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

Abstract

Direct-contact heat exchangers that involve energy exchange between gas and liquid streams have a variety of applications, including waste heat recovery, thermoelectric power plant cooling, and thermal desalination. Directcontact heat exchangers are appealing as they may help mitigate potential corrosion, fouling, and scaling of solid surfaces and enhance heat transfer effectiveness. In this study, we experimentally investigate the thermohydraulic characteristics of an economic lightweight direct-contact heat exchanger that incorporates an array of strings of diameter of the order of 0.1~1 mm to sustain flows of thin liquid films. We constructed a 1.6 m-tall prototype heat exchanger with an array of as many as 112 vertically aligned strings. Thin films of a non-evaporating liquid are flown down the strings by gravity and exchange thermal energy with a counterflowing gas stream. We obtained axial liquid temperature profiles and frictional loss in the gas stream for different combinations of liquid and gas flow rates and two different string pitches. Numerical simulation is also performed to help interpret and indirectly validate our experimental results. The overall, gas-side, and liquid-side heat transfer coefficients extracted from the experimentally measured temperature profiles are examined to evaluate the impact of instability in liquid film flows and inter-bead spacing. The applicability of the Reynolds analogy is also assessed using the measured gasstream pressure drops and air-side heat transfer coefficients. The present study helps improve our understanding of heat transfer and gas-stream pressure drop in string-based direct-contact heat exchangers and provides an experimental database to help systematically optimize their design. KEY WORDS: Direct-contact heat exchanger, interfacial heat transfer, dry cooling, thin film flows Liquid films develop wavy patterns as they flow down strings Gas streams (in) Fan Gas streams (out)

Water vapor capturing using an array of traveling liquid beads for desalination and water treatment

“…This was confirmed in an independent experimental study of heat transfer in a multistring heat exchanger using nonevaporating liquids. In that study, the Reynolds analogy was shown to hold reasonably well, indicating that the frictional component to the pressure drop dominates over the component associated with form drag ( 18 ).…”

“…Despite offering high interface-to-volume ratios, the multistring array introduces small pressure drops for gas flows in part because it provides straight, open and contiguous gas flow paths and in part because low-profile traveling liquid beads present relatively small form drag ( 15 – 18 ).…”

A highly effective multi-string humidifier with a low gas stream pressure drop for desalination

“…This is in part due to the additional form drag caused by liquid beads traveling down the strings (see Fig. 7) [29], which is not accounted for in the engineering correlation for smooth rod bundles. The acceleration pressure drop due to evaporation contributes 10 -15% of the total pressure drop at mass flow rates higher than 10 kg/h where evaporation rate is correspondingly higher.…”

“…A previous study using a non-volatile liquid as a working fluid demonstrated that a direct-contact heat exchanger of the same geometric design can deliver the same heat transfer effectiveness at a much lower pressure drop than traditional parallel-plate structured packings [29,30]. Earlier studies of multi-string exchangers also suggested their applications in CO 2 absorption and hydrazine vapor separation [31][32][33].…”

Thermally-driven coalescence in thin liquid film flowing down a fibre

“…Thin liquid films flowing down a vertical fibre subject to thermal effects have been previously studied owing to their importance in a variety of industrial applications, which include dry cooling systems (Zeng et al. 2017; Zeng, Sadeghpour & Ju 2018), and heat and mass exchangers for vapour, CO and particle capture (Sadeghpour et al. 2019, 2021; Zeng, Sadeghpour & Ju 2019).…”