Air intake An intake, or tube, is needed in front of the compressor to help direct the incoming air smoothly into the moving compressor blades. Older engines had stationary vanes in front of the moving blades. These vanes also helped to direct the air onto the blades. The intake is also shaped to minimise any flow losses when the compressor is accelerating the air through the intake at zero and low aircraft speeds, and to slow the flow down for the compressor when the aircraft is operating above Mach 1. The air flowing into a turbojet engine must always be subsonic, regardless of the speed of the aircraft itself. Compressor The compressor is driven by the turbine. It rotates at high speed, adding energy to the airflow and at the same time squeezing (compressing) it into a smaller space. Compressing the air increases its pressure and temperature. The smaller the compressor the faster it turns. At the large end of the range the GE-90-115 fan rotates at about 2,500 RPM while a small helicopter engine compressor rotates at about 50,000 RPM. In most turbojet-powered aircraft, bleed air is extracted from the compressor section at various stages to perform a variety of jobs including air conditioning/pressurization, engine inlet anti-icing and turbine cooling. Bleeding air off decreases the overall efficiency of the engine, but the usefulness of the compressed air outweighs the loss in efficiency. Compressor types used in turbojets were typically axial or centrifugal. Early turbojet compressors had overall pressure ratios as low as 5:1. Aerodynamic improvements including splitting the compressor into two separately rotating parts, incorporating variable blade angles for entry guide vanes and stators, enabled later turbojets to have overall pressure ratios of 15:1 or more. For comparison, modern civil turbofan engines have overall pressure ratios of 44:1 or more. After leaving the compressor, the air enters the combustion chamber. Combustion chamber The burning process in the combustor is significantly different from that in a piston engine. In a piston engine the burning gases are confined to a small volume and, as the fuel burns, the pressure increases. In a turbojet the air and fuel mixture burn in the combustor and pass through to the turbine in a continuous flowing process with no pressure build-up. Instead there is a small pressure loss in the combustor. The fuel-air mixture can only burn in slow moving air so an area of reverse flow is maintained by the fuel nozzles for the approximately stoichiometric burning in the primary zone. Further compressor air is introduced which completes the combustion process and reduces the temperature of the combustion products to a level which the turbine can accept. Less than 25% of the air is typically used for combustion, as an overall lean mixture is required to keep within the turbine temperature limits. Turbine Hot gases leaving the combustor expand through the turbine. Typical materials for turbines include inconel and Nimonic.[9] The turbine vanes and blades have internal cooling passages. Air from the compressor is passed through these to keep the metal temperature within limits. In the first stage the turbine is largely an impulse turbine (similar to a pelton wheel) and rotates because of the impact of the hot gas stream. Later stages are convergent ducts that accelerate the gas. Energy is transferred into the shaft through momentum exchange in the opposite way to energy transfer in the compressor. The power developed by the turbine drives the compressor as well as accessories, like fuel, oil, and hydraulic pumps that are driven by the accessory gearbox. Nozzle Main article: propelling nozzle After the turbine, the gases expand through the exhaust nozzle producing a high velocity jet. In a convergent nozzle, the ducting narrows progressively to a throat. The nozzle pressure ratio on a turbojet is high enough at higher thrust settings to cause the nozzle to choke. If, however, a convergent-divergent de Laval nozzle is fitted, the divergent (increasing flow area) section allows the gases to reach supersonic velocity within the divergent section. Additional thrust is generated by the higher resulting exhaust velocity. Thrust augmentation Thrust was most commonly increased in turbojets with water/methanol injection or afterburning. Some engines used both at the same time. Afterburner Main article: afterburner An afterburner or reheat jetpipe is a combustion chamber added to reheat the turbine exhaust gases. The fuel consumption is very high, typically four times that of the main engine. Afterburners are used almost exclusively on supersonic aircraft, most being military aircraft. Two supersonic airliners, Concorde and the TU-144, also used afterburners as does Scaled Composites White Knight, a carrier aircraft for the experimental SpaceShipOne suborbital spacecraft.
Posted on: Sun, 24 Aug 2014 23:48:02 +0000
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