Computational investigation of flow phenomena in an isolated axial fan rotor using Navier-Stokes procedure

A finite element based Navier-Stokes solver is applied to investigate the three-dimensional viscous flow in a low speed isolated axial rotor with tip clearance at design condition. An higher-order anisotropic eddy viscosity model is used for closure, with wall function treatment able to simulate stationary and moving boundaries. The presented comparisons with experimental data include three-dimensional flow structure behind the rotor as well as the tip leakage flow behaviour developing through the rotor. It is shown that the code predicts well the flow structure observed in the experiments. A critical discussion of predicting limits is also carried out in order to address possible improvements. Introduction Turbomachinery flows modeling requires built-in capabilities for the assessment of viscous and turbulent effects on secondary flows (endwall losses, flow turning, flow separation, etc.). The ordinary first moment closure turbulence model lacks of sensitivity due to the forced isotropy of normal Reynolds stress components, thus the adoption of more physical stress-strain constitutive relation scheme is needed. The computational approach, here proposed for steady incompressible internal flows, adopts a non linear k ε model (Craft et al., 1993) with wall function treatment of solid boundaries to preserve acceptable level of discretization. The simulation of internal flows in Turbomachinery is here approached on the basis of modified Galerkin finite element (FE) formulation, developed in order to control the main instability sources that affects incompressible flows numerical prediction. The developed FE formulation is based on a stabilized Petrov-Galerkin method, supported on 3-D elements by mixed or equal order trial functions. The consistent stabilization scheme is tuned in order to act as discontinuous streamline balancing diffusive term in the advection-dominated flow (Brooks et al., 1982), and as an incompressibility constraint relaxing term in diffusive-dominated flow behaviour (Tezduyar, 1992). As far as the solution strategy is concerned, satisfactory computational compromise has been achieved by adopting 'in core' semi-iterative solver (Generalized Minimal Residual GMRes Saad et al. 1986) with compact management of the stiffness matrix stored using addressing arrays (Borello et al., 1997b). The developed numerical tool is here used to gain an insight into the complex three-dimensionl (3D) flow developing in an axial fan rotor of non free vortex (NFV) design, with special concern to the tip leakage flow region. The numerical model of rotor employs a conventional H-grid topology with the adoption of a 'thin blade approximation' for the tip gap (Basson et al. 1993), and the set of applied boundary conditions is based on the detailed experimental investigations carried out by Vad and Bencze (1996). In this study, the Navier-Stokes prediction of the tested fan rotor flow is first validated against the benchmark LDA measurements on the flow field downstream of the rotor provided by Vad (1997) and Vad and Bencze (1996), (1998a). Then, the predicted tip leakage flow behaviour at the design flow rate is compared to literature experimental studies available for rotor as well as moving or stationary cascade flows. The results demonstrate the actual capabality of the proposed numerical approach for complex turbomachinery flows. A critical discussion of predicting limits is also carried out, in order to address the possible improvements for both flow and rotor modeling. DMA-URLS R&D (TéT) Cooperation Project I-28/98