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Cambridge Fluids Network


Flow behaviour and cleaning by impinging jets

Liquid jet impinging    Liquid jet impinging close-up

Credit: Rajesh Bhagat

Liquid jets created by spray balls and nozzles are widely used in industry for cleaning the surfaces and internals of process vessels. With vertical or inclined walls, the jet creates a thin, fast moving film characterised by strongly radial flow which terminated by a film jump similar to the hydraulic jump seen with vertical jets striking horizontal surfaces. The liquid then drains vertically under gravity. The flow behaviour in the thin film is important for determining the rate of cleaning of any soil on the surface where this is influenced by shear forces or convection. The flow in the draining film determines how much of the surface is contacted with cleaning solution and thus exposed to lower shear stresses and reaction with cleaning agents. This project is employing various techniques, including high speed photography, to understand the flow behaviour of these jets.  We are also considering the coupled problem of the interaction of the liquid film with the soil layer, for a range of soil rheologies and reactivity.

Left: Liquid jet impinging on vertical surface at an oblique angle: surface waves and unstable rope just before the rope becomes a draining film.

Right: Liquid jet impinging perpendicularly on a vertical surface - early stage. Unstable circular perimeter, before formation of a draining film.

Crack propagation through a drying colloidal dispersion

 Thin colloid film drying and cracking

 Credit: Lucas Goehring

 A dispersion of silica particles is dried on a flat substrate. At the top of the image the dispersion is still fluid, with a volume fraction of around 10 %. The particles consolidate into a solid at the compaction front which is distinguishable by a sharp change in colour. A little distance below this a fracture front can be seen. The cracks follow the consolidation front through the drying material. Notice the regularity in crack spacing. Even lower in the film the particles become dry and a dewetting front is observed.