Saturday, December 31, 2011

A typical rotor blade showing twisted contour


Fig. 3-12 A typical rotor blade showing
twisted contour.
26. The rotor blades are of airfoil section (fig. 3-12) and usually designed to give a pressure gradient along their length to ensure that the air maintains a reasonably uniform axial velocity. The higher pressure towards the tip balances out the centrifugal action of the rotor on the airstream. To obtain these conditions, it is necessary to ’twist’ the blade from root to tip to give the correct angle of incidence at each point. Air flowing through a compressor creates two boundary layers of slow to stagnant air on the inner and outer walls. In order to compensate for the slow air in the boundary layer a localized increase in blade camber both at the blade tip and root has been introduced. The blade extremities appear as ifformed by bending over each corner, hence the term’end-bend’.Stator vanes
27. The stator vanes are again of airfoil section and are secured into the compressor casing or into stator vane retaining rings, which are themselves secured to the casing (fig. 3-13). The vanes are often assembled in segments in the front stages and may be shrouded at their inner ends to minimize the vibrational effect of flow variations on the longer vanes. It is also necessary to lock the stator vanes in such a manner that they will not rotate around the casing.
OPERATING CONDITIONS
28. Each stage of a multi-stage compressor possesses
certain airflow characteristics that are dissimilar from those of its neighbour; thus to design a workable and efficient compressor, the characteristics of each stage must be carefully matched. This is a relatively simple process to implement for one set of conditions (design mass flow, pressure ratio and rotational speed), but is much more difficult when reasonable matching is to be retained with the compressor operating over a wide range of conditions such as an aircraft engine encounters.
Fig. 3-14 Limits of stable airflow.
Fig. 3-13 Methods of securing vanes to compressor casing.
29. If the operating conditions imposed upon the compressor blade departs too far from the design intention, breakdown of airflow and/or aerodynamically induced vibration will occur. These phenomena may take one of two forms; the blades may stallbecause the angle of incidence of the air relative tothe blade is too high (positive incidence stall) or too low (negative incidence stall). The former is a front stage problem at low speeds and the latter usually affects the rear stages at high speed, either can lead to blade vibration which can induce rapid destruction. If the engine demands a pressure rise from the compressor, which is higher than the blading can sustain, ’surge’ occurs. In this case there is an instantaneous breakdown of flow through the machine and the high pressure air in the combustion system is expelled forward through the compressor with a loud ’bang’ and a resultant loss of engine thrust.Compressors are designed with adequate margin to ensure that this area of instability (fig. 3-14) is avoided.

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