Investigation of an in-line, non-destructive ultrasonic method to study flow and rheology of complex fluids

Ultrasound pulsed Doppler and ultrasound pulsed Doppler velocimetry (UPDV) imaging have been implemented to study the flow and rheological properties of complex fluids undergoing steady pressure driven tube flow. This in-line shear viscosity determination method relies upon obtaining steady state velocity profiles using UPDV and simultaneous pressure measurements. The shear rate is determined as a function of radius by differentiating the velocity profile and the conservation of linear momentum theory provides a relation between shear stress and radius that is independent of the constitutive relation. Each radial position gives a shear viscosity-shear rate data point where the shear viscosity is defined as shear stress divided by the shear rate at that radial position. Shear viscosity data obtained using the ultrasound pulsed Doppler velocimetry system for a wide range of complex fluids are compared with the results of conventional rheometers. The fluids studied are: Micro crystalline cellulose (MCC) gel, xanthan gum solution, modified and unmodified starch suspensions and gels, tomato concentrates, diced tomato particles in juice, hardwood and softwood paper pulp suspensions, polyacrylamide micro bead suspensions and a polymer melt. The rheological and flow properties of these fluids are investigated including studies of apparent wall slip phenomena and yield stress behavior. The standard deviation of shear viscosity data at low shear rates is determined by resolution of the velocity data. A design curve is generated to predict the deviation of the shear viscosity from the rotational rheometers at low shear rates. A method to study turbulent flows with an analysis of Doppler power and spectral broadening of frequency spectrum in ultrasound pulsed Doppler velocimetry measurements is described. The experimental results reveal that the discrepancies in Doppler power are observed for different fluids but the normalized spectral broadening index provides an indication of turbulence.