Types of jet engines
Known basic types of jet engines:
Powder and liquid rocket engines for its work does not need oxygen from the ambient air, as required for the combustion of oxygen contained in the substances that make up the powder or liquid oxidizer.
During the combustion of the powder or liquid fuel mixed with the liquid oxidizing agent formed combustion products that occupy many times larger volume than the starting materials, so the combustion gases in the form of a high velocity jet nozzles are pulled out from the outside.
By the law of conservation of energy the amount of movement of bodies is constant. The engine and the prisoners therein combustion products are a system of two bodies. And if one of the bodies of the system (combustion) of mass m gets velocity V "CT, m. e. creates a quantity of motion equal to the product, then other body systems (engine) must receive equal in magnitude but opposite in direction of movement. Only in this case the amount of movement of the entire system will not change and will not be broken by the law of conservation of energy. If the engine has a mass, he will get the velocity V in the direction opposite to the gas flow. The number of the motor, equal to the product should be equal to the amount of movement of the combustion products
The use of powder and liquid rocket engines for the helicopter is difficult because of the limited time their actions Mr difficulties throttling. As fuzzy, these engines are developing all the time the same thrust as long as all the fuel will burn.
The liquid rocket engines difficult to regulate the supply of fuel under high pressure, their efficiency is low, and the life is low. Therefore, as a powder and liquid rocket engines can not be used as a motor for rotating the rotor vita.
Ramjet peaktivny engine uses oxygen for combustion 's ambient air and consists of the following main parts: the air intake (the inlet cone), the combustion chamber, the nozzle.
The inlet serves to direct the air flow into the engine. Form input and change in the air intake passage area along the flow are selected such that with minimum hydraulic losses at the inlet to provide a pressure increase in the air on their way to the combustion chamber. To reduce losses at the inlet to the air intake of its front edge is formed as a circular wing profile nose which has a small radius of curvature. To increase the air pressure air intake give the appearance of an expanding channel (diffuser).
Conversion of thermal energy contained in the gas into mechanical work expiration can occur only by gas expansion. Therefore, the air before entering the combustion chamber must be pre-compressed to increase its pressure.
In flight, the air coming to the air intake of the engine at a speed equal to the speed of flight. When hovering helicopter, this speed is the circumferential speed of the blade tip. Before entering the air intake more braked, thereby increasing its pressure, and once in the expanding channel inlet, further reduces its speed, whereby the pressure continues to increase.
Thus, in the ramjet engine the air pressure is increased by the use of the kinetic energy of the incoming air into it. This explains the inability to work ramjet engine in place when the speed of the incoming flow is zero. This also explains the increase in the engine thrust to increase its speed. Helicopter rotor with blades mounted on the ends of the ramjet engine requires so before starting the engine pre-promotion of the external power source.
In the combustion chamber through the fuel injector is continuously fed. When burning fuel heats and expands the air, whereby it increases its speed. The gas exits the nozzle at a rate substantially exceeding the rate of entry. As a result of acceleration of the mass of the gas inside the engine is formed jet thrust.
Ramjet engine can be successfully applied to the helicopter, if the screw to provide preliminary promotion.
Pulsating air peaktivny engine in this respect compares favorably with the continuous-flow, because it can create traction on the ground (without movement of the helicopter), and does not require promotion of the screw.
The pulsating combustion engine does not occur continuously, as in the parallel flow and periodically. Before the combustion chamber of the engine pulsating set grille with flaps. Due to the presence of the difference of air pressure in the air intake valves and the combustion chamber opens and passes into the combustion chamber portion of fresh air. Simultaneously, the combustion fuel is injected and ignited. Heating of the air causes a transient increase in pressure in the combustion chamber, whereby the valves are closed in the lattice. Gases from the combustion chamber at a high velocity flow through a jet nozzle, which causes underpressure
in the combustion chamber, and valves are opened again, letting the cell next portion of fresh air, after which the cycle repeats. Link of the engine is changed from maximum to zero. However, because the frequency of the pulsations is very large, the changes do not affect the thrust almost -on uniform rotation of the rotor. The frequency of the pulsations is inversely proportional to the length of the engine. Thus, if the engine having 610 mm length, running at 270 pulsation cycles per second, the engine having a length 915 mm, - with a frequency 180 cycles per second.
It should be noted that the supply of fuel to the engine at the ends of the blades does not require the use of pumps for the forced feeding. The fact that occurs when rotating rotor centrifugal force drives the fuel itself from the hub to the engines of the fuel-wire, laid along the blade. However, in this case it is difficult to carry out sealing of the movable connection through which fuel from pipeline located on the fixed part of the helicopter is transmitted to the rotating sleeve.
The design of the motor and adjustment of the fuel delivery and ignition timing should be such as to ensure synchronization with pulsation post combustion gases.
Pulsating engine, except what may develop cravings at work at the site, also has the advantage that it is considerably less fuel to create each kilogram traction than other types of jet engines. When selecting a motor to be mounted on the ends of the blades of the helicopter designers often stop "and pulsing the motor also because the engine develops highest thrust value for each unit frontal area.
The main disadvantage of pulsing engine vibratory loads are significant, this explains the small intake valve life (a few hours) and frequent fatigue breakage tail pipe. In addition to disadvantages include the need for compressed air to run (for the initial cycles of work), and finally, the big noise of the engine running.
Turbofan and turboprop engines in the form in which they exist now, at the ends of the blades can not be used. Although these engines and have the lowest specific consumptionfuel per hour per kilogram for each thrust or horsepower, but the proportion of these engines, t. e. the ratio of thrust to weight, still so large that it allows them to effectively use at the ends of the blades. These motors can be applied in a conventional helicopter power plant with mechanical drive to the rotor.
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