Aluf Orell

Pool boiling from large arrays of artificial nucleation sites

Abstract Pool boiling heat transfer from large and controlled arrays of artificial nucleation sites was studied experimentally. Organic liquids were boiled from artificial sites of uniform size, shape and spacing, drilled in superfinished horizontal surfaces at site densities of 8 and 16 per cm 2 . The results confirm the existence of two heat transfer regimes which are characteristic of boiling from a constant number of sites [2]. Appropriate correlations, based on a physical model, were developed for predicting the heat transfer rate in the two regimes over a wide range of site densities and Prandtl numbers. A heat transfer mechanism is proposed and discussed.

Dry patch formation in liquid films flowing in a heated horizontal channel

The hydrodynamic and heat-transfer characteristics of thin liquid films flowing in a horizontal heated channel were investigated experimentally and analytically for the purpose of modelling film breakdown and dry patch formation. A physical criterion for film breakdown was formulated and incorporated in a model, containing one empirical parameter, that successfully predicts the critical heat flux for water and organic liquids for 5 < Re < 100. Equations for predicting axial film thickness profiles for both heated and unheated film flow were derived based on a proposed downstream boundary condition.

CONDENSATION OF BUBBLE TRAINS: AN APPROXIMATE SOLUTION

Abstract Adjacent bubbles affect the flow and temperature fields and the rate of bubble collapse differs from that of single bubbles in an infinite expanse. An exact iterative numerical solutionis quite complicated. Here, the collapse rate of a bubble train is achieved, albeit approximately, by a closed form analytical solution of a single bubble condensing in flow and temperature fields characteristic of the multi-bubble system. This approach allows for a general solution of single-and two-phase bubbles, including the effect of noncon-densables. The average driving force affecting bubble collapse is first approximated by assuming simple functional relationships between the radii of the successive bubbles. A more accurate solution, based on computing bubble size while solving for the temperature along its path, is then presented. Effects of the temperature driving force, number of bubbles in a row and bubble frequency are demonstrated.

ON THE LIQUID FLOW REVERSAL IN BUBBLE COLUMNS

Abstract The maximum range of the radial position within which liquid flow reversal can be expected to occur in zero net liquid flow bubble columns is predicted. It is shown that existing models, that employ this position as an input parameter for predicting the liquid velocity profile, are intrinsically valid only when the flow reversal dimensiontess radius is confined to ihe narrow range of 0.644-0.707. It is demonstrated that radial positions outside this range are unacceptable on physical grounds. Guidelines for evaluating the appropriate location of the flow reversal point for typical bubble column operating conditions are proposed.

Laser Grating technique for measurement of local velocity of large drops and jets in liquid-liquid systems

Abstract Measurements of the local rise velocity of large liquid drops of 3–7 mm in diameter and the local axial wave propagation velocity on a liquid jet surface in liquid-liquid systems were successfully performed using a Laser Grating Velocimeter (LGV). The primary frequency of the photodetector signal that is required for evaluation of the local velocity, was determined unambiguously by a frequency-domain Fast Fourier Transform analysis of the velocity signal. The validity of this technique, which eliminates potential measurement errors, was confirmed by a controlled experiment. The LGV was applied to the determination of the drop terminal rise velocity distribution of a population of 300 drops, and axial distribution of the jet surface wave velocity in order to demonstrate the capability and effectiveness of the technique.