Numerical and experimental investigations on capillary-driven thin-film evaporation with applications in pumped two-phase loop and loop thermosyphon
Thin-film evaporation in solid-liquid-vapor interface has been the focal point of engineering research and industrial applications for decades. The objective of this dissertation is to systematically investigate the surface-tension (capillary)-driven thin-film evaporation in porous media and of a sessile droplet on a solid substrate using computational experimental methods. In Chapter 2, a mechanical-capillary-driven (hybrid) two-phase loop (HTPL) was used as a test platform to study the capillary-driven thin-film evaporation at a system level. Deionized water is the working fluid for the HTPL. A computational model for the HTPL using a thermal-hydraulic network was developed to predict the steady-state thermal performance and the evaporation mode transition in the HTPL. It was found that there are three distinctive evaporation modes (flooded, partially flooded, and capillary) which are determined by the pressure conditions in the liquid and vapor volumes of the evaporator. As the heat input increases, the evaporation mode is shifted from flooded to partially flooded to capillary mode. The capillary limit, the maximum heat flux limit of the thin-film evaporation, is raised by increasing the mechanical pump flow rate. In Chapter 3, a loop thermosyphon as a passive system was used to study the effects of the evaporator wick structure and properties on the capillary-driven thin-film evaporation. The effects of the evaporator wick base thickness and wick permeability on the evaporator thermal resistance and the capillary heat flux limit were numerically analyzed. A monolayer wick, which was made of mono-size sintered copper particles embedded in a single layer of a wire mesh, was used in the evaporator to lower the thermal resistance. The thermophysical and hydrodynamic properties (e.g. thermal resistance and permeability) of the monolayer wick for various liquid levels were calculated by computational fluid dynamics (CFD) simulations. A thermal-hydraulic network model of the loop thermosyphon was used to study the evaporation mode transition in the monolayer wick in the evaporator. The thermal-hydraulic network model was experimentally validated by the experimental results for three different evaporator designs employing multilayer and monolayer wick bases, and porous and tubular wick posts. The evaporation mode transition from flooded to thin-film evaporation was detected by the evaporator thermal resistance which is directly related with the liquid level in the monolayer wick base. In Chapter 4, an active flow control scheme was developed for the HTPL in an effort to control the capillary-driven thin-film evaporation in the evaporator. The relation between the capillary pressure head and the evaporation modes was established and used for the active flow control. An active flow algorithm was developed to control the mechanical pump flow rate to maintain the capillary pressure head in the evaporator constant, while operating under varying heat input. It was found that the active flow control greatly extends the operating range of the HTPL using the capillary thin-film evaporation and significantly reduces the pumping power consumption compared to a constant flow operation. The robustness of the active flow control was confirmed by operating the HTPL under a dynamic (on-off) heat input cycle along with the active control successfully to maintain the capillary pressure head at a preset value without temperature overshooting. In Chapter 5, the surface evaporation of a sessile droplet on solid substrates was numerically and experimentally investigated. A CFD model was developed to analyze the heat and mass transfer for the surface evaporation of a water droplet on solid substrates in ambient. The results from the CFD simulations were in good agreement with the experimental results for a water droplet evaporation on a hydrophobic-coated silicon substrate. It was found from the CFD simulations that the thermocapillary-driven flow (the Marangoni flow) and natural convection play a great role in the droplet evaporation and internal liquid circulation in the droplet. The effects of contact angle, thermal conductivity of the substrate, and ambient humidity on the droplet evaporation were numerically investigated and the results are discussed.