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
前面数级中进口空气的轴向速度相对于转子叶片速度因此变得较低，这就改变了空气流到叶片上的迎角，并达到气流分离和压气机流量下降的程度。要求单转子压气机达到高增压比时，可在该系统的前几级中采用可调静子叶片来解决这个问题。它把空气流到转子叶片上去的迎角矫正到这些叶片能够容忍的程度。另外的办法是采用级间放气，将进入压气机的一部分空气从中间级放走并泄入外涵气流中。虽然这种方法矫正了通过前几缎的轴向速度，但是浪费了能量。所以，人们更喜欢采用可调静子。
在风扇中达到这一点是通过利用很高的叶尖速度(1500英尺／秒)，以及风扇叶片的外涵道部分流过的气流用叶尖处高达马赫数为1.5的超音速进口空气速度工作。得到的压力从叶尖到内径处是逐渐变小的，相对速度最高的叶尖处值很高，在向核心增压的内径处降到较正常的1.3到1.4间的值，那里的气动力设计更类似于常规压气机的级。这种压气机级的能力实现了单位迎风面积的流量高、效率高和一排旋转叶片内的增压比高等循环要求，而且没有进口导向叶片和发动机直径适中的条件下实现的，从而使重量和机械复杂性均保持在良好的水平。
22.高涵道比涡轮风扇发动机的风扇是一个例子，说明轴流压气机已经优化到能满足这种循环的各种特殊要求。虽然从原理上与核心压气机的级相类似，但是，设计的比例是，气体通道内径类似于它后面的核心压气机的通道内径。而叶尖直径则大得多。通过风扇的质量流量典型地6倍于核心通道要求的流量，其余的六分之五从核心外旁路出去，通过它自己的同轴心喷管膨胀，或者在一个共同的喷管里与核心出口气流混合。为了优化这一循环，外涵气流必须将压力提高到进气道压力的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.
结构
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.
转子
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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