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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.

The focus of this study is the design of a collector with the objective of achieving uniform flow in multiple parallel microchannels. This objective is achieved by understanding the limitations of current designs and a novel design is proposed, which is further carefully optimized. The existing collector shape considered is U-type, which is investigated numerically. The creation of a stagnation zone, growth of a boundary layer along the collector wall and low/high velocity zones in the collector are identified as the prime causes of flow maldistribution. A novel design, a dumbbell shape collector, is proposed to overcome the limitations of the earlier designs. The dumbbell shape is evaluated quantitatively and is found to perform better than all existing shapes. This dumbbell shape collector provides a uniform flow distribution with less than 0.4% relative difference from the average flow rate in different channels, which is substantially better than existing collectors with 2.3% relative difference from the average flow rate for Rech  =  32. The uniformity is further confirmed using micro-particle image velocimetry measurements. The dumbbell shape collector is generalized and optimized to cater to heat sinks of different dimensions and to broaden its applicability in both micro and macro dimensions.

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