3.2 Compressor and turbine design

Gas turbine engines rely on compressors and turbines to generate power. Compressors increase air pressure before combustion, while turbines extract energy from hot gases. These components work together to create thrust or mechanical power in aircraft and power plants.

Compressor and turbine design is crucial for engine efficiency and performance. Factors like blade geometry, material selection, and flow management are carefully optimized. Understanding these components helps engineers improve gas turbine engines for various applications.

Axial vs Centrifugal Compressors

Axial Compressor Design and Characteristics

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• Axial compressors consist of multiple stages of rotating blades (rotors) and stationary vanes (stators) that gradually increase the pressure and decrease the volume of the working fluid
• Axial compressors are more efficient and have a higher pressure ratio per stage compared to centrifugal compressors, but require more stages for the same overall pressure ratio (multi-stage axial compressor, single-stage centrifugal compressor)
• The pressure ratio of a compressor is the ratio of the outlet pressure to the inlet pressure and is a key parameter in determining the performance and efficiency of the compressor
• Compressor efficiency is affected by factors such as blade design, tip clearance, and flow separation

Centrifugal Compressor Design and Characteristics

• Centrifugal compressors use an impeller to accelerate the fluid outwards, converting kinetic energy into pressure energy in the diffuser
• Centrifugal compressors are more robust, have a wider operating range, and are better suited for smaller gas turbine engines (auxiliary power units, helicopter engines)
• The impeller in a centrifugal compressor typically has backward-curved blades to efficiently convert kinetic energy into pressure energy
• Centrifugal compressors often have a single stage, which simplifies the design and reduces the overall length of the compressor section

Compressor Stage Design Considerations

Blade Geometry and Airfoil Design

• Compressor blades are designed with airfoil shapes to efficiently turn the flow and increase the pressure
• The blade angle of attack, camber, and thickness are optimized for the specific operating conditions and desired pressure ratio
• Blade tip clearance is minimized to reduce leakage and improve efficiency, but sufficient clearance is necessary to avoid rubbing during operation (abradable coatings, active clearance control)
• Compressor blades are subject to high centrifugal stresses and require careful material selection and design to ensure structural integrity (titanium alloys, nickel-based superalloys)

Stator-Rotor Interaction and Flow Management

• Stators are used to redirect the flow and prepare it for the next rotor stage, minimizing swirl and improving efficiency
• The spacing between the rotor and stator stages affects the flow characteristics and the overall performance of the compressor
• The use of variable stator vanes allows for optimized performance over a wider range of operating conditions (part-load operation, transient response)
• Flow separation and stall can occur if the blade angle of attack is too high or the flow velocity is too low, leading to reduced performance and potential damage

Turbine Stage Function and Design

Turbine Blade Design and Materials

• Turbine blades are designed to withstand high temperatures and stresses while efficiently converting the kinetic energy of the gas flow into mechanical energy
• Turbine blade materials, such as nickel-based superalloys and ceramic matrix composites, are selected for their high-temperature strength and durability (single crystal blades, thermal barrier coatings)
• Cooling techniques, such as internal cooling passages and film cooling, are used to protect turbine blades from the extreme temperatures of the gas flow
• The blade geometry, including the airfoil shape, twist, and taper, is optimized for the specific operating conditions and desired power output

Turbine Stage Configuration and Energy Extraction

• Turbine stages extract energy from the high-temperature, high-pressure gas flow to drive the compressor and provide useful work
• Turbine stages are designed to maximize the pressure drop across each stage while minimizing losses due to factors such as flow separation and tip leakage
• The turbine nozzle guide vanes (NGVs) direct the flow onto the turbine blades at the optimal angle for maximum efficiency
• Multiple turbine stages are used to efficiently extract energy from the gas flow, with each stage operating at progressively lower pressure and temperature (high-pressure turbine, low-pressure turbine)

Compressor and Turbine Performance Analysis

Performance Maps and Operating Envelopes

• Compressor and turbine performance maps are used to characterize the efficiency and pressure ratio over a range of operating conditions, such as rotational speed and mass flow rate
• The operating envelope of a compressor or turbine is limited by factors such as surge, stall, and choking
• Surge and stall are flow instabilities that can occur in compressors when the flow separates from the blade surface, leading to a sudden drop in pressure ratio and efficiency
• The surge margin is the distance between the operating point and the surge line on the compressor map, and it represents the safety margin for stable operation

Off-Design Performance and Cycle Analysis

• Off-design performance of compressors and turbines is affected by factors such as changes in inlet conditions, rotational speed, and blade degradation (fouling, erosion)
• Choking occurs when the flow reaches sonic velocity at some point in the compressor or turbine, limiting the maximum mass flow rate
• The overall efficiency of a gas turbine engine is determined by the combined performance of the compressor, combustor, and turbine components
• Cycle analysis techniques, such as the Brayton cycle, are used to evaluate the thermodynamic performance and efficiency of gas turbine engines under various operating conditions (design point, off-design)