02 ATPL Training Gas Turbine Engines #02 Introduction Part 2

Summary

This script explains the operation of gas turbine engines, focusing on their combustion process, the laws of gases, and how efficiency is achieved through compression, combustion, and expansion phases. It contrasts the gas turbine with the piston engine, highlighting how the constant pressure combustion process and the use of low octane fuels reduce construction weight. Key principles like Boyle's Law and Charles' Law are discussed to explain how temperature, pressure, and volume interact within the engine, emphasizing the role of modern materials and cooling methods in improving engine efficiency and performance.

Takeaways

• 😀 Combustion in a gas turbine engine occurs at constant pressure, ensuring stable engine performance unlike in piston engines, where high pressure requires heavy construction.
• 😀 Gas turbine engines can use low-octane fuels and lighter construction materials due to constant pressure combustion, contrasting with piston engines that require high-octane fuels and stronger materials.
• 😀 The efficiency of a turbojet engine increases with higher combustion temperatures, but this is limited by the materials used in the turbine, nozzle guide veins, and blades.
• 😀 Early German gas turbine engines failed mainly due to the inability to produce materials that could withstand high temperatures, leading to catastrophic engine disintegration after only 10-20 hours of use.
• 😀 Modern turbine engines have higher thermal efficiency due to advanced materials and efficient cooling methods that allow higher gas temperatures to be used safely.
• 😀 Gases have neither fixed volume nor shape, and their behavior is studied using three variables: volume, pressure, and temperature.
• 😀 Boyle's Law states that the volume of a gas at constant temperature is inversely proportional to its pressure (pressure × volume = constant).
• 😀 Charles’ Law states that the volume of a gas at constant pressure is directly proportional to its absolute temperature (volume/temperature = constant).
• 😀 The Combined Gas Law integrates Boyle’s and Charles’ laws to show the relationship between pressure, volume, and temperature, helpful in understanding the behavior of gases in a gas turbine engine.
• 😀 In a gas turbine engine, the main stages of gas behavior are compression, combustion, and expansion, during which pressure, temperature, and volume continuously change.
• 😀 The turbine is most efficient at converting the kinetic energy in the gas stream into mechanical energy, which drives the compressor. Nozzle guide veins shape and control this energy conversion.
• 😀 The gas stream’s velocity is reduced during compression and combustion to ensure efficient combustion and prevent the extinguishment of the flame, while it increases during expansion to convert energy into mechanical work.

Q & A

What is the primary difference between combustion in a gas turbine engine and a piston engine?

In a gas turbine engine, combustion occurs at a constant pressure, achieved through the continuous process of the Brayton cycle. In contrast, a piston engine experiences fluctuating pressure, with peak pressure values exceeding 1,000 pounds per square inch, requiring stronger and heavier construction.

Why can gas turbine engines use lower octane fuels compared to piston engines?

Gas turbine engines can use lower octane fuels because they operate at a constant pressure, unlike piston engines, which require high octane fuels to avoid detonation due to fluctuating pressures.

What is the relationship between heat and efficiency in a turbojet engine?

The greater the temperature retained in combustion, the greater the expansion of gases, which leads to higher efficiency in a turbojet engine. However, there is a limit to the heat that can be released into the turbine, determined by the materials used in the engine.

What were the main problems with early German gas turbine engines?

The main problem with early German gas turbine engines was the inability to produce materials capable of withstanding the high temperatures of the gas stream. This resulted in turbine blade meltdowns and catastrophic engine disintegration after only 10 to 20 hours of use.

How did modern materials improve the performance of gas turbine engines?

The use of modern materials and efficient cooling methods in the nozzle guide veins and turbine blades has allowed for higher gas temperatures in modern engines, resulting in improved thermal efficiency and performance.

What are the three main variables used to measure gases?

The three main variables used to measure gases are volume, pressure, and temperature.

What does Boyle's Law state about the relationship between gas pressure and volume?

Boyle's Law states that, for a gas held at constant temperature, its volume is inversely proportional to its pressure. This means that as the pressure increases, the volume decreases, and vice versa, as long as the temperature remains constant.

What is Charles' Law and how does it relate to gas behavior?

Charles' Law states that if a gas is held at constant pressure, its volume is directly proportional to its absolute temperature. This means that as the temperature of a gas increases, its volume also increases, provided the pressure remains constant.

What is the Combined Gas Law and how is it applied in a gas turbine engine?

The Combined Gas Law integrates Boyle's and Charles' Laws, relating pressure, volume, and temperature in a single equation. In the context of a gas turbine engine, it helps determine the behavior of gases during compression, combustion, and expansion phases, where all three variables change constantly.

How does the compression stage in a gas turbine engine affect the air?

During compression, work is done to increase the pressure and decrease the volume of the air, leading to a rise in its temperature. Higher compression ratios improve thermal efficiency and reduce specific fuel consumption.

What role do nozzle guide veins and turbine rotors play in converting energy in a gas turbine engine?

Nozzle guide veins convert pressure and heat energy into kinetic energy by shaping the gas flow into convergent ducts. Turbine rotors then convert this kinetic energy, along with some remaining pressure and heat energy, into mechanical energy to drive the compressor.