离心式压气机
5．离心式压气机有单面或双面叶轮。有时采用双级单面叶轮，如罗尔斯-罗伊斯公司的“达特”(Dart)发动机。叶轮支承于机匣里面，机匣还包容一圈扩压器导向叶片。如果采用了双面进气叶轮，流向后侧面的空气流要逆向进入叶轮，并需要个稳压室。
工作原理
6．叶轮由涡轮驱动高速旋转，空气连续地吸入叶轮的中心。离心力的作用使空气沿导向叶片径向向外流向叶轮尖部。从而使空气加速，并造成压力升高。发动机进气道上也可装导向叶片，用以给进入压气机的空气提供初始漩流。
Compressors
THE CENTRIFUGAL FLOW COMPRESSOR per sec. By operating at such high tip speeds the air
5. Centrifugal flow compressors have a single or double-sided impeller and occasionally a two-stage, single sided impeller is used, as on the Rolls-Royce Dart. The impeller is supported in a casing that also contains a ring of diffuser vanes. If a double-entry impeller is used, the airflow to the _rear side is reversed in direction and a plenum chamber is required.
Principles of operation
6.
The impeller is rotated at high speed by the turbine and air is continuously induced into the centre of the impeller. Centrifugal action causes it to flow radially outwards along the vanes to the impeller tip, thus accelerating the air and also causing a rise in pressure to occur. The engine intake duct may contain vanes that provide an initial swirl to the air entering the compressor.
7.
The air, on leaving the impeller, passes into the diffuser section where the passages form divergent nozzles that convert most of the kinetic energy into pressure energy, as illustrated in fig. 3-3. In practice, it is usual to design the compressor so that about half of the pressure rise occurs in the impeller and half in the diffuser.
8.
To maximize the airflow and pressure rise through the compressor requires the impeller to be rotated at high speed, therefore impellers are designed to operate at tip speeds of up to 1,600 ft.
进口
速度
压力
叶轮
出口
扩压器
图3-3离心压气机中的压力和速度变化
velocity from the impeller is increased so that greater energy is available for conversion to pressure.
9. To maintain the efficiency of the compressor, it is necessary to prevent excessive air leakage between the impeller and the casing; this is achieved by keeping their clearances as small as possible (fig. 3-4).
Fig. 3-4 Impeller working clearance andair leakage.
Construction
10. The construction of the compressor centres around the impeller, diffuser and air intake system. The impeller shaft rotates in ball and roller bearings and is either common to the turbine shaft or split in the centre and connected by a coupling, which is usually designed for ease of detachment.
Impellers
11. The impeller consists of a .forged, disc with integral, radially disposed vanes on one or both sides (fig. 3-5) forming convergent passages in conjunction with the compressor casing. The vanes may be swept back, but for ease of manufacture straight
图3-4叶轮工作间隙和漏气
叶轮
11．叶轮含有一锻造的盘，在一侧或两侧上有整体式径向配置的导向叶片(图3-5)，与压气机机匣一起形成了收敛通道。导向叶片可以是后掠的。但为了易于制造，通常采用径向平直导向叶片。为了使空气从进气道中的轴向气流易于进入旋转的叶轮。叶轮中心部分的导向叶片做成向旋转方向弯曲。弯曲部分可以与径向导向叶片为一整体，或者单独制成，以使制造更加容易并且制造得更为精确
7．空气离开叶轮后进入扩压器段，那里的通道呈扩张形，将大部分动能转化成压力能，如图3-3所示。实际上，通常将这种压气机设计得大约一半压力升高发生在叶轮中，另一半在扩压器中。
8.为了尽量提高通过压气机的空气流量和压力升高，要求叶轮高速旋转。因此，叶轮被设计成在高达1600英尺／秒的叶尖速度下工作。通过在这样高的叶尖速度下工作，增大了从叶轮流出的气流速度，于足得到的可转换成压压力的能量就更多。
9．为了保持压气机的效率，必须防止叶轮和机匣之间漏气过多；将它们之间的间隙保持尽量小即可达此目的(图3-4)。
结构
10．压气机结构主要是叶轮、扩压器和进气系统。叶轮轴在球轴承和滚棒轴承中旋转，或者与涡轮轴共用一轴，或者在中间分开，用联轴节相连，这一般是从易于分解角度设计的。
12．扩压器组件可以和压气机机匣是整体件，或者是一单独连接的组件。不管是整体件还是连接组件，它都有许多导向叶片，这些叶片作成与叶轮相切。导向叶片通道呈扩张形，以便将动能转换成压力能，而且，导向叶片的内边缘与从叶轮流出的合成空气流的与向相一致(图3-6)。叶轮和扩压器之间的间隙是一个重要参数，因为此间隙太小会形成空气动力抖动冲击，此冲击会传给叶轮。造成不稳定的气流以及振动。
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