Local wall shear stress variations predicted by computational fluid dynamics for hygienic design

Food producers depend to a great extent on the availability of easy-to-clean processing equipment. This paper presents initial work carried out to improve the application of commercially available computational fluid dynamics (CFD) programs as a tool to improve hygienic design of food processing equipment. In the present paper, previous work on using CFD to improve hygienic design are combined with work that shows the importance of measured local mean wall shear stresses and the fluctuations in these wall shear stresses with respect to cleaning effects of the flow. Results of wall shear stress and the fluctuations acquired by the measuring technique are difficult to apply directly for actual design optimization. Instead, steady-state CFD simulations using STAR-CD were applied to predict the fluctuation of equivalent mean wall shear stresses on the entire surface of pipes with various diameter changes (gradual and sudden expansions or contractions). Measured fluctuation rates of wall shear stress are shown to be analogous to the turbulence intensity in the near-wall region predicted by CFD simulations. The results are promising; hence, an important step has been taken towards setting up a method for evaluating cleanability by steady-state CFD simulations based on evaluation of the mean wall shear stresses and the turbulence intensity.