The role of forward sweep in axial fan rotor aerodynamics in design and off-design operations

A. Corsini, F. Rispoli

Dipartimento di Meccanica e Aeronautica, Università di Roma “La Sapienza”, Via Eudossiana 18, Roma, I00184, IT

Abstract:

The influence of rotor blade stacking line geometries on fluid dynamic behaviour and performance of highly loaded axial flow fans is herein discussed. The paper reports a numerical study on two highly loaded rotors of non-free vortex design with unswept and forward-swept blades. Effects of forward sweep on the rotor aerodynamics have been investigated using an in-house developed parallel finite element Navier-Stokes solver. In order to establish a reasonable comparison between the two rotors, their basic geometrical characteristics, global design parameters, flow and ideal total head rise coefficients are identical. The flow structure developing through the blade passages and downstream of the rotors as well as loss distributions have been analysed. Improved flow characteristics due to forward blade sweep have been pointed out.

Synopsis

The forward sweep of axial flow rotor blades has been proven to be a geometrical manipulation able to improve the turbomachinery unit operational characteristics. Axial flow compressors and fans with forward swept blades have been investigated experimentally by many authors (e.g. [1-3]). Experimental comparative analyses on linear cascades of various stacking lines are also reported [4]. Few recent works concerned also with concerted numerical and experimental studies (e.g. [5, 6]). Such studies confirmed that stage performance and efficiency can be improved by sweeping the blades forward. Most of the authors highlighted favourable tendencies due to forward blade sweep such as the suppression of secondary losses in the blade passages and in the blade tip region, or the reduction of corner stall phenomenon. Wennerstrom and Puterbaugh [7] and Lakshminarayana [8] referred to the experience that blade sweep can reduce the onset of compressibility effects and the related shock losses. Wright and Simmons [2], Kodama and Namba [9], and Srivastava and Mehmed [10] stated that also rotor noise can be reduced by forward blade sweep.

As emphasized in [11], favourable effects of forward blade sweep were found in literature often in the cases when a spanwise gradient of blade circulation was present in the rotor, either due to the non-free vortex (NFV) design concept (see e.g. [12]) or/and due to part flow rate. Mohammad and Raj [1] pointed out improved performance characteristics due to forward sweep in the part flow rate operational range. Studies by Wright and Simmons [2] showed that forward sweep resulted in increased total pressure rise peak, shift of stall margin towards lower flow rate, and increased pressure rise together with improved characteristics of blade boundary layers. The compressor rotor studied by Yamaguchi et al. [3] was designed for half-vortex operation, in this case the authors reported that sweeping the compressor rotor blades in the forward direction resulted in an increased efficiency, and in a more accurate realisation of design radial distribution of flow coefficient and relative outflow angle. The research program carried out on swept blades by Beiler and Carolus [5, 6] regarded also rotors of NFV design. For a rotor in which a slight blade circulation gradient is present, computations by Glas [13] predicted that optimum fluid dynamic characteristics and increased efficiency can be achieved if the blades are swept slightly forward. On the basis of literature review, it is anticipated that rotor blading of appropriate forward sweep may accommodate the three-dimensional (3D) interblade flow corresponding to shed vorticity due to NFV operation or part flow rate, resulting in improved flow characteristics. Such a complexity is even more emphasized in the case of NFV rotors, widely used in high specific performance turbomachine units (e.g. [8]), where the interaction between vorticity induced by NFV circulation distribution and 3D flow effects due to swept stacking line occurs [14]. The spanwise change in blade circulation results in characteristic 3D blade-to-blade flow, manifesting itself as a torsion of interblade stream surface segments. In spite of early theoretical works (e.g. [15]) the harmonisation of swept blade geometry and 3D blade-to-blade flow lacks of generally applicable concepts. To this end the authors developed an original design concept for the accommodation of 3D NFV rotor flow by the blade geometry with involvement of sweeping the blades forward (outlined in [11, 17, 18]).

The aim of the paper is to contribute to understand the influence that forward swept blade stacking line exerts on the 3D inter-blade flow, establishing casual relations for the aerodynamic improvement in high performance axial rotors. For this reason, comparative numerical and experimental investigations were carried out on two rotors with different stacking lines: a NFV rotor with straight blades; and a forward-swept NFV rotor designed as an evolution of the straight-bladed one. The studied operating conditions include rotor design point, near-pressure-peak and near-choke conditions. The flow structure within the blades and behind the rotors as well as loss distributions were investigated using an in-house developed parallel Navier-Stokes solver with strongly consistent stabilised Finite Element Methodology (FEM) [19]. The results confirm the improved flow characteristics related to the incorporation of forward sweep in NFV rotor design.

REFERENCES

1 Mohammad, K. P., and Raj, D. P. (1977). Investigations on Axial Flow Fan Impellers With Forward Swept Blades. J. of Fluids Engineering, 543-547.

2 Wright, T., and Simmons, W. E. (1990). Blade Sweep for Low-Speed Axial Fans. J. of Turbomachinery, Vol. 112, January 1990, 151-158.

3 Yamaguchi, N., Tominaga, T., Hattori, S., and Mitsuhashi, T. (1991). Secondary-Loss Reduction by Forward-Skewing of Axial Compressor Rotor Blading. Proc. Yokohama Int. Gas Turbine Cong., II.61 - II.68.

4 Sasaki, T., and Breugelmans, F. (1998). Comparison of Sweep and Dihedral Effects on Compressor Cascade Performance. J. of Turbomachinery, 120, 454-464.

5 Beiler, M. G. (1996). Untersuchung der dreidimensionalen Strömung durch Axialventilatoren mit gekrümmten Schaufeln (in German). Doctoral Thesis, VDI Verlag GmbH, Düsseldorf.

6 Beiler, M.G., and Carolus, T.H. (1999). Computation and Measurement of the Flow in Axial Flow Fans with Skewed Blades. J. of Turbomachinery, 121, 59-66.

7 Wennerstrom, A. J., and Puterbaugh, S. L. (1984). A Three Dimensional Model for the Prediction of Shock Losses in Compressor Blade Rows. J. of Eng. for Gas Turbines and Power, 106, 295 - 299.

8 Lakshminarayana, B. (1996). Fluid Dynamics and Heat Transfer of Turbomachinery. J. Wiley & Sons, Inc.

9 Kodama, H., and Namba, M. (1992). Lifting Surface Theory for Steady Aerodynamic Analysis of Ducted Counter Rotation Propfan. ASME Paper 92-GT-14.

10 Srivastava, R., and Mehmed, O. (1993). On the Static Stability of Forward Swept Propfans. AIAA paper 93-1634-CP.

11 Vad, J. (2000). Incorporation of Forward Blade Sweep in Non-Free Vortex Design Methods of Axial Flow Turbomachinery Rotors. Periodica Polytechnica, Mechanical Engineering Series. Ed.s Budapest University of Tech. and Econ. (accepted for publication).

12 Vad, J., and Bencze, F. (1998). Three-Dimensional Flow in Axial Flow Fans of Non-Free Vortex Design. Int. J. of Heat and Fluid Flow, Vol.19, 601 - 607.

13 Glas, W. (2000). Optimierung Gepfeilter Pumpenschaufeln mit Evolutionaeren Algorithmen (in German). Doctoral Thesis, Technical University of Graz, IHFM.

14 Csanady, G.T. (1963). discussion in: Smith L. H. Jr., and Yeh H.. Sweep and dihedral effects in axial-flow turbomachinery. J. of Basic Eng., 401-416.

15 Smith, L.H.Jr., and Yeh, H. (1963). Sweep and dihedral effects in axial-flow turbomachinery. J. of Basic Eng., 401-416.

16 Glas, W., and Kuhn, K. (1998). Vergleich der Festigkeit von gepfeilten und ungepfeilten Schaufeln in axialen hydraulischen Turbomaschinen (in German). Beitragstexte (V17 bis V39) zum XXX. Kraftwerkstechnischen Kolloquium, 110-119, Dresden, Germany. (eliminato)

17 Vad, J., Bencze, F., Corsini, A., and Rispoli, F. (1999). Non-Free Vortex Flow Effects in an Axial Flow Rotor. Periodica Polytechnica, Mechanical Engineering Series. Ed.s Budapest University of Tech. and Econ. (accepted for pubblication).

18 Vad, J., Bencze, F., Corsini, A., and Rispoli, F. (2000). Design Aspects of Three-Dimensional Flow in High Performance Axial Flow Ventilating Fans. Ventilation 2000-6th Int. Symp. on Ventilation for Contaminant Control, Helsinki.

19 Borello, D., Corsini, A., and Rispoli, F. (2002). A finite element overlapping scheme for turbomachinery flows on parallel platforms. to appear in Computers and Fluids, Elsevier Science Ltd (MS#00-145).

20 Corsini, A., Rispoli, F., Bencze, F., and Vad, J. (1999). Concerted Experimental and Numerical Studies on Axial Flow Fan Rotor Aerodynamics. IMechE paper C557/106/99, 3rd European Conf. on Turbomachinery Fluid Dynamics and Thermodynamics, London, UK, 519-531.

21 Corsini, A. (1996). 3D analysis of Francis turbine runner: performance prediction using a finite element technique with unassembled stiffness matrix treatment (in Italian). Doctoral Thesis, DMA, University of Rome "La Sapienza".

22 Brooks, A.N., and Hughes, T.J.R. (1982). Streamline Upwind/Petrov-Galerkin formulations for convection dominated flows with particular emphasis on the incompressible Navier-Stokes equations. Comp. Meth. Appl. Mech. Eng., 32, 199-259.

23 Tezduyar, T.E. (1992). Stabilized finite element formulations for incompressible flow computations. In: Advances in Applied Mechanics. Academic Press Inc..

24 Sani, R.L., Gresho, P.M., Lee, R.L., and Griffiths, D.F. (1981). The cause and cure (?) of spurious pressures generated by certain FEM solutions of the incompressible Navier-Stokes equations: part 1. Int. J. Num. Meth. in Fluids, 1, 17-43.

25 Pacheco, P.S. (1995). A user guide to MPI.

26 Corsini, A., and Rispoli, F. (1999). Numerical simulation of three-dimensional viscous flow in an isolated axial rotor. The Archive of Mechanical Engineering, XLVI-4, 369-392.

27 Corsini, A., Rispoli, F., Bencze, F., and Vad, J. (2001). Effects of blade sweep in a high performance axial flow rotor. ATI-CST paper 005/01, 4th European Conf. on Turbomachinery Fluid Dynamics and Thermodynamics, Firenze.