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Jet engines are the engines that derive its propulsive power by the propeller/fan driven by a turbine or by the reacting force produced from the acceleration of gas or liquid through a nozzle or both. The important points that contribute to the propulsion of jet engines are potential energy, kinetic energy and Bernoulli’s theorem. So let us look into these points.
To understand better we have to first look into the Bernoulli’s theorem. Bernoulli’s theorem deals with pressure of gases (potential and kinetic energy). Pressure can be changed by adding or removing heat, changing the number of molecules present or changing the volume in which the gas is contained. Bernoulli’s theorem deals in changing the volume in which the gas is contained and states that when a fluid or gas (at subsonic speed) is supplied at a constant flow rate through a duct, the sum of pressure (potential energy) and velocity (kinetic energy) is constant. That is if potential energy (static pressure) increases than kinetic energy (ram / dynamic pressure) decreases and vice versa. Here pressure, potential energy and static pressure are the same. Also on the other hand velocity, kinetic energy, ram pressure and dynamic pressure are the same. Now let us look at different types of jet engines.
1. Rocket:- is a non air breathing engine i.e. to say it does not require atmospheric air to support combustion but carries its own oxidiser and fuel in liquid or solid form. Combustion of small volume of liquid or solid creates large volume of gases which escape through the exhaust nozzle at extremely high velocity to provide the required thrust. Based on fuel used rockets are classified into the following.
1.1. Solid fuel rocket
1.2. Liquid fuel rocket
2. Athodyd:- aero thermodynamic duct or ramjet is the simplest of all jet engines. It is a duct with very few components, it receive inlet air and convert it to static pressure, add fuel to it and the resultant combustion, expansion and exhaust of gases provide the thrust.
3. Scram jet:- supersonic combustion ramjet is a turbojet engine at low speed and a ramjet engine at high speed, with the airflow reaching supersonic speed during combustion.
4. Pulse jet:- it is similar to ramjet but it has a system of air inlet flapper valves. The back pressure created by combustion closes the flapper valves and creates a pressure which escape through the exhaust nozzle, opening the flapper valve (due to lack of back pressure) and thus continuing the cycle.
5. Gas turbine:- air is the working fluid of gas turbine engine. The air is compressed in the compressor, combustion of air-fuel mixture take place in combustion chamber and finally the turbine extract the energy from the flowing gas to drive the compressor and the accessories, with the remaining gas escaping the exhaust nozzle giving the reactive thrust. Based on how the energy extracted by the turbine is used, gas turbine engine is classified into the following.
5.1. Turboshaft engine- the energy extracted by the turbine is used to drive a shaft (e.g. to drive the shaft of main rotor of helicopter)
5.2. Turboprop engine- the energy extracted by the turbine is used to drive the propellers (mounted in front of the engine) which provide additional thrust.
5.3. Turbofan engine- the energy extracted by the turbine is used to drive the fans (mounted in front of the compressor but unlike propellers, fans are completely ducted or covered by the engine inlet)
5.4. Turbojet engine- here the energy extracted by the turbine is used for driving the compressor only(all gas turbine engine use extracted energy for driving the compressor) and is purely a reactive thrust engine.
Jet engines have transformed the way we travel especially because of its excessive use in aircrafts (although it is used in other systems as well). Air travel has made global connection possible and this contributed to the development of the world.
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