Experimental Study On Porous Mediaduring Drying Process By MicrofocusX－CT Dynamic Imaging

The drying in porous materials is a complex thermal physics process which happenswidely in various industrial fields, natural environments and daily lives of humanbeings. Heat and mass transfer proceeds simultaneously during a drying process in aporous material. There is an urgent need for techniques to measure the moisturedistribution and mass transfer features in porous materials during drying processes.Microfocus X-ray computer tomography has a great potential to be applied toobservation and measurement of the mass transfer processes because it has a highspatial resolution and is a non-invasion measurement technique without interferingthe inside thermal physics fields.The drying processes in loosely packed beds of expanded polystyrene particles andterylene fiber materials were studied by using a microfocus X-ray computertomography system. The results of experiments were obtained by the followingprocedures: (1) A series of multi-direction projection images of the packed bed duringthe drying processes were enhanced and analyzed by an image processing program toreconstruct the two-dimensional sliced images and three-dimensional images. (2) Thepore structure of the packed beds and internal moisture distribution in the porousmedia in the drying processes could be described visually. (3) Reconstructedtomograms were analyzed qualitatively and quantitatively using CT image analysismethod. The distribution and evaluation of liquid surfaces in cross-section versusaxial height of the packed bed at different time were obtained to analyze dryingcurves.This thesis also presented a brief review of the moisture transfer theories in porousmedia during drying processes. The binary images of the reconstructed tomogramswere compared with moisture phase distribution images obtained by computersimulations with an innovative pore network drying model. Fractal drying front,which was observed clearly in the present investigation, was analyzed.