Microtech2010 2010

Experimental Study of Drying/Evaporation in an Effective 2D Porous Medium

A.G. Yiotis, I.N. Tsimpanogiannis, A.K. Stubos
National Center for Scientific Research “Demokritos”, GR

Keywords: evaporation front, microporous media, experimental, phase change


Liquid-to-gas phase change, where the mass transfer that drives the process occurs in the gas phase, and the subsequent growth of the produced gas phase within a porous medium, is involved in a wide variety of processes of significant scientific and industrial interest. The process has applications in energy production, environmental protection, as well as other industrial or health-related applications. This phase-change process, also known as evaporation or drying (depending on the area used) has a plethora of practical applications including in food industry, pharmaceuticals, ceramics, cements and other construction materials, paper and textile industry, and fuel cells. Evaporation is also one of the dominant processes (another one being the capillarity-driven liquid flow) in microfluidic devices that are used to cool electronic equipment with applications ranging from satellites to portable electronics. For the drying/evaporation process significant progress has been achieved in our understanding of the various phenomena occurring at the pore scale including the effects of capillary, viscous, and gravity forces, convective transfer, counter-current diffusion in both gas and liquid phase, and film flows. Processes of interest to this study included the isothermal drying and evaporation in porous media. Experiments of drying in 2-D effective microporous media were conducted. In particular, Hele-Shaw cells packed with glass-microbeads (40 micron) were constructed using thin glass. Two pieces of glass (2.35x1.65 cm2) were put together with epoxy glue (three sides were glued, while the fourth remained open to the environment). The constructed cells had a gap of 550 micron (z-direction) rendering the medium essentially 2-D. Initially the cells were saturated with hexane and were positioned horizontally in order to study the evolution of the liquid gas interface as it receded within the porous medium. The experiments were recorded with a CCD camera and stored for additional studies. The images were captured and went through an image processing procedure. We extracted information that delineates the motion of the gas liquid interface as a function of time. The mass fractal dimension of the gas/liquid interface was measured from the obtained images, with the “box-counting” method, and was found to be in very good agreement with the value obtained from numerical simulations and experimental results. We also calculated the gas/liquid front width. A power-law scaling relation of the front width with the velocity of the interface was also obtained. This capillary number reflects the fact that drying was controlled by diffusion in contrast to external drainage. The scaling exponent predicted for 2-D geometries was in very good agreement with the experimental results reported by Shaw who measured the width of the interface as a function of the velocity in similar experiments. Finally the experimental results were analyzed and further discussed using concepts of immiscible displacements in porous media driven by mass transfer.
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