Flow fields created by impinging liquid jets and applications in cleaning

Cleaning is an essential domestic and industrial operation. It is particularly important in the food and pharmaceuticals sectors, where a large proportion of the total water consumption (as much as 70%) is used just for cleaning. Impinging liquid jets are frequently used for cleaning operations and currently, almost all industrial cleaning systems are based on empirical results. In an effort to develop efficient and sustainable cleaning systems, I have studied the flow field created by impinging liquid jets and their application in cleaning. On impingement of a liquid jet onto a surface, the liquid spreads radially outwards until it reaches a point where the liquid film changes its thickness abruptly. This transition from a thin film to a thick film is demarcated by a hydraulic jump. The supercritical thin film flow is also associated with higher momentum and shear stress, and is therefore, key for cleaning and heat transfer applications. For more than a century, it has been believed that these thin film hydraulic jumps are created due to gravity. However, in this dissertation it is shown both experimentally and theoretically, that these hydraulic jumps result from energy losses due to surface tension and viscous forces alone and gravity plays no significant role. The new theory allow the size of thin film region (location of the hydraulic jump) to be predicted and manipulated. The location where flow in the thin film becomes turbulent is also considered. The average-velocity and the location demarcating the laminar to turbulent transition were measured and showed good agreement with the model. The model for cleaning by normally impinging jets (by peeling mechanism) (Wilson et al., 2014) was then extended to consider cleaning by oblique impinging jets and compared with experimental data which showed a good agreement. Finally, cleaning scenarios arising in industrial systems where the liquid jets moves across a soiled surface were modelled. In these scenarios the liquid jet impinges obliquely and moves with varying velocity. A simple mathematical framework was developed for these systems on which more detailed models can build on.The Commonwealth Scholarship commissio