Effect of industrial scale stand-off distance on water jet break-up, cleaning and forces imposed on soil layers
The effect of stand-off distance (the distance between a stationary nozzle and a soiled surface, L) on cleaning by a water jet generated by a 3 mm nozzle (diameter, dN) was studied. Two nozzle gauge pressures, pN = 2.0 bar and 4.0 bar, giving jet Reynolds numbers of ReJet = 43800 and 61600, were investigated, for four soils with different cleaning behaviour (petroleum jelly, ketchup, mustard, starch + diatomaceous earth). For long times, of tc = 10 min and 20 min, the area cleaned increased with L due to jet break-up increasing the area exposed to impact. For soils which are easy to clean (here ketchup) at shorter time scales (tc < 0.1 min) a stand-off distance of L = 2 m gave optimal cleaned area. Soils which are hard to clean required small stand-off distances with short cleaning times. The cleaning results are compared with jet break-up, impact force and impact pressure results presented by Fuchs et al. (2018). Jet break-up gives a larger impact area which results in a more efficient cleaning for most soils. However, results of the cleaned area only correspond to measurements of jet impact area in the case of soils that are not swellable and hard to clean. Binding forces within the soils were determined with a micromanipulation apparatus. Results show that the basic cleaning behaviour of the soils can also be determined on a laboratory scale without using a water jet. This allows the prediction of cleaning behaviour without extensive cleaning tests as well as preselection of cleaning devices.