例子-氣流通過渦輪導向器葉片
圖2-3通過擴散和收斂涵道的空氣流
Fig. 2-3 An airflow through divergent and convergent ducts.
9．從圖2-5中的氣流圖可以看出空氣的溫度和壓力在一臺發動機中的變化。由于氣流是連續的，速度變化時就出現體積的變化。
10．發生這些變化時的效率將決定壓力、體積和溫度間所要求的關系能達到何等程度。就一效率較高的壓氣機而言，輸入給定的功，即在空氣的溫度升高一定的條件下產生的壓力就比較高。反之，渦輪利用膨脹燃氣的效率愈高，在給定的燃氣壓降下輸出的功也就愈多。
Working cycle and airflow
11．當空氣在100％的效率下受到壓縮或膨脹時，此過程稱之為絕熱過程。因為這種變化意味著過程中沒有能量損失，既無摩擦、無傳導或者紊流損失，這顯然實際上是無法實現的。壓氣機和渦輪有90％的絕熱效率就滿好了。
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通過收斂-擴散噴管或文氏管的超音速氣流
氣流在喉道處增加到音速
速度和壓力的變化
12．在空氣流過發動機的過程中，從氣動力和能量的要求來看需要空氣的速度和壓力發生變化。例如在壓縮過程中，只要求空氣的壓力升高，并不要求其速度增加。在燃燒后，空氣已經受熱并且其內部能量增加，就需要燃氣的速度增加來驅動渦輪旋轉。在推進噴管處，要求高的出口速度，因為就是空氣動量的這種變化為飛機提供了推力。氣流的局部減速也是需要的，例如，在燃燒室中，要提供一個低速區供火焰燃燒。 中國航空網 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(8)
