One example of an axial flow rotary machine is a turbofan gas turbine engine. A turbofan engine typically has a fan section, a compressor section, a combustion section and a turbine section.
Two annular flow paths for working medium gases extend through the fan section. The first, a primary flow path for working medium gases, extends through the fan section and thence through the compressor combustion and turbine sections. The second, a secondary flow path, is outwardly of the primary flow path and extends only through the fan section.
An example of a turbofan engine having the two annular flow paths is shown in U.S. Pat. No. 3,375,971 entitled "Attachment Means for Turbofan Low Compressor Assembly" issued to Fitton. As shown in Fitton, the fan section includes a stator assembly and a fan rotor assembly. The fan rotor assembly has an array of fan blades which extend outwardly across the two flow paths. A fan casing in close proximity to the fan blades extends circumferentially about the secondary flow path. The fan casing provides an outer boundary to the secondary flow path and supports a nacelle which shields the engine from its environment.
An annular engine casing is downstream of the fan blades and inwardly of the fan casing. The engine casing forms an inner boundary for the annular secondary flow path and the outer boundary of the annular primary flow path. A plurality of struts downstream of the fan blades extend radially across the secondary working medium flow path. The struts are joined to the fan casing and the engine casing to position the fan casing about the engine casing.
As the fan rotor assembly rotates about its axis of rotation, the fan blades drive the working medium gases into the primary flow path and into the secondary flow path. A plurality of exit guide vanes downstream of the fan blades direct the flow exiting the fan blades and provide for the efficient discharge of the pressurized gases from the secondary flow path from the engine through the struts downstream of the exit guide vanes.
The tip of each fan blade is spaced by a clearance gap from the fan casing to avoid destructive interference between the rotating fan blades and the fan casing. The fan casing also has an abradable rubstrip outwardly of the rotor blades which can accept limited penetration by the fan blades as cowl loads, gust loads and maneuver loads deform the fan casing with respect to the path of the whirling fan blades.
As shown in Fitton, the rubstrip on the fan casing is positioned by the plurality of struts which extend from the engine casing to the fan casing. The exit guide vanes extend radially inwardly from the fan casing to a ring which is splined to the engine casing. Thus, the exit guide vanes perform an aerodynamic function and are not used to position the rubstrip from the engine casing.
As in Fitton, the current approach to the design of an exit guide vane is to use a vane that performs an aerodynamic function only. This is in part caused by the high aspect ratio of the vane which enables the vane to perform its aerodynamic function. As a result of the high aspect ratio, the fan blade has a low radial spring rate and a slenderness ratio (length divided by the radius of gyration of the column) that causes the guide vanes to act as a long column, see generally R. Roark "Formulas for Stress and Strain" (McGraw Hill Book Co., 4th Edition 1965) pages 259-270. Consequently, the vanes would fail in a classical Euler buckling mode if subjected to the high radial compressive loads required to locate the fan rubstrip. As a result, the exit guide vane is typically secured to the fan casing only and is free at its inmost end to move radially with respect to the engine casing.
The control of the clearance between the tip of the fan blade and the rubstrip on the fan casing is important in modern high by-pass ratio gas turbine engines to achieve the required levels of fan stability and engine efficiency. The clearance is affected both by the relative position of the fan casing to the engine casing and the concentricity of the fan casing about the fan blades. Because the fan casing is positioned by the struts which support the fan casing from the engine casing, one suggested approach for more positively locating the fan casing (and its rubstrip) about the rotor blades is to increase the size of the struts. This aids the engine in maintaining the required clearance when subjected to cowl loads and maneuver loads, but significantly increases the weight of the struts. As a result, the improvement in efficiency from clearances is offset by the performance penalty associated with the gain in weight of the engine.
Accordingly, scientists and engineers are seeking to develop a structure which does not significantly increase the weight of the engine and yet which stiffens the fan casing during operation against deflections which are caused by maneuver loads, cowl loads and gust loads.