Experimental investigation of interface deformation in free surface flow of concentrated suspensions

It is well known that during the free surface flow of concentrated suspension of non-colloidal particles, the suspension-air interface becomes highly corrugated. This surface corrugation changes the interfacial area which could have important implications in various applications involving heat and mass transfer across the interface. Surface corrugation in free surface flow has been studied in the past, but its mechanism is not fully understood. We report detailed experiments on quantitative measurement of the surface deformation of concentrated suspension of non-colloidal particles in open channel flow. The motion and location of the interface and the velocity field of the bulk flow beneath the free surface were measured using the particle image velocimetry technique. Experiments were performed to study the effect of particle size, particle concentration, and viscosity of suspending fluid on the corrugation. The interface fluctuation was found to increase linearly with the flow rate. The deformation of the interface increased with increase in particle concentration until an optimum concentration is reached and thereafter it decreases. Our observation supports the previous studies on surface corrugation interpreted from the power spectra of the reflected light from the interface. Suspension of larger particles and less viscous fluid gives larger deformations of the suspension-air interface. These results can be used to determine the optimum parameters to control the interfacial area in free surface flow of concentrated suspensions. Published by AIP Publishing.