Compressors Part 3 - Aircraft Gas Turbine Engines #07

Summary

This script discusses the intricacies of engine compressors, focusing on the relationship between compression ratio, air mass flow, and RPM. It explains the significance of the surge stall line and the normal operating line, which define the compressor's efficiency and safety margins. The discussion covers various blade fixing methods, including the BLISK design, and material choices for rotor and stator blades, influenced by temperature and contamination. It also touches on the impact of contaminants on engine efficiency and different cleaning methods to maintain performance, concluding with a look at abrasive grit injection for deep cleaning.

Takeaways

• 📈 The surge stall line on a compressor graph represents the boundary where operation above it leads to compressor stall and surge.
• 🔍 The normal operating line on the graph connects points of proportionally matched compression ratio and air mass flow at various RPMs, indicating efficient engine operation.
• 🚧 The stall margin zone is the area between the surge stall line and the normal operating line, providing a buffer for efficiency reduction and environmental fluctuations.
• ⚙️ The design point on the normal operating line is the optimum operating point for the compressor, typically aligned with the engine's cruising speed and altitude.
• 🛠️ Compressor blades can be fixed to the rotor disk using methods like dovetail joints, which allow for minor alignment variations and become secure under centrifugal force during operation.
• 🔩 The BLISK (Blade-Integrated Disk) design integrates the rotor blades with the disk, reducing weight and enhancing structural integrity.
• 🌡️ Material choice for compressor components varies with temperature and pressure; aluminum alloys are used in low-pressure areas, while nickel-based alloys are necessary for high-temperature sections.
• 🛡️ Titanium is used for low-pressure rotor blades due to its strength and lightness, but it is not suitable for high-temperature, high-pressure stages.
• 🔄 Contaminants like salt and pollution can reduce compressor and turbine efficiency, necessitating regular cleaning procedures like desalination washes.
• 🧼 Performance recovery washes using emulsion cleaners can restore efficiency in contaminated compressors and turbines, sometimes extending service life.

Q & A

What is the surge stall line on a compressor graph?

The surge stall line on a compressor graph is a line that connects points representing the maximum compression ratio and maximum air mass flow at various RPMs. Operating above this line can lead to compressor stall and surge.

What is the normal operating line in the context of a compressor graph?

The normal operating line is a line on the compressor graph that connects points where the engine's compression ratio and air mass flow are proportionally matched at different RPMs, indicating the engine's normal operating conditions.

What is the purpose of the stall margin zone in a compressor?

The stall margin zone is the area between the surge stall line and the normal operating line in a compressor. It allows for compressor efficiency reduction over time and accommodates fluctuations in atmospheric conditions and engine fuel system management.

What is the design point of a compressor?

The design point is the optimum operating point on the normal operating line, representing a specific compression ratio, compressor speed, and mass flow rate. Ideally, this point corresponds to the engine's cruising speed and altitude.

Why do compressor efficiency decrease over time?

Compressor efficiency decreases over time due to factors such as blade deterioration, contamination, and other wear and tear issues that can cause the surge stall line to move closer to the normal operating line, reducing the stall margin zone.

What is the BLISK in the context of compressor blades?

A BLISK is a rotor blade and disk combination where the blade is integral with the disk, designed to overcome challenges in designing practical compressor blade fixing methods while maintaining minimum rotor disk weight.

Why are materials like nickel-based alloys used in the high-pressure section of a compressor?

Nickel-based alloys are used in the high-pressure section of a compressor because they can withstand the high temperatures of compressed air, which aluminum alloys cannot.

What is the purpose of the shrouding at the inner ends of the stator vanes?

The shrouding at the inner ends of the stator vanes prevents vibration-induced damage that can be caused by the velocity of the airflow over them.

Why are fan blades in high bypass ratio engines often made from titanium?

Fan blades in high bypass ratio engines are made from titanium because it combines strength and lightness, which is essential for withstanding out-of-balance forces that could occur if a fan blade fails.

What is a desalination wash in the context of engine maintenance?

A desalination wash is a procedure where the compressor is rinsed with fresh water to remove salt and other contaminants, typically performed while the engine is motoring over on the starter or running at idle speed.

What is the purpose of a performance recovery wash in an engine?

A performance recovery wash is a cleaning procedure where an emulsion cleaner is sprayed into the engine intake to remove contamination and restore aerodynamic efficiency, improving engine performance.