Flow Analysis and Deterministic Decoupling in a Squirrel Cage Fan

Squirrel cage fans are 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, that makes it quite difficult to be studied. In particular, 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. The deterministic stress analysis becomes an interesting tool in analyzing the main flow features from an existing numerical model. In this paper, the analysis of the internal flow and the periodic phenomena resulting from a deterministic study is intended. To accomplish this goal, a series of numerical routines were performed for different flow rates, including also failing operating modes of the fans (blocked inlets as a typical problem for this type of arrangements). The squirrel cage fan studied is a small centrifugal fan with a twin impeller configuration, each with 23 forward curved blades. The blades chord is 0.013 m and each impeller has a diameter of 0.08 m and a width of 0.09 m. The performance curves (head, power and efficiency versus flow rate) was obtained numerically in previous works and confirmed experimentally in normalized test campaigns. These tests have shown a nominal flow rate at around 352 m³/h and a specific speed n_s = 1.9. The main goal of the paper consists on 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.