CFD STUDY OF THE LEAKAGE FLOW IN LOW-SPEED AXIAL-FANS WITH ROTATING SHROUD

The fundamental aim of this thesis is to investigate the behavior of leakage flow within axial fans featuring rotating shrouds. The study encompasses two distinctive geometries. The first scenario, referred to as the prototype geometry, entails an L-shaped configuration in cross-section, facilitating the penetration of leakage flow towards the upstream side. In contrast, the second scenario introduces an updated geometry, which incorporates a rim on the stationary casing to mitigate the impacts of leakage flow. In the prototype geometry, the rotor undergoes axial deformation during operation, leading to alterations in the gap geometry. CFD simulations at the design point are employed to scrutinize the characteristics of leakage flow and its interaction with the main flow fields. Given that the rotor is made of low-stiffness material and experiences axial deformation during operation, Fluid-Structure Interaction (FSI) simulations are utilized to capture the deformed geometries at various rotational speeds, thereby evaluating the effects of this deformation on the leakage flow pattern. The study's findings offer valuable insights into the characteristics of leakage flow within the gap, which have not been previously investigated either experimentally or numerically. It is evident that modifying the gap geometry has substantial implications for the properties of leakage flow, whereas variations in rotational speed at a fixed operating point do not yield significant impacts. The analysis of different flow parameters did not reveal their direct influence on leakage flow behavior. Instead, it is evident that only modifications to the gap geometry, whether altering its shape or adjusting the position of the rotating wall relative to the stationary one, can effectively address this phenomenon. However, it is important to note that while gap geometry modifications can mitigate the adverse effects of leakage flow, they may not completely eliminate them. The results indicate a distinct periodic pattern in leakage flow, consistently associated with specific angular positions of the blades, irrespective of whether the rotor has evenly spaced blades. The influence of this periodicity on the leakage flow is prominently observed in the phase-averaged data. It becomes apparent that increased interaction of the leakage flow with upstream flow corresponds to a higher occurrence of random coherent structures in the leakage flow. Conversely, when the interaction is minimized, the fluctuating properties are reduced. Furthermore, the level of interaction between the leakage flow and upstream flow directly correlates with the suppression of leakage flow tangential effects. Consequently, there is a clear relationship between the tangential effects of leakage flow and the pressure disturbance on the blade sections. In the prototype geometry at Ω=2400 rev/min, where there is an attached leakage flow to the shroud, the effects are pronounced, significantly impacting tip blade sections up to 85% of the normalized blade's length. In contrast, at Ω=3000 rev/min, the leakage flow generates a large circulation zone at a radial distance from the shroud, resulting in weaker effects that cover a range of blade sections from 50% span to the tip. In the second scenario involving the updated geometry, the influence of the novel gap configuration on leakage flow is compared against the former arrangement. With the implementation of a rim in the updated geometry, leakage flow effects are notably contained, and the flow tends to encircle the rotating ring. Notably, the pressure disturbance on the blade's section is confined to approximately 5% of the blade's section, particularly at the tip area, showcasing a substantial 96% reduction compared to the prototype geometry at Ω=2400 rev/min. This underscores the efficacy of the rim application in effectively mitigating the impact of leakage flow, particularly in limiting its effects on the blade sections. Moreover, to address fan fluttering during operation in the updated geometry, a rigid body motion technique is employed to simulate such fluttering phenomena. The resulting effect of this rigid motion on leakage flow characteristics is then compared to the non-flutter case, providing further insights into the interplay between fan dynamics and leakage flow behavior.