Determining Key Variables of the Kinetic Theory of Granular Flow Using DWS

The granular temperature, pressure and viscosity are three key variables that underpin the kinetic theory of granular flows while they are also playing an increasing role in other contexts such as modelling heat transfer, segregation, erosion, attrition and aggregation in various granular processing technologies. The scarcity of experimental measurements of these quantities is a significant barrier to exploitation of these theories and models. Diffusing wave spectroscopy (DWS) is now a well-established technique for measurement of granular temperature in dense granular systems. Using a liquid-fluidized bed as a test-case, we demonstrate here that DWS can be used to obtain the granular temperature and, via kinetic theory of granular flow expressions for the granular pressure and viscosity. The determined granular temperature and pressure variation with solids fraction compares well with theoretical results of others. The analysis also shows that the granular viscosity estimates obtained from the various kinetic theory models available vary significantly, particularly qualitatively at the low coefficients of restitution thought typical of liquid fluidized beds.