Vapor-Liquid Interfaces and their Implications on Phase-Change Inside and Outside Microchannels

摘要

Vapor-liquid interfaces encountered during condensation of refrigerants in micro- and mini- channels are discussed in this paper. Flow mechanisms documented through the use of novel visualization techniques were used to develop flow regime maps for condensing flows in microchannels. The variation of these flow regime maps with tube geometry, and departures from similar phenomena in larger diameter tubes are addressed. Insights obtained from the knowledge of these interfacialphenomena are used for the development of flow regime-based pressure drop and heat transfer models for the mass flux range 150 < G < 750 kg/m{sup}2-s, which includes regimes such as intermittent, wavy, annular and dispersed flows. These flow regime-based models yield substantially better predictions than the traditionally used correlations that are primarily based on air-water flows for large diameter tubes. Interfaces that develop during gravity-driven flow of binary fluids over microchannel tube banks are also presented. High-speed flow visualization of the flow of LiBr-H{sub}2O and NH-H{sub}2O solutions on tubes is used to demonstrate that the falling-film process consists of stages such as droplet evolution, elongation to liquid threads, formation of satellite droplets, impact on to subsequent tubes, and formation of axially non-uniform and spreading saddle waves on the tube surface. A semi-automated edge detection-based image analysis technique is developed to quantify the interfacial area and volume of the droplets during their formation, detachment, fall and impact. In addition, a fixed-grid 3-D computational model of the same process is developed using the volume-of fluid method to track the liquid-vapor interface. This 3-D numerical model accurately matches all the important experimentally observed characteristics of droplet formation, detachment and impact Models based on these integrated experimental and computational studies are used to develop miniaturized heat and mass transfer components for binary-fluid absorption and desorption processes in thermally activated heat pumps.