Cleaning in a backward facing step

Abrupt changes in geometry such as sharp expansions, e.g. a backward facing step, are normally notrecommended under hygienic design guidelines.In some cases such features are unavoidable and it is important to understand how cleaning in such spaces differs from that in simple duct geometries.The removal of model diffusive food soil (based on instant coffee) from 304 stainless steel surfaces was studied in a rectangular duct cell at Reynolds numbers spanning the transition to turbulence (3 800-11 000).Removal was monitored visually and the evolution of the size and shape of the cleaned area as well as the local erosion rate were calculated.The cleaning behaviour observed immediately downstream of the expansions differed noticeably from that upstream, and further downstream.Insights into this are provided by 2D computational fluid dynamics simulations, and in some cases these are able to give reasonable estimates of the experimental cleaning rates.