EFFECT OF ROTOR-STATOR CONFIGURATION IN THE GENERATION OF VORTICAL SCALES AND WAKE MIXING IN SINGLE STAGE AXIAL FANS. PART I: LES MODELLING AND EXPERIMENTAL VALIDATION

The unsteadiness and vortical structures associated to impinging wakes convected through rotor and stator passages. have been studied in detail in a single-stage low-speed axial fan, with 9 blades and 13 inlet/outlet guide vanes. In particular, in this first part, the effect of complementary rotor-stator (RS) and stator-rotor (SR) configurations has been addressed in terms of wake mixing and generation of vortical structures in both absolute and relative frames of reference. A LES simulation of the midspan section (in a 2.5D model) is introduced to resolved the largest scales of the vortical motion within the wakes, related to vortex shedding, especially at off-design conditions. Chopping mechanisms and periodic interactions of the coherent turbulent structures are described and the presence of turbulent spots due to wake-wake interactions is revealed. Another relevant flow pattern, like the advection of leading edge separation through the downstream passage, is also identified and linked to the periodic potential interaction of upstream vanes (SR) or blades (RS). Additionally, complete experimental databases of the time-resolved and the turbulent scales of the flow are available for both configurations by means of hot-wire anemometry measurements. Wake transport and viscous mixing are identified in corresponding measuring windows, and primary flow structures at midspan are also recovered and compared with the numerical results for validation. From the comparison of experimental and numerical results it can be concluded that the numerical modeling is able to reproduce accurately the unsteady phenomena that occur inside the axial fan, and shows the potentiality of LES techniques to resolve with high fidelity the main turbulent structures present in the flow, especially at off-design flow rates.