Cleaning viscous soil layers off walls by intermittent impinging jets
The removal of viscous soil layers from vertical walls by an intermittent water jet by was investigated using a novel apparatus featuring a moving interrupter plate to disrupt, periodically, a continuous water jet impinging on a Perspex® wall (2 L min-1, nozzle diameter 2 mm). The jetting period was varied by using different interrupter plates and the speed at which the plate moved. Four soil materials were studied, namely a petroleum jelly, tomato ketchup (both viscoplastic) and two types of toothpaste (one viscoplastic, one thixotropic). The jetting periods (60 ms and 630 ms) were slightly longer than the time taken to form the hydraulic jump (22 ms). The use of intermittent jets was found to provide little improvement on the cleaning of these soil layers in the absence of soaking. The removal behaviour of the three viscoplastic soil materials was described well by the model of Fernandes et al. (2019) Chem. Eng. Sci., 207, 752-758, whereas the initial removal behaviour of the thixotropic toothpaste was not. These results indicate that previous reports of the enhancement of cleaning by intermittent jets are influenced by cleaning in the region below the point of jet impingement, where removal by the draining film is influenced more strongly by soaking effects and wave formation.