incorporated into the free surface boundary conditions through the computation of the capillary pressure. The required curvature is evaluated by fitting a least squares arc of circunference to the free surface using the tracking markers in the cell and in its closest neighbours. To avoid short wavelength perturbations on the free surface, a mass-conserving local 4-point stencil filter is employed. This filter is a combination of the Trapezoidal Sub-grid Undulations Removal (TSUR) method (Castelo et al., 1999), which conserves area, and an appropriate mapping, in order to conserve volume and therefore mass. The TSUR technique consists of modifying the positions of the two "internal" markers of the stencil in such a way that the surface length and the curvature are minimized, while still preserving area. This technique was implemented in the GENSMAC code (Tome & McKee, 1994), and it has been proved to be robust. The code is shown to produce accurate results when compared with exact solutions of selected fluid dynamic problems involving surface tension. Additionally, it is demonstrated that the method is applicable to complex free-surface flows.

Keywords: Numerical simulation, Axisymmetric flows, Free-surface flows, Surface tension.
 
 

Luben Cabezas Gómez - e-mail: lubencg@sc.usp.br
Fernando Eduardo Milioli - e-mail: milioli@sc.usp.br
Núcleo de Engenharia Térmica e Fluidos, Escola de Engenharia de São Carlos, USP. Av.
Trabalhador São-carlense, N° 400-Centro, São Carlos, SP. CEP 13566-590.

Mathematical modelling and numerical simulation are developed for two-phase gas-solids flow through the riser of a circulating fluidized bed using a two-fluids model. Two methods are considered for treating the solids phase viscosity. Simulation is performed aiming to study the effect of the solids phase viscosity over the flow hydrodynamics. Validation is performed by comparing the results of simulations with experimentals data. Key-Words: Two-fluids model, numerical modeling, two-phase gas-solid flow, circulating fluidized bed, solid viscosity.
 
 

F. Schiavon - schiavon@sc.usp.br
P. Seleghim Jr. - Seleghim@sc.usp.br
Núcleo de Engenharia Térmica e Fluidos
Departamento de Engenharia Mecânica
Escola de Engenharia de São Carlos - USP
Av. Dr. Carlos Botelho, 1465
13560-970 São Carlos - SP, Brasil

The formal calibration procedure of a phase fraction meter is based on registering the outputs resulting from imposed phase fractions at known flow regimes. This can be straightforwardly done in laboratory conditions, but is rarely the case in industrial conditions, and particularly for on-site applications. Thus, there is a clear need for less restrictive calibration methods regarding to the prior knowledge o the complete set of inlet conditions. A new procedure is proposed in this work for the on-site construction of the calibration curve from total flown mass values of the homogeneous dispersed phase. The problem is formulated as a set of integral equations, which could be formally solved by setting an appropriate approximation for the calibration curve. However, due to an intrinsically ill conditioned characteristic, these equations cannot be solved in practical situations because of the severe influence of experimental errors. A solution to this problem is also proposed in this work. The method is based on minimizing an error functional constructed from a set of redundant measurements, which restores the lost information associated to the integration of the instantaneous mass flow rate in the one-dimensional one-velocity flow model. Numerical simulations performed for increasing errors demonstrate that acceptable calibration curves can be reconstructed, even from total mass measured within a precision of up to 4%. Thus, the method can readily be applied, especially in on-site calibration problems in which classical procedures fail due to the impossibility of having a strict control of all the input/output parameters.

Keywords: multiphase flow, instrumentation, phase meter, calibration
 
 

Vítor T. Lacerda - vitor@nrva.ufsc.br
Álvaro T. Prata - prata@nrva.ufsc.br
Universidade Federal de Santa Catarina, Departamento de Engenharia Mecânica
88040-900 - Florianópolis, SC, Brasil

Fabian Fagotti
Empresa Brasileira de Compressores - EMBRACO
89219-901 - Joinville, SC, Brasil

In the present investigation the refrigeration oil flow with refrigerant outgassing is explored experimentally. A mixture of oil saturated with refrigerant is forced to flow in two straight horizontal tubes of constant diameter. One tube is used for flow visualization and the other is instrumented for pressure and temperature measurements. At the tubes inlet liquid state prevails and as flow proceeds the pressure drop reduces the gas solubility in the oil and outgassing occurs. Initially small bubbles are observed and eventually the bubble population reaches a stage where foaming flow is observed. The flow visualization allowed identification of the two-phases flow regimes experimented by the mixture. Pressure and temperature distributions are measured along the flow and from that the mixture void fraction is estimated. General aspects of the mixture flow are addressed in the context of lubrication models to be used in analysis and simulation of lubricating process occurring inside the compressor.

Keywords: Lubrication, Oil-refrigerant iteration, Refrigerant outgassing from oil
 
 

Rubem da Cunha Reis - rreis@eq.pucrs.br
Pontifícia Universidade Católica do Rio Grande do Sul, Depto. de Eng ª Química
Av. Ipiranga 6681- CEP 90619-200, Porto Alegre, RS

César Antônio Leal - leal@vortex.ufrgs.br
Univ. Federal do Rio Grande do Sul, Depto. de Eng ª Nuclear e Programa de Pós-Graduação
em Engenharia Mecânica, UFRGS
Av. Sarmento Leite 425 - CEP 90050-170, Porto Alegre, RS

In this paper, it is presented the modeling of the transient that follows an accidental gas pipeline breakage. It is assumed that before the rupture the gas was flowing under super-critical pressure, something typical for this kind of system, and the aim is to establish the spatial and time variations of the variables mass flow, pressure and temperature, inside the system, during the mentioned transient. The drop in pressure induces a reduction of the gas temperature inside the pipeline and therefore incorporation of heat from the surroundings. The focus of the work is in the evaluation of the importance that this contribution of heat has on the obtained results. The modeling and respective results are presented for situations with and without thermal isolation of the pipeline, that is, models and results with contribution of external heat (convection) and without contribution of heat (isolated system). The situation studied is for a pipeline initially at ambient temperature of 25 0 C and operating pressure of 10,13 MPa, typical values for this type of system. Starting from this initial condition (critical flow) there will be the formation of choked flow and, for this reason, it is also presented a discussion of the use of a thermodynamical state equation for the evaluation of properties and its influence in the calculation of the choked flow. The obtained results indicate that the in-corporation of heat has little influence on the results obtained with the model.

Keywords: choked flow, critical flow, two-phase flow