Wednesday, September 8, 2021

Turbojet engine, Working principle and its parts

An aircraft turbojet engine is basically a gas generator fitted with an inlet and exhaust system. The major principle in all jet engines are the same and they work according to similar concepts as the internal combustion engine. The first part of a turbojet engine is focused on the inlet, where the air is sucked. After that the focus is on the compression of the air, where the inlet air is compressed to a higher pressure.

Then next we have a combustion chamber, where the compressed air is mixed with fuel and then ignited. And finally the outlet of the engine, where the ignited air and fuel-mixture exits at a high velocity. A schematic diagram of a turbojet engine is shown below.
Jeff Dahl, CC BY-SA 4.0, via Wikimedia Commons

The Inlet or Diffuser of a turbojet engine


The main function of an inlet is to diffuse or decelerate the flow, that’s why it is also called a diffuser. Flow deceleration is accompanied by the static pressure rise or in we can say the adverse pressure gradient in fluid dynamics. From the first principle of fluid mechanics, we know that the boundary layers, being of a low-energy and momentum-deficit zone, facing an adverse pressure gradient environment tend to separate.


Therefore, one of the challenges faced by an inlet designer is to prevent “inlet boundary layer separation”. And this can be achieved by changing the geometry of the inlet to avoid rapid diffusion or and it is probable that it can be achieved possibly through variable geometry inlet design.


An ideal inlet is considered to provide a reversible and adiabatic, or in other words isentropic compression of the captured flow to the engine. Isentropic requirement means that there is no heat exchange between the captured stream and the ambient air, through the diffuser walls.

Compressor


In a gas generator the thermodynamic process begins with the compression of air in the compressor. As the compressor discharge contains high energy gases, means the compressed air, therefore it requires external power to operate. And this external power comes from the turbine via a shaft, for the case of an operating gas turbine engine.


Other sources of external power may be used to start the engine, such as electric motor, air turbine, and hydraulic starters. The flow of air in a compressor is considered to be adiabatic process, which means that only a small amount of heat transfer takes place between the air inside and the ambient air outside the engine. Therefore, even in a real compressor analysis, the flow will be treated as adiabatic.


The power delivered in a compressor is achieved by one or more rows of rotating blades known as rotors, attached to one or more spinning shafts, known as spools. Each rotor blade, which changes the spin or swirl, will experience a counter-torque as a reaction to its own action on the fluid.

Burner


In the combustor, the air is mixed with the fuel and a chemical reaction occur which is exothermic, means, it results in a heat release. The ideal burner is considered to behave like a reversible heater, which means very slow burning, with no friction acting on its walls. Under such conditions, the total pressure remains conserved. In a real combustor, due to the wall friction, turbulent mixing and chemical reaction at finite Mach number, the total pressure decreases.

Turbine


The high temperature and pressure gas that leaves the combustor is directed into a turbine. The turbine can be considered as a valve because on one side it has a high-pressure gas and on the other side it has a very low-pressure gas of the exhaust nozzle.


Thus, the flow process in a turbine and exhaust nozzle involves significant pressure drop and, and along with it, the static temperature also drops, which is called flow expansion. The flow expansion produces the necessary power for the compressor and the propulsive power for the aircraft. The turbine is connected to the compressor via a common shaft, which provides the shaft power to the compressor.


Basically, we can think of the expansion process in a gas turbine engine as the counterpart of power stroke in a regular combustion engine. However, in a turbine, the power transmittal is continuous. Due to high temperatures of the combustor exit flow, the first few stages of the turbine, means, the high-pressure turbine, is needed to be cool.

The coolant that is used is air from the compressor, which may be extracted from different compression stages, for example, between the low- and high-pressure compressor and at the compressor exit.


A cooling solution, which uses the engine cryogenic fuel, such as hydrogen or methane, as the coolant to cool the engine and aircraft components is called regenerative cooling and has proven its effectiveness in liquid propellant chemical rocket engines from last few decades. Coolant is typically injected from the blade attachment into the blade, which provides internal convective cooling and usually external film cooling on the blades.

Nozzle of turbojet engine


The primary function of an aircraft engine exhaust system or the nozzle is to accelerate the gas efficiently. The nozzle parameter that is of most importance in propulsion is the gross thrust T. The expression for the net thrust can be written as

T = ̇ ma × Vo + (p − po) × Ae


In the above equation, the first term on the right-hand side is called the momentum thrust and the second term is called the pressure thrust.

As the fluid accelerates in a nozzle, the static pressure drops and a favorable pressure gradient environment is produced in the nozzle. This is in contrast to diffuser lows where an adverse pressure gradient environment induces. Therefore, boundary layers are well behaved in the nozzle and less complex to treat than the inlet.


For a subsonic exit Mach number, the nozzle expansion process will continue all the way to the ambient pressure. This means that in subsonic streams, the static pressure inside and outside of the jet are the same. There is no mechanism for a pressure jump in a subsonic flow, which is in contrast to the supersonic flows where shock waves allow for static pressure discontinuity.


Talking about the parts of a nozzle, the outer shape of the nozzle is known as a boat-tail, which affects the installed performance of the exhaust system and the external aerodynamics of the nozzle installation belongs to the propulsion system integration studies.

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