Mohammad Amir Neshat

This paper presents a numerical investigation of the effect of rotational speed on the development of leakage flow in a low-speed axial fan equipped with a rotating shroud. Rotor deformation induced by centrifugal forces and aerodynamic loading is taken into account through a one-way coupling between steady CFD simulations and static FEM analyses. Aerodynamic and structural results obtained at four rotational speeds are validated against available experimental data collected by the same research group, and the different leakage flow patterns associated with rotor deformation are correctly reproduced. Subsequently, the deformed geometries corresponding to two rotational speeds and operating at the same non-dimensional flow coefficient are used to perform URANS simulations. The numerical results provide insight into the leakage flow behavior within the gap between the rotating shroud and the stationary casing, a region that cannot be experimentally investigated due to optical access limitations. It is shown that the leakage flow rate through the gap shows limited sensitivity to rotor deformation, as it scales with rotational speed, and it is fed by two main contributions: a flow directly extracted from the rotor outlet and a recirculating flow developing along the mounting panel. Conversely, the non-dimensional angular momentum flow rate is larger at the lower rotational speed, corresponding to the case in which the leakage flow is rapidly re-ingested by the rotor. This indicates that the centrifugal effects associated with the leakage flow swirl are not responsible for the observed change in leakage flow pattern. Finally, significant periodic and non-periodic components are identified within the leakage flow. These components are expected to contribute to the formation of the large-scale structures impinging on the rotor blades and thus generating significant noise.

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The axial fans employed in automotive cooling systems are commonly provided with a rotating shroud which improves the volumetric efficiency and strengthens the whole assembly. The tip clearance between the rotating shroud and the stationary casing has a great impact on the development of a leakage flow which is released from the gap and is re-ingested by the fan. As a matter of fact, such a leakage flow is highly turbulent and may be characterized by large scale periodical structures hence affecting largely the operating conditions of the blade portion which is interested by the re-ingestion process. Different gap geometries, which may be resulting from design strategies or from fan deformation may induce highly different leakage flow patterns. The present work reports a detailed CFD analysis on the same geometry which has been experimentally studied in previous work. A coupled CFD – FEM approach has been adopted to evaluate the deformed fan shape in two operating conditions, and the so-obtained geometries have been studied through the employ of a URANS approach. The results have been validated through the available experimental data. The simulated leakage flow has demonstrated to be highly sensitive to slight modifications of the gap geometry due to the fan deformation, leading to completely different recirculation bubbles. As a consequence, the mixing process between the incoming undisturbed flow and the recirculating one is also affected, leading to the generation of largely different turbulent structures which are able to affect the unsteady pressure field of the blades. The blade loading variations have been statistically evaluated and the results indicated that a more compact recirculation bubble, i.e. a more direct re-ingestion of the leakage flow is able to affect largely the blade tip pressure field. For completeness and as a reference case a geometry without gap has been also considered.