1. Field of the Invention
The present invention relates to rotor blades of axial-flow machines for transferring energy to fluid or for receiving energy from fluid, such as axial-flow blowers, axial-flow compressors, axial-flow pumps, axial-flow gas turbines, etc. (throughout this specification and claims, these machines will be generally referred to as "axial-flow machines").
2. Description of the Prior Art
At first, the structure of a rotor blade of an axial-flow machine in the prior art will be described with reference to FIG. 6. In FIG. 6(a), reference numeral 1 designates a blade body of a rotor blade, numeral 2 designates a platform (flange portion), and numeral 3 designates a screw portion. The rotor blade body 1 is fixedly secured to a hub (not shown) by means of the platform 2 and the screw portion 3. In lieu of the screw portion 3, a dovetail could ployed. The respective cross-sectional profiles taken along cross sections A-F perpendicular to the radial direction of the hub are shown in FIG. 6(c), and the points denoted by numeral 5 in this figure are centers of the respective cross-sectional profiles. In addition, reference character Y designates the direction of an airflow, and reference character R designates the direction of rotation of the blade body 1.
The blade body 1 of a rotor blade in the prior art has the centers 5 of the respective cross-sectional profiles aligned in the same straight line. Numeral 6 designates a centroid of centers 5 which form a straight line aligned above the same radial location on the hub. The reason why the respective centers 5 are aligned above the same radial location on the hub, is so that unnecessary stress will not be generated by a centrifugal force acting upon the rotor blade. If the centers of FIG. 5 were not aligned in a straight line, a moment acting in directions other than the radial direction of the hub would be generated by the centrifugal force, and a bending stress would act upon the rotor blade. However, if the centers 5 are aligned above the same radial location on the hub, then theoretically only a tensile stress can act upon the rotor blade. (It is to be noted that, in practice, a bending stress caused by compressed gas as well as a torsion stress on the respective cross-sectional profiles would be also generated.) In other words, the structure of the rotor blade in the prior art was designed only from the view point of mechanical strength.
As described above, in a rotor blade of, for instance, an axial-flow compressor in the prior art, the structure of the rotor blade was designed only from a view point of mechanical strength, and so the respective centers 5 of the cross-sectional profiles of the blade body 1 were aligned above the same radial location on the hub. However, at the tip end portion of the blade body 1, that is, at the portion of the blade body closest to the inner surface of a casing, turbulent complicated flows are formed as the result of a drift by centrifugal forces at a boundary layer along the inner surface of the casing and a boundary layer along the blade surface, or as the result of an accumulation of secondary flows between the respective blade bodies. Hence, fluid having low energy is liable to stagnate, resulting in a decreased action of the blade body 1, and a pressure loss of the flow at the tip end portion that is larger than that of the flow at the central portion of the blade body 1 (a principal flow). Consequently, the efficiency of the rotor blade is low.