Sweep and Dihedral Effects in Axial-Flow Turbomachinery

Abstract

An approximate method for including the effects of sweep and dihedral when designing axial-flow turbomachinery blading is presented. Blades are said to have sweep when the flow direction is not perpendicular to the spanwise direction, and dihedral when the blade surface is not normal to the surface of an end wall. It is shown that blade cross sections should be cut by sectioning surfaces that are tangent to the axisymmetric stream surfaces of the meridional flow, but that these cross sections should be viewed by looking parallel to the axis (stacking line) of the blade. When this is done the observed blade shapes and flow angle distributions are most nearly comparable to those obtained from two-dimensional cascade experiments and analyses. This approach is found to be inadequate at the blade ends, however, and an analytical method is presented which yields a wall correction for blade rows of semi-infinite span. For all practical variations of the parameters involved in the design of axial-flow compressors and turbines, the wall correction can be conveniently calculated from a set of approximate formulas presented in this paper. The importance of an adequate axisymmetric solution (method not presented herein) as the first step in the analysis is pointed out; many of the effects of sweep and dihedral are traceable to the skewness of the force and thickness-blockage fields of the axisymmetric model. Finally, the paper summarizes the blading design procedure and applies the present work within the framework of the overall design. As an example, the method is used to design a swept cascade; previously reported test results for a similar cascade tend to substantiate the validity of the design procedure, but experimental results for a direct comparison with the theory are not available.

Aerodynamic Design of Low-Speed Axial-Flow Fans: A Historical Overview

“…In 1963, Smith and Yeh [37] presented an approximate method to account for the effects of sweep and dihedral on the performance of axial-flow turbomachinery blades; in 1971, Lewis and Hill [38] presented a qualitative description of the effects of these geometrical features in turbomachines and included a development of the actuator disk theory to account for swept cascade. The first experimental systematic investigation on axial fans with forward swept blades was presented by Mohammed and Raj [39]: these authors observed an improvement of the performance and efficiency of the NRS fan with respect to the one with radial blades and concluded that including a small amount of sweep ( 5-10 • ) into the design of the blade can be advantageous in several circumstances.…”

“…In this sense, numeric techniques have been fundamental in the development of design methods for NRS blades. In 1989, Wright and Simmons [50] developed and implemented the method proposed in [37], applying it to two forward-swept rotor-only fans: the experimental tests validated the design approach and showed a significant sound reduction with respect to the unswept fan. Addressing again sound emission, a few years later, Beiler and Carolus [51] presented a numerical and experimental investigation on skewed and swept fan blades, proposing an extension of Eck's design method to easily account for NRS effects.…”

Study of Sweep and Induced Dihedral Effects in Subsonic Axial Flow Compressor Passages—Part I: Design Considerations—Changes in Incidence, Deflection, and Streamline Curvature

“…Theories based on infinite span blades would invariably be different in such cases. Streamline twist [8,9] and spanwise redistribution of flow [4,7] are reported with sweep in such low aspect ratio configurations. Starting from lifting line theory, nearly all the blade design procedures (axisymmetric flow assumption followed by finding a blade-to-blade solution for each axisymmetric stream surface) are not legitimate when the blades are swept.…”

“…Smith and Yeh [8] have shown that the vorticity of a swept blade contains an axial component which according to Squire and Winter [14] will induce a secondary flow in a plane perpendicular to the machine axis. The effect of this secondary flow on the swept back blades is to increase the incidence [8].…”

“…Differences arise in terms of effective flow incidences, meridional velocity distribution, and effective blade angles if the blade stacking is highly three dimensional. Smith and Yeh [8] have shown that designing a swept blade on sections defined by the axisymmetric stream surfaces generated by the throughflow calculation methods is not correct. Instead, sections perpendicular to the stacking axis must be chosen for the design.…”