Cleaning of a soluble soil layer by flow over a flat surface: an example of a Stefan-Graetz problem

• Cleaning of a finite soluble soil is marked by the evolution of cleaning front(s). • Ignoring the moving boundary leads to large overestimation of cleaning times. • Simulations show good agreement with experiments using a coffee soil. • Recirculation zones control behaviour in a radial flow cell. • Turbulent duct flow removal exhibits streaks arising from flow striations. The cleaning of a soluble soil layer by the flow of solvent is an important example of a moving boundary problem coupled with a non-linear flux distribution. Removal of a model dissolving soil, a soluble coffee extract, from flat stainless steel plates by the passage of cold water was investigated experimentally in two geometries: ( i ) a long rectangular duct where the flow is turbulent and well developed, and ( ii ) a radial flow cell with Reynolds numbers < 1000, where the flow is laminar and steady (but not simple). Cleaning was quantified as the time to remove the soil and was found to be strongly location dependent, owing to the moving front, which was compounded further in ( ii ) owing to the non-uniform mass flux profile in this geometry. Cleaning in the rectangular duct featured striations associated with periodic turbulence patterns, while two cleaning fronts were observed in the radial flow cell owing to the presence of recirculation zones. A framework for computing the local cleaning time is presented which shows that neglecting the moving boundary nature of the problem can result in large overpredictions of cleaning times. Detailed simulations of mass transport were used to estimate local cleaning behaviour, with velocity fields calculated by CFD and diffusion coefficients obtained from separate experiments. The results showed good agreement with the experimental values.