Afterburner Engine: Formulation
Figure 10.13 is a schematic diagram showing the station numbers for an AB jet engine. To keep numbers consistent with the turbojet numbering system, there is no difference between Stations 4 and 5, which represent the turbine exit condition. Station 5 is the start and Station 6 is the end of AB. Station 7 is the final exit plane. Figure 10.13 also shows the isentropic AB cycle in a T-s diagram.
AB is deployed only in military aircraft (except in the civil supersonic Concorde) as a temporary thrust-augmentation device to meet the mission demand at takeoff and/or fast acceleration and maneuvers to engage or disengage in combat. AB is applied at full throttle by activating a fuel switch. The pilot can feel the deployment by the sudden increase in the g-level in the flight direction. A ground observer
notices a sudden increase in the noise level, which can exceed the physical threshold. An AB glow is visible at the exit nozzle; in the dark, it appears as a spectacular plume with supersonic expansion “diamonds.” In the absence of any downstream rotating machines, the AB temperature limit can be increased from 2,000 to 2,200 deg K, at the expense of a significant increase in the fuel flow (i. e., a richer fuel-to-air ratio than in the core combustion).
An AB exit nozzle invariably runs choked and requires a convergent-divergent nozzle for the supersonic expansion to increase the gain in momentum for the thrust augmentation. Typically, to gain a 50% thrust increase, fuel consumption increases from 100 to 120%; that is why it is used only for a short period, not necessarily in one burst. It is interesting that AB in bypass engines is an attractive proposition because the AB inlet temperature is lower. In fact, all modern combat-category engines use a low bypass of 1 to 3.
Losses in an AB exit nozzle are high – the flameholders and so forth act as obstructions. It is preferable to diffuse the flow speed at the AB from higher speed to Mach 0.2 to 0.3, which results in a small bulge in the jet-pipe diameter around that area. A combat aircraft fuselage must be able to house this bulge.