2012 Annual Report - Jesus College - University of Cambridge

22 CHEMICAL ENGINEERING I Jesus College Annual Report 2012
Flow pattern created by a horizontal
water jet impinging on a vertical glass
sheet, taken from behind the glass.
Ruler tape markings at cm intervals.
Photograph courtesy of final year
MEng research project students
Liam Stevens and David Faria
In addition to knowing how thick and how strong a deposit layer is, we need to be able to
calculate the size of the forces acting on it to cause removal. This is relatively
straightforward for fluids flowing in filled pipes or ducts. Dishwashers and tank cleaning
systems, however, employ jets of liquid which impinge on a surface and drain away as a
thin film. Material is cleaned away quickly at the point of impingement but most of the
surface only sees the draining film. The photograph shows an example of the flow pattern
created by an impinging liquid jet: it involves a transition from turbulent to laminar flow
as well as surface tension shaping the free surface. These are complex flows and we use
experimental as well as computational techniques to calculate the shape and speed of the
flow and thereby determine the stresses imposed by liquid on the soil and its flow path.
Combining this with our knowledge of the soil’s strength with different detergents should
allow us to predict how long it will take to wash the dishes, without having to spend a
lifetime testing it at the sink. This knowledge can be applied beyond designing and
optimising dishwashers to the cleaning of tanks, disinfection systems and (if you think
about it) urinals.
Jesus College has supported this research in many ways, from hosting conferences and
academic visitors to providing able students and occasions for cross-disciplinary
discussions on the ethics of water supply, surfactant chemistry and the physiological
factors affecting the surface tension of urine. ■