例子-气流通过涡轮导向器叶片
图2-3通过扩散和收敛涵道的空气流
Fig. 2-3 An airflow through divergent and convergent ducts.
9．从图2-5中的气流图可以看出空气的温度和压力在一台发动机中的变化。由于气流是连续的，速度变化时就出现体积的变化。
10．发生这些变化时的效率将决定压力、体积和温度间所要求的关系能达到何等程度。就一效率较高的压气机而言，输入给定的功，即在空气的温度升高一定的条件下产生的压力就比较高。反之，涡轮利用膨胀燃气的效率愈高，在给定的燃气压降下输出的功也就愈多。
Working cycle and airflow
11．当空气在100％的效率下受到压缩或膨胀时，此过程称之为绝热过程。因为这种变化意味着过程中没有能量损失，既无摩擦、无传导或者紊流损失，这显然实际上是无法实现的。压气机和涡轮有90％的绝热效率就满好了。
stages. This relationship applies for whatever means are used to change the state of the air. For example, whether energy is added by combustion or by compression, or is extracted by the turbine, the heat change is directly proportional to the work added or taken from the gas.
8.
There are three main conditions in the engine working cycle during which these changes occur. During compression, when work is done to increase the pressure and decrease the volume of the air, there is a corresponding rise in the temperature. During combustion, when fuel is added to the air and burnt to increase the temperature, there is a corre-sponding increase in volume whilst the pressure remains almost constant. During expansion, when work is taken from the gas stream by the turbine assembly, there is a decrease in temperature and pressure with a corresponding increase in volume.
9.
Changes in the temperature and pressure of the air can be traced through an engine by using the airflow diagram in fig. 2-5. With the airflow being continuous, volume changes are shown up as changes in velocity.
10.
The efficiency with which these changes are made will determine to what extent the desired relations between the pressure, volume and temperature are attained. For the more efficient the compressor, the higher the pressure generated for a given work input; that is, for a given temperature rise of the air. Conversely, the more efficiently the turbine uses the expanding gas, the greater the output of work for a given pressure drop in the gas.
11.
When the air is compressed or expanded at 100 per cent efficiency, the process is said to be adiabatic. Since such a change means there is no energy losses in the process, either by friction, conduction or turbulence, it is obviously impossible to achieve in practice; 90 per cent is a good adiabatic efficiency for the compressor and turbine.
CHANGES IN VELOCITY AND PRESSURE
12. During the passage of the air through the engine, aerodynamic and energy requirements demand changes in its velocity and pressure. For instance: during compression, a rise in the pressure of the air is required and not an increase in its velocity. After the air has been heated and its internal energy increased by combustion, an increase in the velocity of the gases is necessary to force the turbine to rotate. At the propelling nozzle a high exit velocity is required, for it is the change in the momentum of the air that provides the thrust on the aircraft. Local decelerations of airflow are also required, as for instance, in the combustion chambers to provide a low velocity zone for the flame to burn.
13. These various changes are effected by means of the size and shape of the ducts through which the air passes on its way through the engine. Where a conversion from velocity (kinetic) energy to pressure is required, the passages are divergent in shape. Conversely, where it is required to convert the energy stored in the combustion gases to velocity energy, a convergent passage or nozzle (fig. 2-3) is used. These shapes apply to the gas turbine engine where the airflow velocity is subsonic or sonic, i.e. at the local speed of sound. Where supersonic speeds are encountered, such as in the propelling nozzle of the rocket, athodyd and some jet engines (Part 6), a convergent-divergent nozzle or venturi (fig. 2-4) is used to obtain the maximum conversion of the energy in the combustion gases to kinetic energy.
14. The design of the passages and nozzles is of great importance, for upon their good design will depend the efficiency with which the energy changes are effected. Any interference with the smooth airflow creates a loss in efficiency and could result in component failure due to vibration caused by eddies or turbulence of the airflow. 圈2-4通过收敛-扩散喷管或文氏管的超音速气流
气流在喉道处增加到音速
速度和压力的变化
12．在空气流过发动机的过程中，从气动力和能量的要求来看需要空气的速度和压力发生变化。例如在压缩过程中，只要求空气的压力升高，并不要求其速度增加。在燃烧后，空气已经受热并且其内部能量增加，就需要燃气的速度增加来驱动涡轮旋转。在推进喷管处，要求高的出口速度，因为就是空气动量的这种变化为飞机提供了推力。气流的局部减速也是需要的，例如，在燃烧室中，要提供一个低速区供火焰燃烧。 中国航空网 www.aero.cn 航空翻译 www.aviation.cn 本文链接地址：劳斯莱斯喷气引擎－中英(8)
