Variable area final nozzle - When combustion takes place in the reheat jet pipe, the rapid expansion of the gases results in an increase in velocity. If the exit area of the nozzle were not increased to allow the expanding gases to escape, the exit nozzle would cause a restriction and there would be a build up of pressure inside the jet pipe.This increase in pressure is effectively a back pressure which is felt right back through the engine and could cause compressor stall or surge. To prevent this happening, a variable area final nozzle is fitted. The nozzle is normally closed (convergent) when reheat is not operating, and it is opened just sufficient to stop a "back pressure" developing (as sensed by the P3 and Pe sensors). In use the nozzle may be parallel or more likely slightly divergent. The nozzle is moved by a system of hydraulic rams (automatic nozzle control system). Nozzle Control System - This consists of an automatic control unit and a series of rams to move the nozzle itself. The unit receives sensing signals of P3and P6and adjusts the nozzle area by the use of the rams to maintain the correct ratio. Ignition System - injection of the fuel into the jet pipe will not normally cause combustion to take place. Also, the gases are travelling too fast for combustion to be self sustaining even after ignition has occurred. Therefore some form of continuous ignition is required. There are three ways of providing this; Spark Ignition functions in a similar way to normal combustion chamber igniters. Light-up is initiated by a pilot fuel supply, and an igniter plug. A tapping from the main fuel flow supplies fuel for the pilot burner. The burner sprays fuel into a region of low velocity inside a cone forming part of the reheat assembly. The igniter plug is of the spark gap type and projects into the cone adjacent to the pilot burner. When reheat is selected, the ignition system is energized via a time switch. The switch will cut out ignition after a predetermined time. Hot Shot IgnitionIs operated by two fuel injectors, one spraying fuel into one of the combustion chamber "cans", the other spraying fuel into the exhaust system. The streak of flame initiated in the combustion chamber ignites the fuel/air mixture in the reheat jet pipe. The turbine blades are not damaged by the hot streak because of its relatively low energy content and the fact that reheat is used only briefly. Figure 15.2: Hot shot ignition system
Part 66 Training Syllabus Module 15 –Gas Turbine Engine Issue: 1 Issue Date: 15.04.2010 Amdt. / Date: Doc No: BM1_01 Copyright © 2010 - Unpublished Work Part 66 Module 1 Page 216 Catalytic Ignition Consists of a platinum/rhodium element in a case fitted into a housing secured to the burner hub. The housing contains a venture tube, the mouth of which is open to the main gas stream from the turbines. Fuel is taken to the throat of the element of the igniter. A chemical reaction between the fuel/air mixture and the platinum/rhodium element lowers the flashpoint
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- gas turbine engine, engine inlet, Compressor Inlet Pressure