離心式壓氣機
5．離心式壓氣機有單面或雙面葉輪。有時采用雙級單面葉輪，如羅爾斯-羅伊斯公司的“達特”(Dart)發動機。葉輪支承于機匣里面，機匣還包容一圈擴壓器導向葉片。如果采用了雙面進氣葉輪，流向后側面的空氣流要逆向進入葉輪，并需要個穩壓室。
工作原理
6．葉輪由渦輪驅動高速旋轉，空氣連續地吸入葉輪的中心。離心力的作用使空氣沿導向葉片徑向向外流向葉輪尖部。從而使空氣加速，并造成壓力升高。發動機進氣道上也可裝導向葉片，用以給進入壓氣機的空氣提供初始漩流。
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.
進口
速度
壓力
葉輪
出口
擴壓器
圖3-3離心壓氣機中的壓力和速度變化
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
圖3-4葉輪工作間隙和漏氣
葉輪
11．葉輪含有一鍛造的盤，在一側或兩側上有整體式徑向配置的導向葉片(圖3-5)，與壓氣機機匣一起形成了收斂通道。導向葉片可以是后掠的。但為了易于制造，通常采用徑向平直導向葉片。為了使空氣從進氣道中的軸向氣流易于進入旋轉的葉輪。葉輪中心部分的導向葉片做成向旋轉方向彎曲。彎曲部分可以與徑向導向葉片為一整體，或者單獨制成，以使制造更加容易并且制造得更為精確
7．空氣離開葉輪后進入擴壓器段，那里的通道呈擴張形，將大部分動能轉化成壓力能，如圖3-3所示。實際上，通常將這種壓氣機設計得大約一半壓力升高發生在葉輪中，另一半在擴壓器中。
8.為了盡量提高通過壓氣機的空氣流量和壓力升高，要求葉輪高速旋轉。因此，葉輪被設計成在高達1600英尺／秒的葉尖速度下工作。通過在這樣高的葉尖速度下工作，增大了從葉輪流出的氣流速度，于足得到的可轉換成壓壓力的能量就更多。
9．為了保持壓氣機的效率，必須防止葉輪和機匣之間漏氣過多；將它們之間的間隙保持盡量小即可達此目的(圖3-4)。
結構
10．壓氣機結構主要是葉輪、擴壓器和進氣系統。葉輪軸在球軸承和滾棒軸承中旋轉，或者與渦輪軸共用一軸，或者在中間分開，用聯軸節相連，這一般是從易于分解角度設計的。
12．擴壓器組件可以和壓氣機機匣是整體件，或者是一單獨連接的組件。不管是整體件還是連接組件，它都有許多導向葉片，這些葉片作成與葉輪相切。導向葉片通道呈擴張形，以便將動能轉換成壓力能，而且，導向葉片的內邊緣與從葉輪流出的合成空氣流的與向相一致(圖3-6)。葉輪和擴壓器之間的間隙是一個重要參數，因為此間隙太小會形成空氣動力抖動沖擊，此沖擊會傳給葉輪。造成不穩定的氣流以及振動。
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