29.
From the foregoing, it follows that for a particular blade material and an acceptable safe life there is an associated maximum permissible turbine entry temperature and a corresponding maximum engine power. It is not surprising, therefore, that met-allurgists and designers are constantly searching for better turbine blade materials and improved methods of blade cooling.
30.
Over a period of operational time the turbine blades slowly grow in length. This phenomenon is known as 'creep' and there is a finite useful life limit before failure occurs.
31.
The early materials used were high temperature steel forgings, but these were rapidly replaced by cast nickel base alloys which give better creep and fatigue properties.
32.
Close examination of a conventional turbine blade reveals a myriad of crystals that lie in all directions (equi-axed). Improved service life can be obtained by aligning the crystals to form columns along the blade length, produced by a method known as 'Directional Solidification'. A further advance of this technique is to make the blade out of a single
Fig. 5-13 Comparison of turbine blade life圖5-13 渦輪工作葉片壽命特性的比較
properties.
時間
延伸率
等軸葉片
定向凝固葉片
斷裂
單晶葉片
圖5-14 陶瓷渦輪工作葉片
Fig. 5-14 Ceramic turbine blades.
crystal, Examples of these structures are shown in fig. 5-12. Each method extends the useful creep life of the blade (fig. 5-13) and in the case of the single crystal blade, the operating temperature can be sub-stantially increased.
33. A non-metal based turbine blade can be manu-factured from reinforced ceramics. Their initial production application is likely to be for small high speed turbines which have very high turbine entry 30．超過一段工作期間，渦輪工作葉片慢慢地伸長，這種現(xiàn)象被稱為“蠕變”，而且，在損壞之前存在有限的可用壽命極限。
31．早期曾經(jīng)使用高溫鋼鍛件，但這些材料很快便由鑄造鎳基合金所取代。后者具有更好的蠕變和疲勞特性。
29．依據(jù)上面所述，對于某種葉片材料及允許的安全壽命，就會有一個相應(yīng)的最大允許的渦輪進口溫度以及相應(yīng)的最大發(fā)動機功率。因此，并不奇懌，冶金學家和設(shè)計師們總是不斷地在尋求更好的渦輪工作葉片材料和改善葉片的冷卻方法。
temperatures. An example of a ceramic blade is shown in fig. 5-14.
BALANCING
34. The balancing of a turbine is an extremely important operation in its assembly. In view of the high rotational speeds and the mass of materials, any unbalance could seriously affect the rotating assembly bearings and engine operation. Balancing is effected on a special balancing machine, the principles of which are briefly described in Part 25. 平衡
34．渦輪的平衡是其裝配中極端重要的工作。考慮到高旋轉(zhuǎn)速度及材料的質(zhì)量，任何不平衡都會嚴重影響旋轉(zhuǎn)組件的軸承和發(fā)動機的工作。平衡在專用的平衡機上進行，其原理在第25章中簡要敘述。
33．用強化陶瓷可以制造出非金屬基的渦輪工作葉片。它們初始的生產(chǎn)應(yīng)用似乎會用于具有極高渦輪進口溫度的小型高速渦輪。圖5-14示出了一種陶瓷葉片的例子。
32．對常規(guī)渦輪工作葉片的仔細研究發(fā)現(xiàn)，在各個方向(等軸)存在無數(shù)晶體。通過將晶粒沿葉片長度方向排成柱狀可以改善使用壽命，這種方法稱為“定向凝固”。這種技術(shù)的進一步改進是用一個單晶體制造葉片。這些結(jié)構(gòu)的示例見圖5-12。定向凝固，單晶都增長了葉片的蠕變可用壽命(圖5-13)。在單晶葉片的情況下，使用溫度可以大大地增高。
羅爾斯-羅伊斯公司
RB211-535E4發(fā)動機
1943年末，羅爾斯-羅伊斯公司決定研制一種渦輪螺槳發(fā)動機，用于速度為400英里／小時左右的飛機。所制成的發(fā)動機稱為RB50“遄達”(Trent)，它基本上是一臺“德溫特”(Derwent)Ⅱ型，帶有一根驅(qū)動減速器和螺旋槳的撓性的套軸。1945年9月20日，裝有兩臺“遄達”發(fā)動機的格洛斯特公司的“流星”飛機成為全世界第一架投入飛行的以渦輪螺漿為動力的飛機。
Rolls-Royce RB50 Trent
Late in 1943 the decision was taken at Rolls-Royce to build a turbo-prop for aircraft speeds of around 400 mph. The resulting engine, known as the RB50 Trent, was basically a Derwent II with a flexible quillshaft to reduction gear and propeller. On 20 September 1945 a Gloster Meteor, fitted with two Trents, became the world's first turbo-prop powered aircraft to fly.
羅爾斯-羅伊斯公司
RB50“遄達”發(fā)動機
6: Exhaust system
第六章 排氣系統(tǒng)
Contents Page
Introduction 59
Exhaust gas flow 61
緒言
1．航空燃氣渦輪發(fā)動機有一個排氣系統(tǒng)，它將渦輪排出的燃氣以一定的速度和要求的方向排入大氣，提供最后所得到的推力。在渦輪噴氣發(fā)動機中排氣流的速度和壓力產(chǎn)生推力(第5段)。但在渦輪螺槳發(fā)動機中，排氣流只提供少量推力，因為大部分能量已經(jīng)由渦輪吸收，用來驅(qū)動螺旋槳。因此，排氣采統(tǒng)的設(shè)計對發(fā)動機的性能有很大影響。噴管、推進噴管或排氣口的面積影響到渦輪進口溫度、排氣流的質(zhì)量流量、速度及壓力。
2．依據(jù)發(fā)動機類型的不同，進入排氣系統(tǒng)的燃氣溫度在550～850℃之間，采用加力燃燒室時(第11章)可達l500℃，甚至更高。所以需要采用的材料及結(jié)構(gòu)形式應(yīng)能夠抵御撓曲和產(chǎn)生裂紋，并防止向飛機結(jié)構(gòu)的熱傳導(dǎo)。
3.基本的排氣系統(tǒng)示于圖6-1。反推力裝置(第15章)，消聲器(第19章)及雙位推進噴管的使用使系統(tǒng)的結(jié)構(gòu)更為復(fù)雜，參見圖6-2所示。低涵道比發(fā)動機也可以有一個混合器(圖6-4)，用于促進冷熱燃氣流徹底的混合。 中國航空網(wǎng) www.k6050.com 航空翻譯 www.aviation.cn 本文鏈接地址：勞斯萊斯噴氣引擎－中英(28)
