圖16-3 幾種加力燃燒的點火方式
Fig. 16-3 Methods of afterburning ignition.
推進噴管
14．推進噴管的材料和結構與噴管類似，作為一個獨立的裝置固定在噴管上。雙位置的推進噴管有兩個可移動的半圓形調節噴口，由作動筒或氣壓作動筒操縱。使它處于打開或關閉的位置(第4段)。面積可調的推進噴管有一圈聯鎖的魚鱗片，魚鱗片鉸接到外殼上，并用一外罩罩上。魚鱗片通過有動力的作動筒操縱到關小位置，靠氣動載荷吹開到中間或打開位置；魚鱗片位置的控制是通過控制裝置完成的，泵向作動筒提供動力(第18段)。
13．一種類似于噴管材料的環形隔熱屏常常被裝在噴管內壁中以改善噴嘴后部的冷卻。隔熱屏由多段組成，用波紋狀冷卻板連接，形成了單獨的一層。隔熱屏的后部是一些搭接的耐火“板”，沿圓周鉚接(圖16-4)。隔熱屏也防止燃燒的不穩定性產生過大的噪聲和振動，這些過大的噪聲和振動也會造成加力燃燒室裝置的結構損壞。
控制系統
15．顯而易見，為了使加力燃燒室系統達到滿意的工作，燃油流量和推進噴管面積這兩個功能必須協調一致。這兩個功能是根據噴嘴的燃油流量來決定噴口面積而互相關連起來的，反過來也是一樣。駕駛員控制加力燃油流量或噴口面積，它們是與壓氣機出口／噴管壓力傳感裝置(壓力比控制裝置)連在一起的。當加力燃油流量增加時，噴口面積增加；當加力燃油流量減少，噴口面積也減小。無論噴口面積和燃油流量是多少，壓力比控制裝置均保證渦輪前后的壓力比不變，并且也保證了發動機不受加力工作的影響。
16．由于加力燃燒需要較大的燃油流量，因此常使用額外的油泵。這種泵一般為離心式泵或齒輪式泵，當選用加力燃燒時能夠自動供油。該系統為全自動的，在加力燃燒室失效時，具有“故障安全”的特點。控制系統和加力燃燒室噴管之間的相互連接以圖解的方式示于圖16-5。
temperatures up to 700 deg. C., combustion will not take place below 800 deg. C. If however, the conditions were such that spontaneous ignition could be effected at sea level, it is unlikely that it could be effected at altitude where the atmospheric pressure is low. The spark or flame that initiates combustion must be of such intensity that a light-up can be obtained at considerable altitudes.
10. For smooth functioning of the system, a stable flame that will burn steadily over a wide range of mixture strengths and gas flows is required. The mixture must also be easy to ignite under all conditions of flight and combustion must be maintained with the minimum loss of pressure.
CONSTRUCTION
Burners
11. The burner system consists of several circular concentric fuel manifolds supported by struts inside the jet pipe. Fuel is supplied to the manifolds by feed pipes in the support struts and sprayed into the flame area, between the flame stabilizers, from holes in the downstream edge of the manifolds. The flame stabilizers are blunt nosed V-section annular rings located downstream of the fuel burners. An alternative system includes an additional segmented fuel manifold mounted within the flame stabilizers. The typical burner and flame stabilizer shown in fig. 16-4 is based on the latter system.
Jet pipe
12.
The afterburning jet pipe is made from a heat-resistant nickel alloy and requires more insulation than the normal jet pipe to prevent the heat of combustion being transferred to the aircraft structure. The jet pipe may be of a double skin construction with the outer skin carrying the flight loads and the inner skin the thermal stresses; a flow of cooling air is often induced between the inner and outer skins. Provision is also made to accommodate expansion and contraction, and to prevent gas leaks at the jet pipe joints.
13.
A circular heatshield of similar material to the jet pipe is often fitted to the inner wall of the jet pipe to improve cooling at the rear of the burner section. The heatshield comprises a number of bands, linked by
cooling corrugations, to form a single skin. The rear of the heatshield is a series of overlapping 'tiles' riveted to the surrounding skin (fig. 16-4). The shield also prevents combustion instability from creating excessive noise and vibration, which in turn would cause rapid physical deterioration of the afterburner equipment.
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