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7.通過導引發動機周圍的外界層放氣，可將在高馬赫數下飛行的飛機的自燃起火減至最少。但是如果一旦發生起火情況，則必須切斷這一高速空氣流，否則由于滅火劑被快速吹散，它可能會組長火勢，并且降低滅火系統的有效性。
 
外部冷卻和通風
 
8.發動機機艙或吊艙一般由流經發動機四周的大氣中的空氣進行冷卻和通風，然后排出機外（圖14-1）。地面試車時由噴射系統利用一個內部冷卻空氣的出氣口作為引射系統來提供對流冷卻。這一氣流的重要功能是吹除發動機艙中的可燃蒸汽。通過保持最小的空氣流量，則可將動力裝置的阻力減至最小。由于所需的滅火劑劑量與該區域的空氣流量成正比，所以失火的火力將是較弱的。
4.所有的燃油、滑油或液壓油的導管均用耐火／防火材料制成并符合防火規定要求，所有的電氣附件及連接件均為防爆產品。靜電放電引起的火花通過所有的飛機和發動機附件接地來防止。這使所有的附件之間無電位差，使它們不能點燃可燃蒸汽。
 
5.在某些發動機上，發動機“熱區”中輸送可燃液體的管子是雙層結構的。如果液體的主管發生破裂。管子的外層將容納任何泄漏的液體，從而防止任何可能發生的起火。
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.在發動機停車打開滅火瓶并釋放滅火劑之前，迅速探測出著火對盡量縮短著火時間是十分關鍵的。特別重要的是火警探測系統不要給出假警報，假警報通常是由于電氣工作系統受摩擦或濕氣侵入引起短路所致，或是在使用充滿氣體的連續測溫感應頭時，它的毛細管擦傷引起管內氣體損失而造成的。
火警探測
Fig. 14-1 A typical cooling and ventilation system.
12.探測系統可包括許多位于關鍵部位的探測器裝置，或是探測型連續元件（充滿氣體的或電氣的），這種元件可以做成一定形狀并且連接在預先形成的管子上。探測元件可穿過排氣孔布置，例如區域抽氣通風管，以便及早探測出起火（圖14-3）。

13.對于電器系統，起火是通過探測器線路的電氣特性變化來感受，根據探測器的類型，它可是熱敏電阻、熱電偶或電氣連續元件。在這些情況下，溫度的變化產生信號，信號通過放大器啟動報警指示器。

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