Numerical methodology for the assessment of relative and absolute deterministic flow structures in the analysis of impeller-tongue interactions for centrifugal fans

The squirrel cage fan studied is a small centrifugal fan with a twin impeller configuration, each with 23 forward curved blades, a nominal flow rate at around 352 m3/h and a specific speed ns = 1.9. This type of squirrel cage fans is often used as blowers for automobile applications or for small industrial equipment. The flow in this kind of fans happens to be quite complex and with unsteady features. Unsteady flow separation at the machine inlet or at the impeller blades and a variety of flow induced vibrations is found for most of the operation conditions. In this context, the deterministic decomposition becomes an essential tool to analyze the main flow structures, like the evaluation of the non-uniformities induced by the volute tongue over the blade-to-blade distributions within the impeller. As a consequence, fluctuation levels in the blade loadings, derived from deterministic non-uniformities, can be provided in the relative frame of reference. The practical applications of the conclusions do imply a progress in the knowledge of the working parameters for machines that affect in a direct way to the passengers comfort.