D. Moalem Maron

Flow pattern transitions in two-phase liquid-liquid flow in horizontal tubes

Abstract As the various concepts and results experienced in gas-liquid two-phase flows cannot readily be translated to liquid-liquid systems, an attempt is made to form a basis for constructing a general two-fluid flow pattern map. The departure from a steady stratified configuration to other bounding flow patterns is analysed in view of the relationships between the instability criterion and the conditions for reality of characteristics, which evolve from exploring the stability and well-posedness of the governing equations. The transitional boundaries between the other flow patterns encountered in liquid-liquid systems are obtained based on mechanistic models. A parametric study made for wide ranges of geometry and physical properties, as encountered in liquid-liquid systems, is also included. Comparisons of the proposed transitional criteria with (limited) available data in liquid-liquid systems show reasonable agreement. The convergence of the general criteria to the extremes of gas-liquid data, on the one hand, and the data of highly viscous core flows, on the other hand, is satisfactory.

Stability analysis of stratfied liquid-liquid flow

Abstract Linear stability analysis is being widely used in exploring gas-liquid stratified/non-stratified transitions. As the present study relates to liquid-liquid two-phase systems, the stability characteristics of stratified layers are considered. In parallel, the conditions necessary for real characteristics are also explored. The relations between stability conditions and those for well-posedness are discussed in view of the implications for flow pattern transitions. The convergence of the stability conditions to various extremes is discussed. The integrated considerations of stability and well-posedness shed light on the physical sequences in flow pattern transitions, which so far have been studied by stability analysis alone. As liquid-liquid systems are involved, wide ranges of density and viscosity ratios are studied for various operational conditions.

A two-fluid model for stratified flows with curved interfaces

Abstract This study is motivated by the need to develop a practical tool for predicting the interface shape in stratified flow of a general two-fluid system. A configuration of a curved interface is considered. A two-fluid model is used to solve the momentum equations for a variable interface curvature. Energy considerations provide a closure relation for the interface curvature. The analysis identifies all the input dimensionless parameters which determine the solution for the stratified flow pattern. When these are given, a complete solution of the problem is obtained, including the interface shape, in situ hold-up and pressure drop. The validity of the two-fluid model is evaluated by comparing its prediction with available experimental data and with the results of exact analytical solutions for laminar flows with curved interfaces. Thus, the conventional two-fluid model has been extended to tackle stratified flow with curved interfaces and various flow regimes, in which case analytical solutions are complicated and restricted to laminar flows.

Determination of the interface curvature in stratified two-phase systems by energy considerations

Abstract A configuration of a plane interface between two stratified layers is appropriate for two-phase systems which are dominated by gravity, as is the case for large scale air-water systems under earth gravitation. However, for a general two-fluid system, the basic in situ configuration is stratified layers with a curved interface. The prescription of the characteristic interface curvature is required in order to initiate the solution of the flow problem and the associated transport phenomena. Energy considerations are employed to predict the interface configuration. The effect of the fluid physical properties, in situ hold up, tube dimension, wall adhesion and gravitation on the characteristic interface curvature are explored. The prediction of interface curvature provides the closure relation required for a complete solution of stratified flows with curved interfaces for a variety of two-fluid systems.

Analysis of stratified/non-stratified transitional boundaries in inclined gas-liquid flows

Abstract Stability and well-posedness analyses are shown to yield complimentary predictive tools for the stability of stratified gas-liquid flow and the departure to other bounding flow patterns over a wide range of (upward and downward) inclination. The conditions for marginal stability and well-posedness are further shown to coincide with the conditions for stable kinematic and dynamic waves derived from wave theory. A complete stratified/non-stratified transitional boundary is proposed, which shows satisfactory agreement with experimental observations in horizontal and inclined conduits. The observed sensitivity of the departure from the stratified configuration to the flow inclination is well-elucidated, in view of the dramatic effects of inclination on the structure of the stability and well-posedness map. The effects of physical properties, liquid viscosity, phases densities and density differential on the stability and well-posedness map in inclined systems are also explored.

Analytical solution for laminar two-phase flow in a fully eccentric core-annular configuration

Abstract Fully eccentric and concentric core annular flows represent two extremes which are of practical interest with regard to the performance of core flows. The fully eccentric configuration, which is obviously the problematic one, has been tackled herein by introducing a unipolar coordinate system, since the bipolar coordinate system fails to describe the flow field for this extreme. The analytical solution obtained yields the velocity profiles, wall and interfacial shear stresses and the resulting insitu holdup and pressure drop. The determination of the flow characteristics for fully eccentric flows is important as a bound to evaluate the effect of the core eccentricity in annular flows and as a complementary information to previous solutions of stratified flows with curved interface.

Hydrodynamic mechanisms in the horizontal slug pattern

Abstract The slug flow pattern is associated with an intensive mixing zone at its frontal region, where the fast-moving liquid slug front overruns the substrate film ahead. This mixing zone demonstrates intense fluctuations and asymmetry of the various flow characteristics, which clearly can not be treated analytically. An attempt to provide some insight into the complexity of the hydrodynamic mechanisms in the mixing zone is made by applying the k-l , k-ϵ models and numerically evaluating the various hydrodynamic characteristics. Particular boundary conditions at the free-moving interface have been developed and incorporated in the numerical simulation.

Heat and mass transfer in direct contact hygroscopic condensation

A theoretical analysis of direct contact hygroscopic-condensation of cold vapor on hot films is presented. The condensation of the relatively low temperature, low pressure, vapors on a hot film of an hygroscopic brine solution may occur due to the reduced vapor pressure of a sufficiently concentrated solution.The driving force for condensation is the difference between the partial pressure of water in the brine and the partial pressure of the condensing water vapor. The condensation is also governed by simultaneous mass transfer mechanisms, due to a non-isothermal absorption, with a possible opposing thermal driving force in the condensing vapor phase. The overall performance is determined by the accumulating effects of the various resistances to heat and mass transfer. The present study is aimed to elucidate the controlling mechanisms associated with this absorption-condensation process, and suggest overall transfer rates at the laminar and turbulent flow regimes.ZusammenfassungEs wird eine theoretische Analyse der hygroskopischen Kondensation eines kalten Dampfes auf heißem Film bei unmittelbarem Kontakt vorgestellt. Diese Kondensation bei relativ niedriger Temperatur, niedrigem Druck des Dampfes auf heißem Film einer hygroskopischen Sole-Lösung kann auftreten durch Druckerniedrigung über einer genügend hoch konzentrierten Lösung.Die treibende Kraft für die Kondensation ist der Unterschied zwischen dem Partialdruck des Wassers in der Sole und dem Partialdruck des kondensierenden Wasserdampfes. Die Kondensation wird auch durch gleichzeitig auftretende Stofftransportmechanismen gesteuert, resultierend aus einer nichtisothermen Absorption, die durch eine entgegengesetzte, thermische treibende Kraft in der kondensierenden Dampfphase ermöglicht wird. Das gesamte Verhalten wird bestimmt durch die akkumulierenden Effekte der verschiedenen Widerstände auf den Wärmeund den Stofftransport. Die vorliegende Studie hat zum Ziel, diese steuernden Mechanismen, die mit Absorptions- und Kondensationsprozessen verbunden sind, zu klären und es wird ein mittlerer Wärmeübergangskoeffizient für laminare und turbulente Strömungsbereiche vorgeschlagen.

Condensation inside near horizontal tubes in co-current and counter-current flow

Abstract Vapor condensation rate inside horizontal conduits was studied for co-current and counter-current flow of steam and the accumulated condensate at the bottom of the tube. When compared with the co-current flow case, the interfacial shear increases the axial pressure drop in the counter-current case and decreases the effective transfer surface. The effect of changing the inclination angle of the tube from 1° to 2° from the horizontal on the heat transfer rate is relatively unimportant. For a given tube length, co-current flow is the recommended mode of operation when a higher condensate production rate is desired at a given temperature driving force.

Wettability and break-up of thin films on inclined surfaces with continuous and intermittent feed

Abstract The conditions for complete wettability of dry or wet surfaces and film break-up have been studied. The minimum wetting rates and critical rate before break-up have been determined with various operation conditions, including liquid properties, temperature level and surface inclination. Two modes of liquid distribution at the top of the surface are considered, continuous and intermittent. The results are compared and discussed with reference to previous studies.