21.
The more the pressure ratio of a compressor is increased the more difficult it becomes to ensure that it will operate efficiently over the full speed range. This is because the requirement for the ratio of inlet area to exit area, at the high speed case, results in an inlet area that becomes progressively too large relative to the exit area as the compressor speed and hence pressure ratio is reduced. The axial velocity of the inlet air in the front stages thus becomes low relative to the blade speed, this changes the incidence of the air onto the blades and a condition is reached where the flow separates and the compressor flow breaks down. Where high pressure ratios are required from a single compressor this problem can be overcome by introducing variable stator vanes in the front stages of the system. This corrects the incidence of air onto the rotor blades to angles which they can tolerate. An alternative is the incorporation of interstage bleeds, where a proportion of air after entering the compressor is

前面數級中進口空氣的軸向速度相對于轉子葉片速度因此變得較低，這就改變了空氣流到葉片上的迎角，并達到氣流分離和壓氣機流量下降的程度。要求單轉子壓氣機達到高增壓比時，可在該系統的前幾級中采用可調靜子葉片來解決這個問題。它把空氣流到轉子葉片上去的迎角矯正到這些葉片能夠容忍的程度。另外的辦法是采用級間放氣，將進入壓氣機的一部分空氣從中間級放走并泄入外涵氣流中。雖然這種方法矯正了通過前幾緞的軸向速度，但是浪費了能量。所以，人們更喜歡采用可調靜子。
在風扇中達到這一點是通過利用很高的葉尖速度(1500英尺／秒)，以及風扇葉片的外涵道部分流過的氣流用葉尖處高達馬赫數為1.5的超音速進口空氣速度工作。得到的壓力從葉尖到內徑處是逐漸變小的，相對速度最高的葉尖處值很高，在向核心增壓的內徑處降到較正常的1.3到1.4間的值，那里的氣動力設計更類似于常規壓氣機的級。這種壓氣機級的能力實現了單位迎風面積的流量高、效率高和一排旋轉葉片內的增壓比高等循環要求，而且沒有進口導向葉片和發動機直徑適中的條件下實現的，從而使重量和機械復雜性均保持在良好的水平。
22.高涵道比渦輪風扇發動機的風扇是一個例子，說明軸流壓氣機已經優化到能滿足這種循環的各種特殊要求。雖然從原理上與核心壓氣機的級相類似，但是，設計的比例是，氣體通道內徑類似于它后面的核心壓氣機的通道內徑。而葉尖直徑則大得多。通過風扇的質量流量典型地6倍于核心通道要求的流量，其余的六分之五從核心外旁路出去，通過它自己的同軸心噴管膨脹，或者在一個共同的噴管里與核心出口氣流混合。為了優化這一循環，外涵氣流必須將壓力提高到進氣道壓力的1.6倍左右。

Compressors

removed at an intermediate stage and .dumped into the bypass flow. While this method corrects the axial velocity through the preceding stages, energy is wasted and incorporation of variable stators is preferred.
22. The fan of the high by-pass ratio turbo-fan is an example of an axial compressor which has been optimized to meet the specific requirements of this cycle. While similar in principle to the core compressor stage, the proportions of design are such that the inner gas path is similar to that of the core compressor that follows it, while the tip diameter is considerably larger. The mass flow passed by the fan is typically six times that required by the core, the remaining five sixths by-pass the core and is expanded through its own coaxial nozzle, or may be mixed with the flow at exit from the core in a common nozzle. To optimize the cycle the by-pass flow has to be raised to a pressure of approximately 1.6 times the inlet pressure. This is achieved in the fan by utilizing very high tip speeds (1500 ft. per sec.) and airflow such that the by-pass section of the blades operate with a supersonic inlet air velocity of up to Mach 1.5 at the tip. The pressure that results is graded from a high value at the tip where relative velocities are highest to the more normal values of
1.3 to 1.4 at the inner radius which supercharges the core where aerodynamic design is more akin to that of a conventional compressor stage. The capability of this type of compressor stage achieves the cycle requirement of high flow per unit of frontal area, high efficiency and high pressure ratio in a single rotating blade row without inlet guide vanes within an acceptable engine diameter. Thus keeping weight and mechanical complexity at an acceptable level.
結構
Construction
23. The construction of the compressor centres around the rotor assembly and casings. The rotor shaft is supported in ball and roller bearings and coupled to the turbine shaft in a manner that allows for any slight variation of alignment. The cylindrical casing assembly may consist of a number of cylindrical casings with a bolted axial joint between each stage or the casing may be in two halves with a bolted centre line joint. One or other of these con-struction methods is required in order that the casing can be assembled around the rotor.
轉子
Rotors
24. In compressor designs (fig. 3-10) the rotational speed is such that a disc is required to support the centrifugal blade load. Where a number of discs are fitted onto one shaft they may be coupled and secured together by a mechanical fixing but
　
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