Metrovick F2/4 Beryl. Turbo-Union RB199

Transcription

1 Turbo-Union RB199 Metrovick F2/4 Beryl Development of the F2, the first British axial flow turbo-jet, began in f 940. After initial flight trials in the tail of an Avro Lancaster, two F2s were installed in a Gloster Meteor and first flew on 13 November After early problems the F2/4 Beryl was developed which gave up to 4000 lb thrust and was used to power the Saunders Roe SR/A1 flying boat fighter.

2 16: Afterburning Contents Page Introduction 169 Operation of afterburning 170 Construction 173 Burners Jet pipe Propelling nozzle Control system 173 Thrust increase 175 Fuel consumption 178 INTRODUCTION 1. Afterburning (or reheat) is a method of augmenting the basic thrust of an engine to improve the aircraft take-off, climb and (for military aircraft) combat performance. The increased power could be obtained by the use of a larger engine, but as this would increase the weight, frontal area and overall fuel consumption, afterburning provides the best method of thrust augmentation for short periods. 2. Afterburning consists of the introduction and burning of fuel between the engine turbine and the jet pipe propelling nozzle, utilizing the unburned oxygen in the exhaust gas to support combustion (fig. 16-1). The resultant increase in the temperature of the exhaust gas gives an increased velocity of the jet leaving the propelling nozzle and therefore increases the engine thrust. 3. As the temperature of the afterburner flame can be in excess of 1,700 deg. C., the burners are usually arranged so that the flame is concentrated around the axis of the jet pipe. This allows a proportion of the turbine discharge gas to flow along the wall of the jet pipe and thus maintain the wall temperature at a safe value. 169

3 Fig Principle of afterburning 4. The area of the afterburning jet pipe is larger than a normal jet pipe would be for the same engine, to obtain a reduced velocity gas stream. To provide for operation under all conditions, an afterburning jet pipe is fitted with either a two-position or a variablearea propelling nozzle (fig. 16-2). The nozzle is closed during non-afterburning operation, but when afterburning is selected the gas temperature increases and the nozzle opens to give an exit area suitable for the resultant increase in the volume of the gas stream. This prevents any increase in pressure occurring in the jet pipe which would affect the functioning of the engine and enables afterburning to be used over a wide range of engine speeds. 5. The thrust of an afterburning engine, without afterburning in operation, is slightly less than that of a similar engine not fitted with afterburning equipment; this is due to the added restrictions in the jet pipe. The overall weight of the power plant is also increased because of the heavier jet pipe and afterburning equipment. 6. Afterburning is achieved on low by-pass engines by mixing the by-pass and turbine streams before the afterburner fuel injection and stabilizer system is reached so that the combustion takes place in the mixed exhaust stream. An alternative method is to inject the fuel and stabilize the flame in the individual by-pass and turbine streams, burning the available gases up to a common exit temperature at the final nozzle. In this method, the fuel injection is scheduled separately to the individual streams and it is normal to provide some form of interconnection between the flame stabilizers in the hot and cold streams to assist the combustion processes in the cold by-pass air. OPERATION OF AFTERBURNING 7. The gas stream from the engine turbine enters the jet pipe at a velocity of 750 to 1,200 feet per second, but as this velocity is far too high for a stable flame to be maintained, the flow is diffused before it enters the afterburner combustion zone, i.e. the flow velocity is reduced and the pressure is increased. However, as the speed of burning kerosine at normal mixture ratios is only a few feet per second, any fuel lit even in the diffused air stream would be blown away. A form of flame stabilizer (vapour gutter) is, therefore, located downstream of the fuel burners to provide a region in which turbulent eddies are formed to assist combustion and where the local gas velocity is further reduced to a figure at which flame stabilization occurs whilst combustion is in operation. 170

4 Fig Examples of afterburning jet pipes and propelling nozzles. 171

5 8. An atomized fuel spray is fed into the jet pipe through a number of burners, which are so arranged as to distribute the fuel evenly over the flame area. Combustion is then initiated by a catalytic igniter, which creates a flame as a result of the chemical reaction of the fuel/air mixture being sprayed on to a platinum-based element, by an igniter plug adjacent to the burner, or by a hot streak of flame that originates in the engine combustion chamber (fig. 16-3): this latter method is known as 'hot-shot' ignition. Once combustion is initiated, the gas temperature increases and the expanding gases accelerate through the enlarged area propelling nozzle to provide the additional thrust. 9. In view of the high temperature of the gases entering the jet pipe from the turbine, it might be assumed that the mixture would ignite spontaneously. This is not so, for although cool flames form at Fig Methods of afterburning ignition. 172

6 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 is based on the latter system. Jet pipe 12. The afterburning jet pipe is made from a heatresistant 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. Propelling nozzle 14. The propelling nozzle is of similar material and construction as the jet pipe, to which it is secured as a separate assembly. A two-position propelling nozzle has two movable eyelids that are operated by actuators, or pneumatic rams, to give an open or closed position (para. 4.). A variable-area propelling nozzle has a ring of interlocking flaps that are hinged to the outer casing and may be enclosed by an outer shroud. The flaps are actuated by powered rams to the closed position, and by gas loads to the intermediate or the open positions; control of the flap position is by a control unit and a pump provides the power to the rams (para. 18). CONTROL SYSTEM 15. It is apparent that two functions, fuel flow and propelling nozzle area, must be co-ordinated for satisfactory operation of the afterburner system, These functions are related by making the nozzle area dependent upon the fuel flow at the burners or viceversa. The pilot controls the afterburner fuel flow or the nozzle area in conjunction with a compressor delivery/jet pipe pressure sensing device (a pressure ratio control unit). When the afterburner fuel flow is increased, the nozzle area increases; when the afterburner fuel flow decreases, the nozzle area is reduced. The pressure ratio control unit ensures the pressure ratio across the turbine remains unchanged and that the engine is unaffected by the operation of afterburning, regardless of the nozzle area and fuel flow. 16. Since large fuel flows are required for afterburning, an additional fuel pump is used. This pump is usually of the centrifugal flow or gear type and is energized automatically when afterburning is selected. The system is fully automatic and incorporates 'fail safe' features in the event of an afterburner malfunction. The interconnection between the control system and afterburner jet pipe is shown diagrammatically in fig

7 Fig Typical afterburning jet pipe equipment. 17. When afterburning is selected, a signal is relayed to the afterburner fuel control unit. The unit determines the total fuel delivery of the pump and controls the distribution of fuel flow to the burner assembly. Fuel from the burners is ignited, resulting in an increase in jet pipe pressure (P6). This alters the pressure ratio across the turbine (P3/P6), and the exit area of the jet pipe nozzle is automatically increased until the correct PS/PS ratio has been restored. With a further increase in the degree of afterburning, the nozzle area is progressively increased to maintain a satisfactory P3/P6 ratio. Fig illustrates a typical afterburner fuel control system. 18. To operate the propelling nozzle against the large 'drag' loads imposed by the gas stream, a pump and either hydraulically or pneumatically operated rams are incorporated in the control system. The system shown in fig uses oil as the 174

8 Fig Simplified control system. hydraulic medium, but some systems use fuel. Nozzle movement is achieved by the hydraulic operating rams which are pressurized by an oil pump, pump output being controlled by a linkage from the pressure ratio control unit. When an increase in afterburning is selected, the afterburner fuel control unit schedules an increase in fuel pump output. The jet pipe pressure (P6) increases, altering the pressure ratio across the turbine (P3/P6). The pressure ratio control unit alters oil pump output, causing an out-of-balance condition between the hydraulic ram load and the gas load on the nozzle flaps. The gas load opens the nozzle to increase its exit area and, as the nozzle opens, the increase in nozzle area restores the P3/P6 ratio and the pressure ratio control unit alters oil pump output until balance is restored between the hydraulic rams and the gas loading on the nozzle flaps. THRUST INCREASE 19. The increase in thrust due to afterburning depends solely upon the ratio of the absolute jet pipe temperatures before and after the extra fuel is burnt. For example, neglecting small losses due to the afterburner equipment and gas flow momentum changes, the thrust increase may be calculated as follows. 175

9 Fig A simplified typical afterburner fuel control system. 176

10 Fig A simplified typical afterburner nozzle control system. 20. Assuming a gas temperature before afterburning of 640 deg. C. (913 deg. K.) and with afterburning of 1,269 deg. C. (1,542 deg. K.). then the temperature ratio = 1,542 = The velocity of the jet stream increases as the square root of the temperature ratio. Therefore, the jet velocity = ^/T.69 = 1.3. Thus, the jet stream velocity is increased by 30 per cent, and the increase in static thrust, in this instance, is also 30 per cent (fig. 16-8). 21. Static thrust increases of up to 70 per cent are obtainable from low by-pass engines fitted with afterburning equipment and at high forward speeds several times this amount of thrust boost can be obtained. High thrust boosts can be achieved on low by-pass engines because of the large amount of oxygen in the exhaust gas stream and the low initial temperature of the exhaust gases. Fig Thrust increase and temperature ratio. 177

11 22. It is not possible to go on increasing the amount of fuel that is burnt in the jet pipe so that all the available oxygen is used, because the jet pipe would not withstand the high temperatures that would be incurred and complete combustion cannot be assured. FUEL CONSUMPTION 23. Afterburning always incurs an increase in specific fuel consumption and is, therefore, generally limited to periods of short duration. Additional fuel must be added to the gas stream to obtain the required temperature ratio (para. 19). Since the temperature rise does not occur at the peak of compression, the fuel is not burnt as efficiently as in the engine combustion chamber and a higher specific fuel consumption must result. For example, assuming a specific fuel consumption without afterburning of 1,15 lb./hr./lb. thrust at sea level and a speed of Mach 0,9 as shown in fig then with 70 per cent afterburning under the same conditions of flight, the consumption will be increased to Fig Specific fuel consumption comparison. Fig Afterburning and its effect on the rate of climb. 178

12 approximately 2.53 lb./hr./lb. thrust. With an increase in height to 35,000 feet this latter figure of 2.53 lb./hr./lb. thrust will fall slightly to about 2.34 lb./hr./lb. thrust due to the reduced intake temperature. When this additional fuel consumption is combined with the improved rate of take-off and climb (fig ), it is found that the amount of fuel required to reduce the time taken to reach operation height is not excessive. 179