Bo Boye Busk Jensen

Critical wall shear stress for the EHEDG test method

In order to simulate the results of practical cleaning tests on closed processing equipment, based on wall shear stress predicted by computational fluid dynamics, a critical wall shear stress is required for that particular cleaning method. This work presents investigations that provide a critical wall shear stress of 3 Pa for the standardised EHEDG cleaning test method. The cleaning tests were performed on a test disc placed in a radial flowcell assay. Turbulent flow conditions were generated and the corresponding wall shear stresses were predicted from CFD simulations. Combining wall shear stress predictions from a simulation using the low Re k-e and one using the two-layer model of Norris and Reynolds were found to produce reliable predictions compared to empirical solutions for the ideal flow case. The comparison of wall shear stress curves predicted for the real RFC with the empirical solution showed that the empirical solution gives a good prediction even in the real RFC from a radius of 15 mm and outwards.

Prediction of hygiene in food processing equipment using flow modelling

Computational fluid dynamics (CFD) has been applied to investigate the design of closed process equipment with respect to cleanability. The CFD simulations were validated using the standardized cleaning test proposed by the European Hygienic Engineering and Design Group. CFD has been proven as a tool which can be used by manufacturers to facilitate their equipment design for high hygienic standards before constructing any prototypes. The study of hydrodynamic cleanability of closed processing equipment was discussed based on modelling the flow in a valve house, an up-stand and various expansions in tubes. Results show that cleaning can be efficient in complex geometries even when the critical wall shear stress (determined in uni-axial flow) is not exceeded. This renders the need for considerations concerning three-dimensional flow, the degree of turbulence and the type of flow pattern. The controlling factors for cleaning identified were the wall shear stress and the nature and magnitude of recirculation zones present.

Numerical study of influence of inlet turbulence parameters on turbulence intensity in the flow domain: Incompressible flow in pipe system

Abstract The prediction of cleaning in pipe-lines is important for equipment manufacturers, who wish to optimize designs with respect to hygienic performance. Degree of cleaning correlates with the level of fluctuations in the signal recorded in discrete points during wall shear stress measurements using an electrochemical method. For optimization of process equipment with respect to cleaning, the levels of local fluctuations across entire surfaces are needed. Trends of fluctuations in the geometries used can be predicted using computational fluid dynamics (CFD). Two sensitivity studies were carried out to investigate the robustness of the CFD predictions: one with the zone of interest located two 90° bends and a straight pipe with a length to diameter ratio of 14:8 downstream of the inlet (full geometry) and one with only a straight pipe with a length to diameter ratio of 1:7 between the inlet and the zone of interest (shortened geometry). Both studies having a fully developed turbulent velocity inlet profile with changing turbulence parameters: turbulence intensity 0.01-30 per cent and turbulence length scale 7.5-30 per cent of inlet pipe diameter. For the full geometry no sensitivity in the fluctuation profile trend estimated by use of CFD was found, whereas for the shortened geometry a great sensitivity in the estimated fluctuation profile trend when changing inlet conditions was seen. In the process Star-CD and Fluent (used in the present study) were compared and showed comparable predictions of turbulence intensity.