15-01-2016, 04:09 PM
• The jet engine for the first time was demonstrated in Hero’s engine, which was produced as a toy in 120 B.C. This toy was known as aeolipile,it showed how the momentum of steam issuing from a number of jets could impart an equal and opposite reaction to the jets themselves.
• This toy consisted of a boiler or bowl that held a supply of water. Two hollow tubes extended up from this boiler and supported a hollow sphere that was free to turn on these supports. When the water in boiler was boiled, the steam shooting from the two small jets caused the sphere to spin, like the lawn sprinkler is made to spin from the reaction of the water leaving its nozzles.
• Chinese people are regarded as the first user of jet reaction in transportation. A Chinese student has developed some jet cylinders which he installed in his vehicle and then used them to provide motion to his vehicle. He was successful and his this idea gave birth to modern jet engine concept.
• A French engineer ReneLorin was the first person who patented a jet propulsion engine in 1913. At that time, it was unsuitable to manufacture or use as it was an aero-thermodynamic-duct.
• In 1930, Frank Whittle was granted the first patent for using a gas turbine engine to produce a propulsive jet. The Whittle engine formed the basis of modern gas turbine engine and from it was developed the Rolls-Royce Welland, Derwent engines. But it took eleven years before his engine completed his first flight. All present day engines are inevitable development of Whittle’s early engine.
• Von Ohain at the same time in Germany was working on development of jet engine for aircraft. He build and ran his first demonstrated engine in 1937.
• The Whittle and Van Ohain engines lead to successful jet-powered fighter aircraft by the end of World War 2, the Messerschmitt Me262 that was used by German Air Force.
• In 1991, both Whittle and Von Ohain were honored as co-inventor of jet engine and now they are equally recognized.
WORKING PRINCIPLE
Jet propulsion is a practical application of Sir Issac Newton’s third law of motion which states that “for every force acting on a body there is an equal and opposite reaction”. For aircraft, the body is atmospheric air that is caused to accelerate as it passes through the engine. The force required to give this acceleration has an equal effect in opposite direction along the apparatus producing the acceleration. A jet engine produces thrust in a similar way to the engine/propeller combination. Both propel the aircraft by thrusting a large weight of air backwards.
MECHANISM
A gas turbine engine consists of five major sections- an inlet duct, a compressor, a combustor chamber, a turbine wheel and an exhaust duct. Some engines might incorporate water injection system, an afterburner, a variable area exhaust nozzle, a free power turbine, a propeller-reduction gearbox and other additional systems to improve or change engine operation, performance and usage.
The front or inlet duct is almost entirely open to permit outside air to enter the front of engine. The compressor works on this incoming air and delivers it to the combustion or burner section with as much as 20 times or more the pressure the air had at the front. In the burner section fuel, is sprayed and mixed with the compressed air. The air-fuel mixture is then ignited by devices similar to spark plug. When the mixture is once ignited, the igniter can be turned off, as the burning process will continue without further assistance as long as the engine is supplied with proper fuel-air ratio.
The fuel-air mixture burns at relatively constant pressure with only 25% of air taking part in actual combustion process. The balance of air is mixed with the products of combustion for cooling before the gases enter the turbine wheel. The turbine extracts a major portion of the energy in the gas stream and uses this energy to turn the compressor and accessories. After, leaving the turbine, there is still enough pressure remaining to force the hot gases through the exhaust duct and jet nozzle at the rear of the engine at very high speeds. The engine’s thrust comes from taking a large mass of air in at the front end and expelling it from the tailpipe at a much higher speed than it had entered the compressor. Thrust then, is equal to mass flow rate times change in velocity.
CATEGORY ON BASIS OF COMPRESSOR FLOW
In the gas turbine engine, compression of the air before expansion through the turbine is effected by one of two basic types of compressor, one giving centrifugal flow and the other axial flow. Both types are driven by the engine turbine and are usually coupled direct to the turbine shaft.
1.Axial Flow Compressor:
An axial flow compressor consists of one or more rotor assemblies that carry blades of airfoil section. These assemblies are mounted between bearings in the casings which incorporate the stator vanes. The compressor is a multi-stage unit as the amount of pressure increase by each stage is small; a stage consists of a row of rotating blades followed by a row of stator vanes. Where several stages of compression operate in series on one shaft it becomes necessary to vary the stator vane angle to enable the compressor to operate effectively at speeds below the design condition. As the pressure ratio is increased the incorporation of variable stator vanes ensures that the airflow is directed onto the succeeding stage of rotor blades at an acceptable angle.From the front to the rear of the compressor, i.e. from the low to the high pressure end, there is a gradual reduction of the air annulus area betweenthe rotor shaft and the stator casing. This is necessary to maintain a near constant air axial velocity as the density increases through the length of the compressor. The convergence of the air annulus is achieved by the tapering of the casing or rotor. A combination of both is also possible, with the arrangement being influenced by manufacturing problems and other mechanical design factors.
• A single-spool compressor consists of one rotor assembly and stators with as many stages as necessary to achieve the desired pressure ratio and all the airflow from the intake passes through the compressor.
• The multi-spool compressor consists of two or more rotor assemblies, each driven by their ownturbine at an optimum speed to achieve higher pressure ratios and to give greater operating flexibility.
• During operation the rotor is turned at high speed by the turbine so that air is continuously induced into the compressor, which is then accelerated by the rotating blades and swept rearwards onto the adjacent row of stator vanes. The pressure rise results from the energy imparted to the air in the rotor which increases the air velocity. The air is then decelerated in the followingstator passage and the kinetic energy translated into pressure. Stator vanes also serve to correct the deflection given to the air by the rotor blades and to present the air at the correct angle to the next stage of rotor blades. The last row of stator vanes usually act as air straighteners to remove swirl from the air prior to entry into the combustion system at a reasonably uniform axial velocity. The changes are accompanied by a progressive increase in air temperature as the pressure increases. Across each stage the ratio of total pressures of outgoing air and inlet air is quite small, being between 1:1 and 1:2. The reason for the small pressure increase through each stage is that the rate of diffusion and the deflection angle of the .blades must be limited if losses due to air breakaway at the blades and subsequent blade stall are to be avoided. Although the pressure ratio of each stage is small, every stage increases the exit pressure of the stage that precedes it.