燃气涡轮工程手册 Gas Turbine Engineering Handbook 2(18)

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The term ""asymmetrical stage"" is applied to stages with reaction other than 50. The axial-inflow stage is a special case of an asymmetrical stage where the entering absolute velocity is in the axial direction. The moving blades impart whirl to the velocity of the leaving flow which is removed by the following stator. From this whirl and the velocity diagram as seen inFigure7-20， the major part of the stage pressure rise occurs in the moving row of blades with the degree of reaction varying from 60-.0. The stage is designed for constant energy transfer and axial velocity at all radii so that the vortex flow condition is maintained in the space between blade rows.
The advantage of a stage with greater than 50 reaction is the low exit loss resulting from lower axial velocity and blade speeds. Because of the small static pressure rise in the stationaryblades， certain simplifications can be introduced such as constant-section stationary blades and the elimination of interstage seals. Higher actual efficiencies have been achieved in this stage type than with the symmetrical stage.primarily because of the reduced exit loss. The disadvantages result from a low static pressure rise in the station-ary blades that necessitates a greater number of stages to achieve a given pressure ratio and creating a heavy compressor. The lower axial velocitiesand bladespeed， necessary to keep within inlet Mach number limitations， result in large diameters. In stationary applications where the increasedweight and frontal area are not of great importance， this type is frequently used to take advantage of the higher efficiency.

The axial-outflow stage diagram in Figure 7-21 shows another special case of the asymmetrical stage with reaction greater than 50. With this type of design， the absolute exit velocity is in an axial direction， and all the static pressure rise occurs in the rotor. A static pressure decrease occurs in the stator so that the degree of reaction is in excess of 100. The advantages ofthis stage type are low axial velocity and bladespeeds， resulting in the lowest possible exit loss. This design produces a heavy machine of many stages and of large diameter. To keep within the allowable limit of the inlet Machnumber， extremely low values must be accepted for the blade velocity and axial velocity. The axial-outflow stage is capable of the highest actual efficiency because of the extremely low exit loss and the beneficial effects of designing for free vortex flow. This compressor type is particularly well-suited for closed-cycle plants where smaller quantities of air are introduced to the compressor at an elevated static pressure.

While a reaction of less than 50.is possible， such a design results in highinlet Mach numbers to the statorrow， causing high losses. The maximum total divergence of the stators should be limited to approximately 20o to avoid excessive turbulence. Combining the high inlet for the limiting diver-gence angles produces a long stator， thereby producing a longer compressor.
Radial Equilibrium
The flow in an axial-flow compressor is defined by the continuity， momen-tum， and energy equations. A complete solution to these equations is not possible because of the complexity of the flow in an axial-flow compressor. Considerable work has been done on the effects of radial flow in an axial-flow compressor. The first simplification used considers the flow axisym-metric. This simplification implies that the flow at each radial and axial station within the blade row can be represented by an average circumferen-tial condition. Another simplification considers the radial component of thevelocity as much smaller than the axial component velocity， so it can be neglected.
For the low-pressure compressor with a low-aspect ratio， and where theeffect of streamline curvature is not significant， the simple radial equilibrium solution can be used. The simple radial equilibrium solution assumes that the change of the radial velocity component along the axial direction is zero (δVradjδz二 0) and the change of entropy in the radial direction is negligible (δ，jδr二 0). The meridional velocity (V的) is equal to the axial velocity (Vz)， since the effect of streamline curvature is not significant. The radial gradient of the static pressure can be given
　
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