What are jet engine and scramjet engine ?

What are jet engine and scramjet engine ?

Asked on 7th September 2021 in Propulsion.
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Jet engines: Jet engines are used to propel the aircraft forward, with huge amount of thrust. A jet engine consists of fan/ Diffuser, compressor, combustor, turbine, nozzle.

Image displaying jet engine partsJet engine

Diffuser: Diffuser which is present in a turbojet engine is deaccelerating the flow at inlet. In a turbofan engine a fan is present at inlet to accelerate a large mass of air. Thrust produced by a turbofan engine is a sum of thrust produced by fan blades and jet from the nozzle.

Compressor: Compressor compresses the inflow air which results in an increase in air pressure. A compressor is made up of fans with many blades which is attached to a shaft. The high pressure squeezed air is passed into the combustion chamber.

Combustor: Compressed air from compressor is passed into the combustion chamber where fuel is sprayed from the nozzle. The mixture of air and fuel burns and produce a high temperature and high-energy airflow. There is a expansion of the gas.

Turbine: Airflow from combustor enters the turbine, which causes the blades of turbine to rotate. Turbines are on shaft connecting to compressor and fan at the inlet, which turns the blades of the compressor and spin the fan.

Nozzle: Airflow after passing from turbine exhausts from the nozzle producing the thrust. The high-energy of the air-flow is lost after passing the turbine and and there is also addition of cooler air which is bypassed in the engine core, this leads to generate a force when exiting the nozzle.

The Jet engines has made particular increase in aircraft speed in the recent years. Jet engines has made supersonic flights possible. The invention and development of jet engine provided the thrust necessary for everyday supersonic flights. Jet engines are used by almost all large commercial transports and military combat aircrafts at present.

The power available from jet engine is given as product of thrust available and aircraft velocity. Thrust available for the jet engine is nearly constant with the velocity. Therefore, power available also varies linearly with velocity.

The fuel which is consumed by a jet engine physically depends on the thrust produced by the engine. When an airplane is flying at minimum thrust required, the jet airplane has a maximum endurance.

At maximum endurance, jet airplane is flying at a velocity where \(C_{L}/C_{D}\) is maximum and maximum range for jet airplane occurs when it is flying at a velocity where \(C_{L}^{1/2}/C_{D}\) is maximum. Here, \(C_{L}\) is the coefficient of lift and \(C_{D}\) is coefficient of drag.

Range of jet airplane also depends on density of atmosphere. Range is increased if density is low. But at extremely high altitudes, ordinary turbojet engine has a decrease in performance. Range of a jet engine is lowest at sea level and it increases with altitude up to a point.

The cruising altitudes for a subsonic commercial jet transport aircraft is \(30,000\) to \(40,000\, ft\) and for a supersonic jet transport aircraft it is \(50,000\) to \(60,000\, ft\).

So, jet engine is a device which takes in air at a freestream velocity at inlet, there is combustion with fuel inside the duct and then blasts of hot mixture of air and combustion products at the backend at a very large velocity.

So, jet engine creates a change in the momentum of the gas by increasing the velocity of the small mass of intake air.

Ramjet engine: A ramjet engine does not have any rotating machinery like fans or propellers like in a turbojet. It is simply a straight through duct. In a ramjet engine air at a high velocity is inducted through the inlet, which is decelerated in the diffuser and after that burned by the injecting fuel, which later blast out the exhaust nozzle at a very high velocity. Ramjet engine is the best choice for future hypersonic vehicles.

Image displaying ramjet engine partsRamjet engine

At a high Mach number combustion temperature for a turbojet engine is increased. But there is a material limitation for the turbine blades as at a high temperature, it will melt. This high temperature material properties limit the turbojet engine to lower to moderate supersonic velocities.

However, a ramjet does not have any moving parts and turbines, so there is no high temperature material limitation.

There can be a higher combustion chamber temperature and therefore a higher supersonic flight velocities. A ramjet engine can have a Mach number above \(3\) or \(4\).

At a higher Mach number like above Mach \(6\), there is a large increase in temperature that will lead to fail ramjet structurally. Also, if the temperature of air entering the combustion chamber is high, it will decompose rather burn, when fuel is injected in the combustion chamber.

This has led to supersonic combustion ramjet which is called scramjet. In scramjet the diffuser, decelerates the airflow but flow is still supersonic when entering the combustor. Fuel is injected in the combustion chamber where supersonic combustion takes place. Here, static temperature is low, and the material limitation and decomposition of air problem evade.

Image displaying scramjet engine partsScramjet engine

So, a scramjet engine can make the hypersonic flight possible. Scramjet powered flights are envisioned for Mach numbers of at least \(15\).

In order to start and run a ramjet engine, it has to be in motion. A ramjet engine must have enough flight speed to start the engine, and to achieve this a ramjet-powered vehicle is launched with rockets or a second engine of different type to push it to enough flight speed.

In a hypersonic vehicle having a scramjet engine the airflow is passing through one or more shock-wave systems from the leading portion of the vehicle. Therefore, it is required to tailor the aerodynamics of this inflow air to have the most efficient engine performance.

Image displaying scramjet engineAirframe-integrated scramjet

Answered on 29th September 2021.
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