劳斯莱斯喷气引擎－中英(79)

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7.通过导引发动机周围的外界层放气，可将在高马赫数下飞行的飞机的自燃起火减至最少。但是如果一旦发生起火情况，则必须切断这一高速空气流，否则由于灭火剂被快速吹散，它可能会组长火势，并且降低灭火系统的有效性。
 
外部冷却和通风
 
8.发动机机舱或吊舱一般由流经发动机四周的大气中的空气进行冷却和通风，然后排出机外（图14-1）。地面试车时由喷射系统利用一个内部冷却空气的出气口作为引射系统来提供对流冷却。这一气流的重要功能是吹除发动机舱中的可燃蒸汽。通过保持最小的空气流量，则可将动力装置的阻力减至最小。由于所需的灭火剂剂量与该区域的空气流量成正比，所以失火的火力将是较弱的。
4.所有的燃油、滑油或液压油的导管均用耐火／防火材料制成并符合防火规定要求，所有的电气附件及连接件均为防爆产品。静电放电引起的火花通过所有的飞机和发动机附件接地来防止。这使所有的附件之间无电位差，使它们不能点燃可燃蒸汽。
 
5.在某些发动机上，发动机“热区”中输送可燃液体的管子是双层结构的。如果液体的主管发生破裂。管子的外层将容纳任何泄漏的液体，从而防止任何可能发生的起火。
However, if ignition should occur, this high velocity air stream may have to be shut off, otherwise it would increase the flame intensity and reduce the effective-ness of the extinguishing system by rapid dispersal of the extinguishant.
External cooling and ventilation
8.
The engine bay or pod is usually cooled and ventilated by atmospheric air being passed around the engine and then vented overboard (fig. 14-1). Convection cooling during ground running may be provided by using an internal cooling outlet vent as an ejector system. An important function of the airflow is to purge any flammable vapours from the engine compartment. By keeping the airflow minimal, the power plant drag is minimized and, as the required quantity of fire extinguishant is in proportion to the zonal airflow, any fire outbreak would be of low intensity.

9.
On some engines a fireproof bulkhead is also provided to separate the 'cool' area or zone of the engine, which contains the fuel, oil, hydraulic and electrical systems, from the 'hot' area surrounding the combustion, turbine and exhaust sections of the engine. Differential pressures can be created in the two zones by calibration of the inlet and outlet apertures to prevent the spread of fire from the hot zone.

10.
Fig. 14-2 shows a more complex cooling and ventilation system used on a turbo-fan engine. Air is induced from the intake duct and also delivered from the fan to provide multi-zone cooling, each zone having its own calibrated cooling flow.

Fire protection

FIRE DETECTION
11.
The rapid detection of a fire is essential to minimize the fire period before engine shut-down drill and release of extinguishant is effected. It is also extremely important that a fire detection system will not give a false fire warning resulting from short circuiting caused by chafing or the ingress of moisture in the case of electrically operated systems and chafes of the capillary resulting in loss of the contained gas in the case of the gas filled continuous element sensing type,

12.
A detection system may consist of a number of strategically located detector units, or be of the continuous element (gas filled or electrical) sensing type that can be shaped and attached to pre-formed tubes. The sensing element can be routed across outlet orifices, such as a zone extractor ventilation duct, to give early detection of a fire (fig. 14-3).

13.
In the case of electrical systems the presence of a fire is signalled by a change in the electrical char-acteristics of the detector circuit, according to the type of detector, be it thermistor, thermocouple or electrical continuous element. In these cases the change in temperature creates the signal which, through an amplifier, operates the warning indicator.

14.
Both the thermocouple and thermistor detectors have properties making them ideally suited to this application. The thermocouple comprises two dissimilar metals which are joined together to form two junctions. As the temperature difference between the two junctions increases an E.M.F. is produced in the circuit and it is this E.M.F. that triggers the fire
防火隔板

 

11.在发动机停车打开灭火瓶并释放灭火剂之前，迅速探测出着火对尽量缩短着火时间是十分关键的。特别重要的是火警探测系统不要给出假警报，假警报通常是由于电气工作系统受摩擦或湿气侵入引起短路所致，或是在使用充满气体的连续测温感应头时，它的毛细管擦伤引起管内气体损失而造成的。
火警探测
Fig. 14-1 A typical cooling and ventilation system.
12.探测系统可包括许多位于关键部位的探测器装置，或是探测型连续元件（充满气体的或电气的），这种元件可以做成一定形状并且连接在预先形成的管子上。探测元件可穿过排气孔布置，例如区域抽气通风管，以便及早探测出起火（图14-3）。

13.对于电器系统，起火是通过探测器线路的电气特性变化来感受，根据探测器的类型，它可是热敏电阻、热电偶或电气连续元件。在这些情况下，温度的变化产生信号，信号通过放大器启动报警指示器。

区域2流经不密封
整流罩的空气流
　
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