Hydrodynamics of impinging liquid jets used in cleaning
Impinging liquid jets are widely used in industrial cleaning-in-place (CIP) systems to remove residual product or soil layers from the internal surfaces of process equipment such as tanks and vessels. The optimisation of these cleaning operations is often done empirically as a large number of parameters are needed to define the problem. Three aspects of jet behaviour were investigated as a step towards enabling the systematic design and optimisation of CIP systems.
The effect of jet length and wall curvature were explored. The flow patterns generated by the impingement of a coherent, turbulent, horizontal water jet on a flat, vertical target were characterised as a benchmark and compared with existing models that predict the shape of the radial flow zone (RFZ). As the jet length increased, some liquid was lost to splatter through jet breakup into droplets and rebound of liquid droplets off the target. The shape of the RFZ agreed with existing models once the fraction of liquid lost to splatter was accounted for. Tests on horizontal and vertical cylinders with curvatures in the range 6.9 to 20 m⁻¹ showed that wall curvature did not have a significant effect on the shape of the RFZ unless the liquid film wrapped around the inside of the cylinder, observed at high jet flow rates. There was no appreciable effect of wall curvature on cleaning behaviour. Soaking a water-soluble soil prior to cleaning increased its cleaning rate.
In many CIP systems, the liquid jet impinges at an oblique angle to the target surface and an understanding of the liquid flow distribution created by inclined jets is needed to predict the cleaning behaviour. Tests with inclined jets were carried out to establish the shape of the hydraulic jump formed and their cleaning behaviour. Three flow distribution models were developed and compared with the experimental data, providing an insight into the liquid flow distribution, but further work remains to enable the liquid flow distribution to be predicted a priori.
The cleaning of viscous soil layers (petroleum jelly, tomato ketchup and two toothpastes) from vertical walls by an intermittent water jet was investigated by using a moving interrupter plate to periodically disrupt the impingement of a continuous water jet. The use of intermittent jets was found to provide no improvement in the cleaning of these soil layers in the absence of soaking.