13．這些不同的變化受空氣流過發(fā)動(dòng)機(jī)時(shí)的涵道尺寸和形狀的影響。在要求速度(動(dòng))能轉(zhuǎn)換成壓力的地方，通道呈擴(kuò)張形。反之，在要求將燃?xì)庵袃?chǔ)存的能量轉(zhuǎn)換成速度能的場合，便采用收斂通道或噴管工作循環(huán)和氣流(圖2-3)。這些形狀適用于氣流速度是亞音速或音速(即當(dāng)?shù)芈曀?的燃?xì)鉁u輪發(fā)動(dòng)機(jī)。在遇到超音速的場合，如火箭的推進(jìn)噴管，沖壓式空氣噴氣發(fā)動(dòng)機(jī)和某些噴氣發(fā)動(dòng)機(jī)(第6章)。便采用收斂-擴(kuò)散噴管即文氏管(圖2-4)，以便將燃?xì)庵械哪芰孔畲笙薅鹊剞D(zhuǎn)換成動(dòng)能。
低壓壓氣機(jī)
高壓壓氣機(jī)
回流燃燒系統(tǒng) 自由動(dòng)力渦輪
雙軸軸流式渦輪螺槳發(fā)功機(jī)
高壓壓氣機(jī)
低壓壓氣機(jī)
14．通道和噴管的設(shè)計(jì)至關(guān)重要，因?yàn)槟芰哭D(zhuǎn)換時(shí)的效率就取決于它們的良好設(shè)計(jì)。對平滑氣流的擾動(dòng)會(huì)使效率損失，并且，由于氣流渦流或紊流引起振動(dòng)，還可能導(dǎo)致部件發(fā)生故障。
高壓壓氣機(jī)
雙級離心式渦輪螺槳發(fā)動(dòng)機(jī)
低壓壓氣機(jī)
典型單軸輔流式渦輪噴氣發(fā)動(dòng)機(jī)
壓縮 燃燒 膨脹 排出
進(jìn)氣道
推進(jìn)噴管
Working cycle and airflow
圖2-5-1 幾種氣流系統(tǒng)
Fig. 2-5-1 Airflow systems.
雙軸渦輪軸發(fā)動(dòng)機(jī)(帶自自動(dòng)力渦輪的)
與排出的燃?xì)饬?br />
混合的外涵道氣流
低壓壓氣機(jī) 高壓壓氣機(jī) 外涵道氣流
Working cycle and airflow
圖2-5-2 幾種氣流系統(tǒng)
Fig, 2-5-2 Airflow systems.
雙軸軸流式對轉(zhuǎn)后風(fēng)扇
(帶自自功率渦輪)
對轉(zhuǎn)風(fēng)扇
軸流式對轉(zhuǎn)螺槳風(fēng)扇
(帶自自功率渦輪)
壓氣機(jī)
對轉(zhuǎn)螺槳風(fēng)扇
三軸軸流式前風(fēng)扇渦輪噴氣發(fā)動(dòng)機(jī)(高涵道比)
高壓壓氣機(jī)
中壓壓氣機(jī)
雙軸軸流式內(nèi)外涵渦輪發(fā)動(dòng)機(jī)動(dòng)(低涵道比)
低壓壓氣機(jī)
AIRFLOW
15.
The path of the air through a gas turbine engine varies according to the design of the engine. A straight-through flow system (fig. 2-5) is the basic design, as it provides for an engine with a relatively small frontal area and is also suitable for use of the by-pass principle. In contrast, the reverse flow system gives an engine with greater frontal area, but with a reduced overall length. The operation, however, of all engines is similar. The variations due to the different designs are described in the subsequent paragraphs.
16.
The major difference of a turbo-propeller engine is the conversion of gas energy into mechanical power to drive the propeller. Only a small amount of 'jet thrust' is available from the exhaust system. The majority of the energy in the gas stream is absorbed by additional turbine stages, which drive the propeller through internal shafts (Part 5).
17.
As can be seen in fig. 2-5, the by-pass principle involves a division of the airflow. Conventionally, all the air taken in is given an initial low compression and a percentage is then ducted to by-pass, the remainder being delivered to the combustion system in the usual manner. As described in Part 21, this
18．內(nèi)外涵發(fā)動(dòng)機(jī)的一個(gè)重要設(shè)計(jì)特點(diǎn)是涵道比，即通過外涵道的冷空氣流量與通過高壓系統(tǒng)的空氣流量之比。涵道比低，比如在1：1的量級時(shí)，這二股氣流通常在從發(fā)動(dòng)機(jī)排出之前混合在一起。風(fēng)扇發(fā)動(dòng)機(jī)可視為內(nèi)外涵原理的擴(kuò)展。高達(dá)5：1的高涵道比的要求大多是用雙軸或三軸結(jié)構(gòu)中的前風(fēng)扇求滿足(風(fēng)扇就裝于軸上，實(shí)際上成為低壓壓氣機(jī))。兩股氣流可以混合。也可以不混合。很高的涵道比(在15：1的量級)是用螺槳風(fēng)扇來實(shí)現(xiàn)。它是渦輪螺槳理論的演變，但具有能在高飛行速度下高效率工作的先進(jìn)技術(shù)螺旋槳。
19在某些前風(fēng)扇發(fā)動(dòng)機(jī)中，外涵道氣流可以直接在風(fēng)扇后邊通過短涵道排出機(jī)外，也可以通過較長的涵道在發(fā)動(dòng)機(jī)后部排出，并因而稱為“涵道風(fēng)扇”。另一種是后風(fēng)扇方案，但極少采用。
氣流
15．流過燃?xì)鉁u輪發(fā)動(dòng)機(jī)的氣流通道按照該發(fā)動(dòng)機(jī)的設(shè)計(jì)變化。直流氣流系統(tǒng)(圖2-5)是基本設(shè)計(jì)，因?yàn)樗鼮榘l(fā)動(dòng)機(jī)提供比較小的迎風(fēng)面積，并且對于應(yīng)用內(nèi)外涵原理的氣流系統(tǒng)也同樣適用。相反，回流系統(tǒng)使發(fā)動(dòng)機(jī)具有較大的迎風(fēng)面積，但總長度較小。然而，所有發(fā)動(dòng)機(jī)的工作都是類似的。下面幾段介紹了不同設(shè)計(jì)帶來的變化。
16．渦輪螺槳發(fā)動(dòng)機(jī)的主要區(qū)別是將燃?xì)獾哪芰哭D(zhuǎn)換成機(jī)械功以驅(qū)動(dòng)螺旋槳。從排氣系統(tǒng)得到的只是少量的“噴氣推力”。燃?xì)饬髦械拇蟛糠帜芰勘活~外幾級渦輪吸收，并通過內(nèi)軸(第5章)來驅(qū)動(dòng)螺旋槳。
17.正如從圖2-5所能看到的那樣，內(nèi)外涵原理涉及到氣流的分流。通常，所有吸入的氣流經(jīng)過最初的低壓壓縮，然后，一定百分比的氣流流入外涵道，其余部分以常規(guī)方式流入燃燒系統(tǒng)。正如第21章所述，這一原理有益于改善推進(jìn)效率和耗油率。
Working cycle and airflow
principle is conducive to improved propulsive efficiency and specific fuel consumption.
18.
An important design feature of the by-pass engine is the by-pass ratio; that is, the ratio of cool air by-passed through the duct to the flow of air passed through the high pressure system. With low by-pass ratios, i.e. in the order of 1:1, the two streams are usually mixed before being exhausted from the engine. The fan engine may be regarded as an extension of the by-pass principle, and the requirement for high by-pass ratios of up to 5:1 is largely met by using the front fan in a twin or triple-spool configuration (on which the fan is, in fact, the low pressure compressor) both with and without mixing of the airflows. Very high by-pass ratios, in the order of 15:1, are achieved using propfans. These are a variation on the turbo-propeller theme but with advanced technology propellers capable of operating with high efficiency at high aircraft speeds.
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