What is jet engine: -
A jet engine is a type of reaction engine that discharges a fast-moving jet of heated gas (usually air) that generates thrust by jet propulsion. Jet engine is popular due to its compact design, impressive power-to-weight ratio, and extraordinary speed capabilities. The fan draws in a large volume of air, which is then efficiently compressed by a series of blades attached to a rotating shaft. This process increases both the air's pressure and temperature, setting the stage for the subsequent power generation. The compressed air is then mixed with fuel in the combustion chamber, where the mixture is ignited by an electric spark. The ensuing combustion takes place at a constant pressure, resulting in the rapid release of high-temperature gases. These gases flow into a turbine, a critical component connected to the same shaft as the compressor. As the gases expand through the turbine, they transfer their energy to the shaft, which drives not only the compressor but also various auxiliary equipment, such as generators and hydraulic pumps.
Fig :- Basic components of turbo-jet engine
All the gas turbine engines operate in the ideal thermodynamic cycle known as the brayton cycle. However the jet engines operate in a jet propulsion cycle in real life. The jet propulsion cycle differs from the ideal Brayton cycle primarily in that it involves a continuous flow of air and fuel, while the ideal Brayton cycle is a theoretical, idealized model for a closed-loop gas turbine.
In a jet propulsion cycle, such as in a jet engine air continuously enters the engine, is compressed, mixed with fuel, ignited, and expelled as a high-speed exhaust jet. This process occurs continuously during operation.
In contrast, the ideal Brayton cycle is a simplified thermodynamic model that assumes a closed-loop system where the working fluid (usually air) undergoes a series of processes in a closed system. It doesn't consider the continuous inflow of air and exhaust as seen in actual jet engines. It is a theoretical concept used for analysis and doesn't account for the dynamic, open-system nature of jet engines.
The main components of jet engine are discussed below:-
Compressor
In the engine core, the compressor is the first part. The compressor is mounted on a shaft and is composed of fans with several blades. The compressor increases the air pressure by forcing the air that enters it into progressively smaller spaces. As a result, the energy potential of the air increases. The compressed air is driven into the combustion chamber.
Combustor
A combustion chamber is a vital component of a jet engine where combustion takes place. It is also known as a burner or flame holder. It is located in between the compressor and turbine. Air and fuel are combined in the combustor before being ignited. Numbers of nozzles are used to spray gasoline into the airstream. Air and fuel combined ignite and catch fire. This creates an airflow with a high temperature and energy level. Compressed air oxygen and the fuel combine to burn, creating hot, expanding gasses. To create a heat-resistant chamber, ceramic materials are frequently used to line the interior of the combustor.
Turbine
The turbine section consists of rotating blades powered by high-pressure air exiting the combustor. These blades capture the fast-moving airflow, causing them to rotate. This rotation, in turn, drives a central shaft responsible for spinning the engine's fan and compressor located at the front of the engine.
Nozzle
The nozzle is the exhaust duct of the engine. This is the portion of the engine that really generates the plane's thrust. The nozzle accelerates the gases to an extremely high velocity, converting their thermal energy into kinetic energy. The resulting high-velocity jet of exhaust gases is expelled from the rear of the engine, propelling the aircraft forward in a dramatic display of Newton's third law of motion.
This project aims to study and analyze the combustion chamber. There are mainly three types of combustion chamber Can-combustor, second is Can-annular combustor and third type is annular combustor. In this project the annular combustor chamber is selected for study and analysis. Annular combustion chambers are used in modern aircraft engines primarily due to their superior combustion efficiency, better flame stability, and reduced emissions compared to other types of combustion chambers.
In jet engines, the combustion chambers are high compression systems where high pressure air and fuel are mixed and burned at a constant pressure. The combustion process is quite crucial and it directly affects the operational efficiency of the jet engine system. The chamber is lined with the combustor liner, which is subjected to extremely rapid temperature rise and drop after loss of pressure.
Combustion Requirements?
Ensure high combustion efficiency for complete fuel utilization and reduced emissions.
Achieve reliable ignition, even in extreme conditions and after high-altitude flameouts.
Maintain a stable flame across varying pressures and air/fuel ratios.
Minimize pressure loss within the combustor for efficient energy conversion.
Tailor outlet temperature distribution to maximize turbine component lifespan.
Prioritize low emissions of smoke and pollutants.
Eliminate pressure fluctuations and combustion-induced instability.
Design for cost-effectiveness, manufacturability, and ease of maintenance.
Ensure compatibility with diverse fuel sources, including petroleum, synthetics, and biomass-based options.
Why is the annular combustion chamber used for turbo jet engines?
A concentrically placed annular liner is housed inside an annular casing in an annular combustion chamber. It has various advantages over other types of chambers, including a small unit with lower pressure loss than those produced by other combustor types because of its clear aerodynamic layout. The annular configuration was well-established by the 1960s as the standard option for all new aviation engines. The GE CF6, P&W JT9D, and RR RB211 engines were equipped with the most efficient annular combustors from this time period into the 1980s. The technological and financial success of each of these engines was exceptional. The annular combustors used in all three engines represent the most recent developments in fuel injection and wall-cooling methods.
The Brayton Cycle which is air standard cycle for gas turbine engines is composed of four internally reversible processes:
1-2 Isentropic compression (in a compressor)
2-3 Constant-pressure heat addition
3-4 Isentropic expansion (in a turbine)
4-1 Constant-pressure heat rejection
It is ideal cycle for all gas turbine engines
Ideal turbo jet engine:-
Gas-turbine engines are widely used to power aircraft because they are light and compact and have a high power-to-weight ratio.
Aircraft gas turbines operate on an open cycle called a jet-propulsion cycle. The ideal jet-propulsion cycle differs from the simple ideal Brayton cycle in that the gases are not expanded to the ambient pressure in the turbine. Instead, they are expanded to a pressure such that the power produced by the turbine is just sufficient to drive the compressor and the auxiliary equipment.
The net work output of a jet-propulsion cycle is zero. The gases that exit the turbine at a relatively high pressure are subsequently accelerated in a nozzle to provide the thrust to propel the aircraft.
Aircraft are propelled by accelerating a fluid in the opposite direction to motion. This is accomplished by either slightly accelerating a large mass of fluid (propeller-driven engine) or greatly accelerating a small mass of fluid (jet or turbojet engine) or both (turboprop engine).
Fig Basic components of a turbojet engine and the T-s diagram for the ideal turbojet cycle.
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