On the improvement of permeability assessment of fibrous materials

Abstract

The focus of this work is to understand the current state of permeability measurement and prediction methods for fibrous, porous media and to suggest improvements. For this purpose the most widely used and accepted measurement technique, the channel flow method, is used to experimentally investigate the effects of fiber sizing and fluid viscosity on the permeability of glass and carbon fibers. Experiments have shown that the variation in permeability occurs due primarily to the fluid viscosity and not the nature of fluid, which other researchers have proposed. Studies were also carried out on both sized and unsized fibers to show that significant permeability variation occurs when fluids of different viscosity are used. Further, experimental studies on the effect of secondary flow have revealed that, for fiber products representative of the aerospace industry, secondary flow has little effect, which challenges models proposed by other researchers. Previous studies had shown a dual scale flow for fiber products with a significantly lower fiber volume fraction. A novel acoustical method based on standardized impedance tube measurements has been developed to predict physical properties—both permeability and characteristic length—of the porous medium. The predicted permeability values from the acoustical method for the range of porosity studied in this work compare well enough with existing permeability models’ predictions to warrant further study. The acoustical method is quick and repeatable, and when compared with the existing flow methods may provide a convenient alternative. It also provides a measure of fiber arrangement (via the “viscous characteristic length”) that should be studied further to explain variations in permeability measurements due to alternative fiber product architecture.