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．工作期間，轉子由渦輪帶動高速旋轉，于是空氣被連續不斷地引入壓氣機。爾后，旋轉著的葉片使空氣加速，并將其推向后排相鄰的一排靜子葉片。轉子傳給空氣的能量使壓力升高，并提高了空氣的速度。然后，空氣在隨后的靜子通道中減速(擴壓)并將動能轉換成壓力。靜干葉片還將轉子葉片加于空氣的偏斜起矯正的作用，并將空氣以正確的角度送到下一級轉子葉片上去。最后一排靜子葉片通常起空氣矯直器的作用，除去空氣的漩流，然后使之以比較均勻的軸向速度進入燃燒系統。氣流通過壓氣機時壓力和速度的變化圖示于圖3-9。隨著壓力提高，這些變化又伴之以空氣溫度的逐漸升高。
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 轉子葉片   靜子葉片

Fig. 3-9 Pressure and velocity changesthrough an axial compressor.

圖3-9 軸流壓氣機中壓力和速度的變化
20．每一級中進出口空氣的總增壓比很小。僅在1：1和1：2之間。每一級的壓力升高這樣小的原因是，如果要避免空氣在轉子葉片上的分離和在隨后的轉子葉片上的失速引起損失的話，擴壓度和轉子葉片的偏轉角必須是很有限的。雖然每一級的壓比很小，但是，每一級的出口壓力都比它前面一級提高。所以，盡管一臺壓氣機的第一級只能使壓力提高3至4磅／平方英寸，但在一個30：1的壓縮系統的后面級中，級壓力升高可以高達80磅／平方英寸。由于已有能力設計出這樣一些多級軸流式壓氣機，它們的空氣速度是可以控制的，氣體是直通流動的，因此可將壓氣機的損失減至最低限度，并導致高的效率，從而得到低的燃油消耗。這使它進一步優于離心式壓氣機。這些狀態是離心壓氣機根本不容易達到的。
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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