Figure 1: View of the initial design of centrifugal fan casing

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1 AERODYNAMIC OPTIMISATION OF CENTRIFUGAL FAN CASING USING CFD * Peter GAŠPAROVIČ, ** Mária ČARNOGURSKÁ * Národná akadémia obrany, Oddelenie prípravy leteckého personálu, Demänová 393, Liptovský Mikuláš, Slovenská republika Phone:+ (421) (905) pgasparovic@gmail.sk ** Technical University of Košice, Department of Power Engineering Equipment Vysokoškolská 4, Košice, Slovenská republika Phone:+ (421) (55) Fax: + (421) (55) maria.carnogurska@tuke.sk Initial design of centrifugal fan casing is studied using CFD simulation. Casing has one inlet and two outlets and distribution of mass flow into the both outlets is determined only by aerodynamic conditions. Outflow of both outlets is very inhomogeneous, therefore new design of the casing is proposed based on CFD results. The method of optimized design which is presented preserves ratio of mass flow through both outlets and utilizes CFD post-processing capabilities of CFD system Ansys CFX 11. Keywords: numerical simulation; centrifugal fan casing; optimization 1 INTRODUCTION The considered objective is centrifugal fan from freezer unit. The fan is inserted into casing with two outlets (figure 1). The aim of the study is to obtain results of flow field in both outlets, and subsequently to optimise the shape of casing in order to homogenize flow field in outlets assuming that mass flow through outlets is preserved. In both analysis and optimisation the Computation Fluid Dynamics (CFD) tools were used. Figure 1: View of the initial design of centrifugal fan casing 2 CFD SIMULATION OF ORIGINAL DESIGN In order to obtain results of flow field in outlets of original design the analysis through CFD simulation was performed. CFD is basically numerical tool, therefore computational mesh was needed. Manufacturing level CAD model was simplified to get rid of unimportant geometry details. The mesh was created in two independent parts. The rotor part of model is highly 3-dimensional (3D), and this necessitates transient numerical analysis, where the rotor part of the model must change its position in relation to stator part during the computation. The rotor part is modelled by unstructured tetrahedral mesh. Boundary layer on the rotor surface is not very important in this case. However the stator part must be discretised with boundary layer details in mind. It results in block structured hexahedral grid with gradually denser elements in surface vicinity (so called Navier-Stokes grid). 1

2 Figure 2: Geometric CAD model of centrifugal fan with casing Figure 3: Detail of numerical mesh Boundary conditions in inlet are modelled by cylinder extension mesh with imposed normal velocity on its planar end. Total pressure condition in outlet is numerically unstable, but real conditions lead to the total pressure. This conflict was solved sufficiently by large exterior domains behind outlets (see figure 4) with static pressure boundary condition. Large size of domain also solves problem of possible backflow in outlets. The mesh was generated with limit of academic licence of Ansys CFX in mind. Limit of the size of mesh is nodes, however final mesh didn t approached this value. Its size is nodes, elements. Computational mesh was created in ICEM CFD system. CFD computation was carried out in Ansys_CFX 11. Flow field was obtained by computation of continuity, momentum, and turbulence equation. Heat transfer wasn t modelled; therefore simple model of air with constant density and viscosity could be used. Numerical scheme is implicit, solved by Finite Volume Method (FVM). Advection scheme is High resolution which ensures both quick convergence in first steps and high precision in final steps. Transient terms are computed with second order backward Euler scheme. Turbulence model is of Bousinesq hypothesis type and it is Menter s k-omega Shear Stress Transport (SST) with automatic wall function and transitional Gama-Theta model. The mesh detail level in boundary layer of stator is kept at y+ value lower than 1. To minimise roundoff error, the reference pressure is set at level Pa, so only small relative pressure values were computed. 2

3 Figure 4: Exterior domains behind outlets Unsteadiness of flow field because of rotation of fan is realised by transient type of simulation in conjunction with Transient rotor frame change model and General Grid Interface (GGI). Frame change model ensures that the mesh of rotor is at the beginning of each time step rotated corresponding to rotational speed, and GGI ensures that the flow field variables are transferred between non-compliant faces on common interface between both frames. Time step was based on rotational speed such way that each passage of fan blade correspond to 10 time steps. The speed of rotation is 1950 rev./minute and fan has 11 blades, therefore time step is set at seconds. Convergence was judged by monitoring of progress of inlet total pressure. Steady oscillations of pressure were reached after approximately 200 time steps which correspond to 1.8 revolutions. Numerical solver allocated more than 600 MB of RAM, and duration of the run was approximately 20 hours on AMD Athlon CPU. The Boundary condition in exterior outlet was as static pressure Pa. The boundary condition in inlet was as normal velocity, which was set differently in specific case. Simulated were only two cases corresponding to 2.5 and 2.0 m/s in inlet which are very close to condition with free outlets (open atmosphere). The ratio of mass flow through both outlets was expected to not change (based on results of [2]), therefore one simulation was sufficient to provide all details of flow field. The second case can be used together with the first case to extrapolate parameters to regime with free outlets. 3 RESULTS AND DISCUSSION OF CFD SIMULATION Integral values for both cases are in Table 1. The details of flow field inside casing and in outlets are on the next figures. It is evident, that velocity field in outlets is very inhomogeneous (figure 5, figure 6). This is in contrast with older results (see [1]), and the reason is better resolution of outlet boundary in the latest model. Therefore optimisation of the case design will be followed. Very important information for this optimisation is the precise location of dividing streamlines between outlets. In the figure 6 it is evident, that separation of both streams is very irregular. Therefore middle plane between top and bottom cap of casing was chosen for searching dividing streamline (see figure 7). The streamline is marked by two points. 3

4 Table1: Integral values of results Case velocity in inlet (m.s-1) mass flow inlet (kg.s-1) mass flow outlet 1 (kg.s-1) mass flow outlet 2 (kg.s-1) total pressure inlet (Pa) total pressure outlet 1 (Pa) total pressure outlet 2 (Pa) ratio of massflow (outlet 1 / outlet 2) (1) increase of total pressure (outlet 1 inlet) (Pa) I. 2,5 0, , , , , ,95 2,19 2,54 Figure 5: 3D views - velocity vectors in the outlets Figure 6: 3D views - streamlines inside 4 II. 2,0 0, , , , , ,87 2,23 11,99 Experiment 2,3 0,0133 0,0094 0,0039 2,40

5 Figure 7: Flow field (streamlines and velocity contours) in the middle offset plane of casing 4 CFD SIMULATION OF OUTLET VARIANTS In order to design optimised casing several variants of outlet were chosen, and they were analysed using CFD. The main question is how they perform in directing flow perpendicular to outlet plane, and what is their total pressure loss. Figure 8: Comparison of CFD results of outlet variants What works the best is classical grid of directing vanes, although they must be fitted to specific condition of low Re number in this scenario. Longer vanes design (last case in the figure 8) works better because its long rear part leads the flow into the narrow channel, therefore it isn t compromised by low performance of suction side in low Re number regime. 5

6 5 CONCLUSION AND REMARKS The design of improved casing (see figure 9) was dictated by requirement of preserved ratio of mass flow through outlets and by requirement of improved homogeneity and perpendicularity of outflow. Divide of the total mass flow into two streams in original design is dictated by subtle hydraulic balance, and distortion of outflow from fan wheel disturbs this weak balance. Therefore it was decided to impose rigid distribution of mass flow into outlets by inserting wedge. Precise location of wedge was dictated by location of dividing streamline which was found out by post-processing CFD results. However wedge must be located far enough from fan, because the unsteady flow from fan (expressed in magnitude and direction) can easily cause periodic separation of vortices from the apex of the wedge. From the same reason, and because low Re number of the flow, the apex has rounded shape. It is also hypothesised that steady hydraulic balance in original casing isn t sustainable because of large spreading angle. In the simulation of outlet variants the same problem is evident. Therefore optimised design has spreading angle limited to 13 degrees. Outlet s width is very large, therefore directing vanes are acting also like another diffuser, and in the case of separated flow on suction side of vanes, they can act at least like distributors of the mass flow along the width of outlets. Figure 9: Comparison of old and new (optimised) design of fan casing REFERENCES [1] [2] [3] [4] [5] Gašparovič, P., Čarnogurská, M.: Aplikácia Ansys_CFX pri skúmaní prúdových pomerov v telese s umiestneným ventilátorom. Acta Mechanica Slovaca, 11/2007, s. 359 Návrh ventilátorového stupňa motora DV-2. Technická správa 119/80-DV-2-PB, Považské strojárne Letecké motory, Považská Bystrica. ANSYS_CFX, V11 User s guide ANSYS Inc TU Košice Malcho, M., Jandačka, J., Kapusta, J., Lábaj, J.: Optimalizácia vzduchotechnickej trasy vetracej šachty aplikáciou CFD metódy. Acta Mechanica Slovaca, 3/2001, s. 569 Blejchař, T., Kozubková, M.: Mathematical Modelling of Non-stationary Flow, Cavitation and Accustic in Hydraulic Valve. In Proceeding The 19th International Conference on Hydraulics and Pneumatics, Prague, May 30 31, 2006 ACKNOWLEDGEMENTS The presented paper was financially supported by research project AV 4/004/07. 6