16.
The multi-spool compressor consists of two or more rotor assemblies, each driven by their own
turbine at an optimum speed to achieve higher pressure ratios and to give greater operating flexibility.
17. Although a twin-spool compressor (fig. 3-7) can be used for a pure jet engine, it is most suitable for the by-pass type of engine where the front or low pressure compressor is designed to handle a larger airflow than the high pressure compressor. Only a percentage of the air from the low pressure compressor passes into the high pressure compressor; the remainder of the air, the by-pass flow, is ducted around the high pressure compressor. Both flows mix in the exhaust system before passing to the propelling nozzle (Part 6). This arrangement matches the velocity of the jet nearer to the optimum requirements of the aircraft and results in higher
工作原理
19．工作期间，转子由涡轮带动高速旋转，于是空气被连续不断地引入压气机。尔后，旋转着的叶片使空气加速，并将其推向后排相邻的一排静子叶片。转子传给空气的能量使压力升高，并提高了空气的速度。然后，空气在随后的静子通道中减速(扩压)并将动能转换成压力。静干叶片还将转子叶片加于空气的偏斜起矫正的作用，并将空气以正确的角度送到下一级转子叶片上去。最后一排静子叶片通常起空气矫直器的作用，除去空气的漩流，然后使之以比较均匀的轴向速度进入燃烧系统。气流通过压气机时压力和速度的变化图示于图3-9。随着压力提高，这些变化又伴之以空气温度的逐渐升高。
Compressors
propulsive efficiency, hence lower fuel consumption. stator passage and the kinetic energy translated into
For this reason the pure jet engine where all the airflow passes through the full compression cycle is now obsolete for all but the highest speed aircraft.
18. With the high by-pass ratio turbo-fan this trend is taken a stage further. The intake air undergoes only one stage of compression in the fan before being split between the core or gas generator system and the by-pass duct in the ratio of approximately one to five (fig. 3-8). This results in the optimum arrangement for passenger and/or transport aircraft flying at just below the speed of sound. The fan may be coupled to the front of a number of core compression stages (two shaft engine) or a separate shaft driven by its own turbine (three shaft engine).
Principles of operation
19. During operation the rotor is turned at high speed by the turbine so that air is continuously induced into the compressor, which is then accelerated by the rotating blades and swept rearwards onto the adjacent row of stator vanes. The pressure rise results from the energy imparted to the air in the rotor which increases the air velocity. The air is then decelerated (diffused) in the following 转子叶片 静子叶片
Fig. 3-9 Pressure and velocity changesthrough an axial compressor.
图3-9 轴流压气机中压力和速度的变化
20．每一级中进出口空气的总增压比很小。仅在1：1和1：2之间。每一级的压力升高这样小的原因是，如果要避免空气在转子叶片上的分离和在随后的转子叶片上的失速引起损失的话，扩压度和转子叶片的偏转角必须是很有限的。虽然每一级的压比很小，但是，每一级的出口压力都比它前面一级提高。所以，尽管一台压气机的第一级只能使压力提高3至4磅／平方英寸，但在一个30：1的压缩系统的后面级中，级压力升高可以高达80磅／平方英寸。由于已有能力设计出这样一些多级轴流式压气机，它们的空气速度是可以控制的，气体是直通流动的，因此可将压气机的损失减至最低限度，并导致高的效率，从而得到低的燃油消耗。这使它进一步优于离心式压气机。这些状态是离心压气机根本不容易达到的。
pressure. Stator vanes also serve to correct the deflection given to the air by the rotor blades and to present the air at the correct angle to the next stage of rotor blades. The last row of stator vanes usually act as air straighteners to remove swirl from the air prior to entry into the combustion system at a reasonably uniform axial velocity. Changes in pressure and velocity that occur in the airflow through the compressor are shown diagrammatically in fig. 3-9. The changes are accompanied by a progressive increase in air temperature as the pressure increases.
20.
Across each stage the ratio of total pressures of outgoing air and inlet air is quite small, being between 1:1 and 1:2. The reason for the small pressure increase through each stage is that the rate of diffusion and the deflection angle of the .blades must be limited if losses due to air breakaway at the blades and subsequent blade stall are to be avoided. Although the pressure ratio of each stage is small, every stage increases the exit pressure of the stage that precedes it. So whilst this first stage of a compressor may only increase the pressure by 3 to 4 lb. per sq. in., at the rear of a thirty to one compression system the stage pressure rise can be up to 80 lb, per sq. in, The ability to design multi-stage axial compressors with controlled air velocities and straight through flow, minimizes losses and results in a high efficiency and hence low fuel consumption. This gives it a further advantage over the centrifugal compressor where these conditions are fundamentally not so easily achieved.
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