Gas Turbine Engines: The Heart of Aviation

Gas Turbine Engines: The Heart of Aviation Explained

Since 2020, Kalkan Endüstri has been active in the defence industry, robotic technologies, automation, and machinery and equipment supply.

In our series of articles on gas turbine engines, we will discuss the working principle of gas turbine engines, their sections and the processes that take place in them. I believe it will be useful to start from the basics in order to better understand this article and those I will write in the future. Let us begin with what gas turbine engines are and where they are used.

Among the aircraft engines that have been so important throughout the history of aviation, the foundations of the gas turbine engine were laid by Frank Whittle when he obtained his patent in 1930. However, the real advances began after the Second World War, once the Germans had also become involved. There is no difference in working logic between the engine in Figure 1.1 and the engine in Figure 1.2. The general working principle of the engine is to convert heat energy into mechanical energy. The most important law of physics used in gas turbine engines is the action–reaction principle, Newton’s Third Law of Motion. In gas turbines operating on the Brayton cycle, air is compressed and burned with fuel, and the cycle ends when the very high energy produced is converted into mechanical power by means of the turbines. Gas turbine engines can be divided into 4 basic groups according to their purpose: Turbojet, Turboprop, Turbofan and Turboshaft.

1. Turbojet Engines

Turbojet engines expel the high-energy gas they produce rearwards with great force. They are generally used in the military field. They are very noisy and efficient at high speeds. The best examples of turbojet engines are the KTJ-3200, produced by Türkiye’s KALE-ARGE, and the TJ-90, produced by TEI.

2. Turboprop Engines

All engines other than the turbojet are derivatives of the turbojet. Put simply, the only difference in turboprop engines is that they include a propeller and a gearbox. In other words, the energy obtained from the turbine is transferred to the propeller via the gearbox and shaft, generating the required thrust. In the image, you can see the PT6A-68T turboprop engine used in the Hürkuş Basic Trainer Aircraft.

3. Turbofan Engines

Turbofan engines, generally used in passenger aircraft, also have a fan at the front to generate additional thrust. We can say that a turbofan is made simply by placing a fan in front of a turbojet engine. Turbofan engines are also known as bypass engines. In Figure 1.3 you can see the F135, the engine of the F-35 fighter aircraft.

4. Turboshaft Engines

Used primarily as a helicopter engine, the turboshaft engine differs from the turbojet in having a transmission-like power transfer system that turns the rotor blades. Through the transmission, it turns the rotor blades using the mechanical energy taken from the turbine. It offers a very good thrust-to-weight ratio compared with piston engines. The T129 ATAK helicopter, produced by TAI and recently added to the inventory of the Turkish Armed Forces, uses the CTS800 turboshaft engine from LHTEC, a Honeywell and Rolls-Royce partnership.

Gas turbine engines generally consist of five sections: air intake, compressor, combustion chamber, turbine and exhaust (nozzle). In my later articles I will go into specific sections such as compressor, combustion chamber, turbine and exhaust design. For now, therefore, let us give some simple definitions to help understand the subject.

Air Intake:

This is the part where air enters the engine. Although it is very simple, its design is very important to ensure that the air is not choked at the front of the engine and that there is no shortage of air.

Compressor:

The compressor compresses the air taken from the air intake by sending it first to the low-pressure compressor (LPC) and then to the high-pressure compressor (HPC). Compressors have a rotor and a stator. A rotor–stator pair is called a stage. The air is compressed further at each compressor stage. The compressor blades become smaller from the LPC towards the HPC.

Combustion Chamber:

The air arriving at the combustion chamber with very high energy is burned together with fuel. The burning air moves along the engine and passes through the turbine. As it passes through the turbine, it gives up part of its energy there. Turbojet engines generally operate at a fuel-to-air ratio of 1:40; in other words, 40 units of air are needed to burn 1 unit of fuel. Kerosene (JP-8) is mostly used as fuel.

Turbine:

The working principle of the turbine is in fact the reverse of the compressor. Whereas the compressor uses work to increase pressure, the turbine converts high-energy air into work, that is, into mechanical energy. Turbine design is similar to compressor design, and turbines also carry blades. The turbine uses the mechanical energy it receives to drive the compressor by means of a central shaft.

Exhaust:

The burned, high-energy air generates thrust as it leaves through the exhaust. Newton’s Third Law, which we mentioned at the beginning, applies here. The air leaving rearwards with very great force provides thrust to the aircraft as a reaction.

Working Principles:

A gas turbine engine is in fact a heat engine that uses air as its working fluid to provide thrust. To provide this thrust, the air passing through the engine must be accelerated; in other words, the velocity or kinetic energy of the air must be increased. The working principle of gas turbine engines is similar to that of four-stroke piston engines. In terms of the laws of thermodynamics, combustion takes place at constant pressure in gas turbine engines and at constant volume in piston engines. The thermodynamic working principle of gas turbine engines is shown in Figure 2.2 (P-V diagram of the Brayton cycle).

Looking at the cycle on the pressure–volume diagram, point A represents the atmosphere. Along line AB, the pressure of the compressed air increases. Heat is added to the air between B and C by burning it at constant pressure, so the volume of the air increases. The reason we describe combustion in the combustion chamber as taking place at constant pressure is that we assume ideal conditions. To obtain the temperatures, pressures and volumes in real conditions, we need to know the values under ideal conditions. For ease of explanation, therefore, we assume ideal conditions and consider that there is no pressure loss. In real conditions there will always be some pressure loss during combustion. Returning to the diagram, the gases produced by combustion expand between C and D, starting from the turbine. Meanwhile, as part of the cycle, some of the energy in the gases is converted into mechanical power by the turbine. The remaining gases are discharged to the atmosphere, providing propulsive thrust. Since gas turbine engines are heat engines, the higher the combustion temperature, the greater the power produced as the gases expand in the turbine. However, the limiting factors here are the materials used in the design of the combustion chamber and turbine and the mechanical properties of the system. The combustion temperature must not exceed these limits.

Pressure–Temperature–Volume Relationship

During the operation of a turbine engine, the airflow or working fluid (air) absorbs and releases heat. As a result, changes occur in temperature, volume and pressure. These changes are closely related to Charles’s and Boyle’s laws, which we know well from chemistry lessons. There are 3 main conditions under which these changes occur in the engine’s operating cycle:

1. Compression: When compression by the compressors is complete, the pressure of the air has risen and its volume has decreased. The rate at which pressure and volume rise and fall is equivalent to the rate at which its temperature rises.
2. Combustion: After fuel is added to the air and burned, the temperature rises. This rise is equivalent to an increase in volume. We must remember that the pressure remains almost constant during this phase.
3. Expansion: Since work is extracted from part of the gas flow by the turbine during expansion, there will be a drop in temperature and pressure. This drop is equivalent to the increase in volume.
   To interpret these principles (pressure–volume and temperature) on a diagram, refer to the P-V and T-S diagrams of the Brayton cycle.

Efficiency is also very important in turbine engines. It is very important that the compressor pressure is raised using a given amount of work and, conversely, that pressure is reduced in the turbine to produce a given amount of work. In every machine, we try to produce maximum power using minimum power. Turbine engines work in the same way. If we obtain 100% efficiency when air is compressed and expanded, this process is called an adiabatic process. In an adiabatic process, we disregard phenomena that would cause losses, such as friction, convection and turbulence, and consider it as a state in which there are no energy losses. Of course, it is known that such a process does not exist in reality. However, in order to obtain maximum efficiency from the compressor, combustion chamber and turbine, we must consider the ideal case. In engineering companies that manufacture aircraft engines, an efficiency of 90% or above is considered successful.

Velocity and Pressure Distribution

The velocity and pressure of the airflow along the engine vary according to aerodynamic and energy requirements. For example, during compression in the compressor, pressure increases but no increase in velocity is needed. After the air has been heated and its internal energy increased by combustion, the gas needs velocity because force is required to turn the turbine. At the exhaust outlet, a high exit velocity is required.

It is the change in momentum that provides thrust in the air. In some regions, we want the air velocity to be greatly reduced. The best example of this is the flame burning in the combustion chamber. The region we call the primary zone is one where we want low velocity. We will cover this in more detail when we discuss the combustion chamber and the combustion process.

To achieve these changes in velocity, attention is paid to the size and shape of the ducts through which the air passes inside the engine. If kinetic energy is to be converted into pressure, a divergent duct is used; if we want to convert the energy stored in the combustion gases into kinetic energy, we use a convergent duct. The design of these passages and nozzles is very important, because it is crucial for energy efficiency.

Airflow

Before discussing the airflow in the engine, I would like to define the term bypass, which we will mention frequently in future articles. To define bypass, you can see a cross-section of the F119-PW-100 engine below. In bypass engines, the air splits into 2 paths at the fan exit. Part of it continues through the compressor, combustion chamber, turbine and exhaust (which may also be called the core engine), while the rest continues between the compressor casing and the outer casing. The ratio of the amount of air passing around the outside to the amount of air passing through the core engine is called the bypass ratio. We can also refer to these as dry thrust and wet thrust. If the bypass ratio is between 0.2 and 1, we can call it a low-bypass engine; if it is 5 or above, a high-bypass engine.

Oğuz Kılıç

Source: The Jet Engine | Rolls-Royce