Experimental study of heat transfer between thin liquid films flowing down a vertical string in the Rayleigh-Plateau instability regime and a counterflowing gas stream

Abstract

Direct contact heat exchangers based on wetted string columns offer an intriguing alternative to packed beds and spray columns. We experimentally examine the flow and heat transfer characteristics of thin liquid films flowing down strings of a diameter approximately 1 mm against a counterflowing air stream. Numerical simulations are also performed to help interpret and validate our experimental results. The Rayleigh-Plateau instability caused by interplay among surface tension, gravity, and viscous forces leads to the formation of uniformly spaced drop-like liquid beads traveling down a string. The liquid mass flow rate and also the nozzle radius influence the radius and spatial/temporal frequency of liquid beads. Aerodynamic drag exerted by the counterflowing air stream deforms liquid beads. The relationship between flow characteristics and heat transfer effectiveness are examined experimentally for different combinations of the air velocities, liquid mass flow rates, and nozzle radii. We show that the liquid mass flow rate and the air velocity are two primary factors influencing heat transfer effectiveness whereas details of the liquid flow instability affect local bead-to-air heat transfer coefficients. We also compare the heat transfer effectiveness and the pressure drop between a wetted string column that consists of an array of vertical strings and a well-established structured packing that consists of vertical plates. The wetted string column is shown to deliver comparable heat transfer performance but at a lower air pressure drop than the structured packing. The present work helps improve our understanding of the flow and heat transfer performance of string-based direct-contact heat exchangers and helps build a foundation for their systematic design and optimization. .

Thermohydraulic characteristics of a multi-string direct-contact heat exchanger

“…A previous study by Nozaki et al [7] and our numerical simulation study [34] indicated that spatial temperature variations within a liquid bead is less than 2% of the bead mean temperature. At measurement locations, secondary flows generated by interactions with the micro-thermocouples are expected to further…”

“…The actual liquid-side heat transfer coefficient would be higher than this value because internal circulation within each bead enhances heat transfer [33]. Note that, for a fixed nozzle diameter, liquid films flowing in the Rayleigh-Plateau regime at different flow rates produce liquid beads of approximately the same radii but different inter-bead spacings [21,34].…”

“…Hattori et al [33] presented an approximate analytical model for temperature distribution along a liquid film flowing down a string in the presence of cross-flows of a cooling gas. A modified version of the model in the counterflow configuration was validated in our previous study using a single-string [34]. A later study [7] constructed an experimental setup consisting of a single string and determined the liquid-to-gas overall heat transfer coefficient.…”

Absolute and convective instabilities of a film flow down a vertical fiber subjected to a radial electric field

“…In industrial applications, the coating processes may be operated in more complex environments and it is highly desirable to control the stability of the film by technological attempts. Recent and ongoing theoretical and experiment studies concentrate on the coating flows with thermocapillarity [14], in contact with current gas flow [15,16], flowing over a slippery surface [17]. When the coating flow is electrified by a concentric outer electrode, both the linear stability characteristics and nonlinear dynamics are altered.…”

Liquid‐bridge flow in the channel of helical string and its application to gas–liquid contacting process

“…Therefore, the liquid is forced to run on defined flow paths and no radial flow occurs that could lead to maldistribution. Many researches have been done to study the interphase mass or heat transfer for such wetted‐wire when applied in gas separation . Grunig et al used the watch chain, armor chain, bead chain and locking wire to improve the liquid flow pattern on the surface of the wire.…”