Interfacial friction and transport in stratified flows

Transport of coarse solids (medium sand and coarser) in slurry pipes is often at least partially stratified, and flowing slurry tends to form a layered structure with a real or virtual interface between two layers. Modelling of friction and transport conditions associated with the interface is crucial to predictive models for the slope of the energy grade line in settling slurry flows. Bed friction and transport are interrelated. At the interface, high bed shear agitates solids transport and transported solids can significantly increase bed friction. The paper discusses the authors' recent results of experimental work and modelling efforts in describing bed friction and solids transport. The proposed friction and transport formulae are implemented in the authors' layered model for stratified flows above and below the deposition-limit velocity (the threshold velocity at incipient sliding of the bed). A comparison with validation data indicates a very reasonable capability of the model to predict the slope of the energy grade line, the deposition-limit velocity, and in case of flow above stationary deposit also the thickness of the deposit.