SESSION 4
MULTIPHASE MEDIA AND PHASE CHANGE 1
TWOPHASE FLOW INSTABILITIES IN HORIZONTAL AND VERTICAL INTUBE FLOW BOILING SYSTEMS
Sadık Kakaç and Claudia O. Gavrilescu
Department of Mechanical Engineering
University of Miami
Coral Gables, FL 33124, USA
In this work, twophase flow instabilities in boiling vertical and horizontal systems are presented. In the first phase of this study, the experimental results of densitywave type oscillations in a single channel, forced convection boiling upflow system using water are given. The limiting heat flux and quality of the onset of densitywave type oscillations were determined; their dependence on system pressure, mass flux, inlet subcooling and exit restriction was found and presented graphically. Also a correlation is formulated for the prediction of the limiting heat flux and quality of densitywave type oscillations. The second phase of this study involves the prediction of the low frequency oscillations in a horizontal system using R11. The steadystate system pressuredrop characteristics are determined by a numerical solution of the governing equations as derived from the DriftFlux model. The transient characteristics of the twophase flow are obtained for various parameters. The numerical solutions are determined using an explicit finite difference scheme. A satisfactory agreement between the theory and experiments is obtained.
FREQUENCY RESPONSE CHARACTERISTICS OF A TUBETYPE CONDENSER WITH TIME VARYING HEAT FLUX AND TWOPHASE PRESSURE DROP
B. L. Bhatt and G. L. Wedekind
Oakland University
Rochester, Michigan USA
In a tubetype condenser involving complete condensation, small changes in the inlet vapor flowrate momentarily cause very large transient surges in the outlet liquid flowrate. Sinusoidal inlet flowrate variations of small amplitude also cause outlet flowrates of large amplitude. Compressibility effects tend to attenuate the amplitude of these oscillations. The System Mean Void Fraction (SMVF) model was used to predict the frequency response characteristics with and without compressibility. In addition to compressibility effects, the SMVF model is extended to include time varying heat flux, a transient twophase pressure drop and liquid inertia. The SMVF model predictions are compared with the available experimental data and the theoretical models. SMVF model predictions represent the data better.
TAYLORTYPE INSTABILITY EFFECT ON THE TWOPHASE FLOW PATTERN IN THE HEATED POROUS MEDIA
L. A. Bol'shov, V. V. Likhanskii, A. V. Moskovchenko and V. F. Strizhov
Russian Academy of Sciences, Nuclear Safety Institute,
52 Bol'shaya Tul'skaya, 113191 Moscow, Russia
Heat transfer capability of the convective boiling mechanism in the porous media is limited by the flooding phenomenon or/and frictional coupling between the phase flows. This limitation is strongly influenced by the twophase flow pattern establishment which depends upon a set of factors, such as the boundary conditions and pore size. In this work it is considered a situation where subcooled liquid penetrates from above in the porous layer heated from below. It is shown that Taylor type instability may initiate a nonhomogeneous liquid penetration in the layer in the form of columns which are spatially separated by a scale of wavelength of more fastly growing instability perturbation. This effect make less critical flooding limitation in the case of relatively small pore sizes because of phase flows friction diminishing in the separated phaseflow mode.
ON THE TRANSIENT HEAT AND MASS TRANSFER MODELING OF DIRECT CONTACT EVAPORATORS
E. M. Queiroz, C. M. Hackenberg
EQ , COPPER/UFRJ  Chemical Engineering Dept.
C.P. 68502, Rio de Janeiro, RJ , 21945970  Brazil
The dynamical modeling to describe the transient heat and mass transfer processes during operation of direct contact evaporators is presented in this work, including a comparison to experimental results that were measured in a pilot plant, specially built to utilize combustion gas and water solutions, in the Thermofluid Dynamics Laboratory at COPPE/UFRJ. The dispersed phase temperature and solvent vapor concentration fields are described by non coupled diffusive models for spherical bubbles of radius, a, with angular symmetry assumption. This system of equations is solved by a functional method that yields the interfacial temperature and concentration transient behavior. An additional equilibrium surface condition is used to couple the heat and mass transfer mechanisms and to determine the mass evaporation flux at the interface. This modeling may be extended to apply to other geometric configurations and to the simulation of submerged combustion units as well.


