Design Improvements of Sirocco Type Fans by Means of Computational Fluid Dynamics and Stereo Particle Image Velocimetry

A3 - Numerical Optimisation of Centrifugal Fan Performances

The performance of Sirocco type fans can be tested on conventional chamber test rigs with low uncertainties concerning flow-rate, pressure rise and efficiency. However, the conventional optimization of these systems require a lot of experimental effort and is very time consuming. Hence, optimization by numerical means such as Computational Fluid Dynamics would be favourable. In order to do so a reliable data base for the assessment of numerical results has to be provided. 2D Particle Image Velocimetry measurements can be performed with relatively low experimental effort. Comparison between velocity fields from experiment and simulation are already quite consistent. However, the third, out-of-plane velocity component remains undetected in principle. Hence, Stereoscopic Particle Image Velocimetry is applied that provides instantaneous 3D velocity fields in the measurement plane. Thus, besides the main flow the secondary vortex flow can be detected and the overall quality of measurement increases significantly. However, this is at the cost of a higher experimental effort. Furthermore, for this kind of internal flow not the whole velocity field of interest can be evaluated due to the shadowing of the scroll, the tongue and the rotor. Nevertheless, data is obtained that could not be achieved otherwise.
Numerical calculations are performed using steady RANS equations with SST turbulence model and highly resolved grids. Both structured and unstructured grids with hexahedral and polyhedral cells respectively can be applied. For a cell based solver and a comparable number of elements, both mesh types will yield equally good results at a similar numerical effort. However, mesh generation is much more flexible and significantly faster with unstructured grids. Comparison between numerical and experimental data shows deviations of less than five per cent for pressure rise and efficiency for a wide operating range. Direct and point wise comparison of the velocity fields show a high degree of consistency between the two methods. Once the numerical method has been established and proved to be accurate it can be used for the virtual optimization of e.g. Sirocco fans before building prototypes. Starting from a previous design an optimized design variant could be developed, that has a considerably smaller size and yields a much higher static efficiency at the best efficiency point. Therefore, Computational Fluid Dynamics in conjunction with Particle Image Velocimetry is a useful way to reliably improve Sirocco type fans.

Key Words: Sirocco Type Fan, Efficiency, CFD, Stereo PIV, Design Rules