Turbulence effect on direct‐contact heat transfer with change of phase: Effect of mixing on heat transfer between an evaporating volatile liquid in direct contact with an immiscible liquid medium

Abstract

The concept of local isotropy in turbulent agitation together with the particular characteristics of volatile drops evaporating while in dispersion were used to derive a simple and practical correlation relating the specific power input with the temperature driving force and the heat flow rates for all conceivable mixing regimes. The correlation was verified by experiments which indicate that dilute evaporating dispersions are controlled by viscous shear stresses.The generality of the proposed correlation is demonstrated by its application to pilot plant and laboratory data obtained over a wide range of turbulent mixing intensities. A t its lower limit the correlation reduces to the well-known relationship of surface-boiling heat transfer.Direct-contact heat transfer between a volatile dispersed phase evaporating into the continuous phase is a relatively new mode of heat transfer operation. It is at present mainly associated with water desalination projects based on direct-contact freezing. Apart from the usual advantages of direct-contact operations under dispersion (high transfer area per unit volume, uniform driving forces, and absence of scale), this technique permits appreciably low mass flow of the coolant, lower temperature gradients between the phases, and convenient continuous separation of the phases; its transfer coefficients are one order of magnitude higher compared with nonevaporating systems (1).The object of this work was a general and simple relationship between the energy input and turbulence in the mixing apparatus, the temperature driving force, and the heat flow rate for the three-phase two-component system. In deriving this relationship, use was made of the wellknown two-phase correlations €or liquid-liquid and liquidgas systems, as well as of the particular characteristics of volatile drops and layers evaporating by direct contact with another immiscible liquid.For the sake of completeness, the established mixing theories used in this work are briefly reviewed. MIXING REGIMES FOR LIQUID-LIQUID AND LIQUID-GAS SYSTEMSDifferent mixing correlations are obtained when one operates under different mixing regimes (2, 3 ) . This explains the wide variety of contradictory mixing correlations proposed in literature, sometimes on the basis of apparently similar systems.When two immiscible fluids are agitated, a dispersion is formed in which continuous breakup and coalescence of droplets (or bubbles) occur simultaneously. The average size of the droplets at equilibrium then depends on the Zvi Barsky is with the Ministry of Development, Jerusalem, Israel.conditions of agitation and physical properties of the components. Drops and bubbles may be broken up by viscous shear forces, especially near the agitator blades, as welt as by turbulent velocity (kinetic energy) and pressure variation along the surface of a single particle. In addition, turbulent flow may accelerate or slow down coalescence of the droplets (3) . Whereas the first phenomenon usually predominates in dilute dispersions, prevention of coal...

Mass Transfer in Gas-Liquid Contacting Systems

“…Empirical considerations based on mass transfer between a liquid and suspended solids led Vermeulen (48) to suggest that kL CC A0'6 ex (P/V)0-2 (11) An almost identical relationship was obtained by Davies, Kilner, and Ratcliff (9) who studied gas absorption rates in exceedingly clean water surfaces in a stirred cell. This is also in fair agreement with the empirical correlation suggested by Yoshida (54), which reduces to kL cc A70-42 cc (P/V)°-u (turbine) (12) kL cc A70-54 cc (P/V)0M (vaned disk) (13) Similar theoretical and experimental relationships have been found for the heat transfer coefficient (6,13,44).…”

“…have been suggested (4,8,22,25,32) and the correlation constants tabulated (44) for various impeller types. More recent work (26,30,50) indicates that the above type correlation is suitable for constant Vs only, since different curves are obtained for each value of Vs.…”

Drop size distribution in agitated liquid‐liquid systems

“…Rietema (11) discusses control of reaction rate in a stirred-tank reactor through control of drop size and interfacial area. Interfacial area control also plays an important role in liquid-liquid extraction (15), dispersion polymerization (7,8), and direct-contact heat transfer (18).…”

Evaluation of Interfacial Heat Transfer Models for Flashing Flow with Two-Fluid CFD

“…However, there is no consistent definition of the Reynolds number and the criterion for transition. On the other hand, Sideman [34] approximated it as a ratio of the bubble diameter d to the fluctuation velocity at a distance of d…”