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

摘要

In this paper a numerical methodology to segregate relative and absolute flow structures, allowing a deep analysis of the impeller-tongue interaction in centrifugal fans, is presented. The procedure, based on a deterministic decomposition of the internal flow fields, is applied for the first time in blade-to-blade planes of radial turbomachinery. Previous numerical results, obtained with a viscous 3D unsteady solver, already validated by the authors and available in the literature, are used as a database for the numerical routines. Interpolation and relocating operations of the velocity fields between CFD meshes and post-processing grids are presented and performed for different flow rates of a squirrel cage fan. This numerical technique is shown as an ideal framework to advance in the physical comprehension of the complexity, three-dimensionality and unsteadiness of the flow structures in the interaction regions of centrifugal fans. Additionally, the different contributors to the unsteady forces on the blades are also addressed, providing a valuable insight to identify the origin of the interaction phenomena for both diagnosis and redesign criteria of impeller and volute geometries. 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 m~3/h and a specific speed n_s = 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.