Direct-contact 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. Direct-contact methods are appealing as they may help mitigate potential corrosion, fouling, and scaling of solid surfaces and enhance transfer effectiveness. The widely used direct contact exchangers these days, such as packed bed and spray columns, suffer from the problems of low gas loading limit or inefficient transfer rate. Alternative exchanger technologies that can circumvent these limitations are urgently needed to enable wider adoption of direct contact methods. This dissertation presents the study on an innovative alternative to the direct contact heat and mass exchanger. The new economic light-weight direct-contact exchanger incorporates an array of strings of diameter of the order of 0.1~1 mm to sustain flows of thin liquid films. Thin liquid films flow down the strings by gravity and exchange thermal energy with a counterflowing gas stream. To enable physics-based systematic design of the multi-string based direct contact exchanger, we need rigorous understanding of the fluid dynamics and interfacial heat and mass transfer of liquid films flowing down strings (i.e. a polymer or cotton string) against counterflowing air streams.

We began our study from numerically investigating the fluid dynamics and heat transfer of a liquid film flowing down a single string. We constructed finite element models using a moving mesh method to solve the time-dependent Navier-Stokes equation and the energy equation to obtain velocity and temperature distributions in the liquid film. The temporal variations in the temperature of travelling beads are analyzed to evaluate the effective heat transfer coefficients and to assess the accuracy of an approximate one-dimensional model.

We then conducted a combined experimental and modeling study of the flow and heat transfer characteristics of thin liquid films flowing down a single string in the presence of a counterflowing cooling gas. We focus on the Rayleigh-Plateau (RP) regime, where uniformly spaced drop-like liquid beads travel on a thin liquid substrate formed along the entire length of a vertical string. Using a high-speed camera and micro-thermocouples, we capture liquid film/bead profiles and temperatures at different air velocities and at different nozzle diameters. Finite element models are also constructed to help interpret and validate experimentally obtained heat transfer characteristics.

Moreover, we extended our experimental study from a single string to a direct-contact multi-string heat exchanger. We constructed a 1.6 m-tall prototype heat exchanger with an array of as many as 112 vertically aligned strings. We limited ourselves to non-evaporating liquids and non-condensing gases (air). We measure axial liquid temperature profiles and gas-stream pressure drop to examine the impact on the thermohydraulic performance of the liquid and air flow rates, instability modes, and string pitch. The applicability of the Reynolds analogy is also examined.

Finally, we adapted our multi-string heat exchanger to a humidifier for thermal desalination. We investigate the evaporation rate as well as gas phase pressure drop. The evaporation process involves simultaneous heat and mass transfer. We report a combined experimental characterization and modeling study to validate our humidifier design for desalination purpose. The effects of the liquid flow rate, air velocity, and liquid salinity on the evaporation rates are experimentally characterized. The gas-stream pressure drop of the multi-string humidifier is measured and compared with existing humidifier designs.