JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech III AIRBREATHING PROPULSION Previous Question Paper (October - 2020 - R16)

The performance and design of a gas turbine are constrained by thermodynamic, material, and aerodynamic limits that dictate the operating envelope and efficiency of the engine.

Limiting Factors:

1. Turbine Inlet Temperature (TIT): The primary limit is the metallurgical capability of the turbine blades. High temperatures increase thermal efficiency but lead to creep, oxidation, and fatigue. Advanced cooling techniques and thermal barrier coatings are required to exceed these limits.
2. Compressor Pressure Ratio: Increasing the pressure ratio improves thermal efficiency but raises compressor discharge temperature, which can approach the metallurgical limits of the downstream components and reduce the cooling effectiveness of the air.
3. Aerodynamic Loading: Blade loading is limited by the onset of flow separation, shock wave interactions in transonic stages, and boundary layer growth, which contribute to aerodynamic losses and reduced stage efficiency.
4. Rotational Speed and Stress: Centrifugal stresses on rotating components (disks and blades) limit the maximum rotational speed, which in turn limits the mass flow rate and power density.

Design Configuration:

Gas turbine configurations are selected based on the mission requirements. A single-spool configuration is simple and lightweight but lacks flexibility. Multi-spool configurations (e.g., dual or triple spool) allow different sections of the compressor and turbine to operate at their optimal rotational speeds, significantly improving part-load efficiency and surge margins. The choice between axial-flow and centrifugal-flow compressors depends on the required mass flow and diameter constraints; axial designs are preferred for high-thrust applications, while centrifugal designs offer robustness and high pressure ratios per stage for smaller engines.

Overall Performance:

Overall performance is characterized by thermal efficiency and propulsive efficiency. Thermal efficiency is governed by the Brayton cycle parameters, primarily the pressure ratio and the turbine inlet temperature. Propulsive efficiency is determined by the exhaust velocity relative to the flight velocity, where high bypass ratios are employed in modern turbofans to increase mass flow and reduce jet velocity, thereby improving propulsive efficiency. The combined effect is expressed as Overall Efficiency (η_o), which is the product of thermal and propulsive efficiencies. Modern design optimization seeks to balance these factors to minimize specific fuel consumption (SFC) while maintaining high power-to-weight ratios.

The selection of a turbine blade profile is a critical design process aimed at balancing aerodynamic efficiency, structural integrity, and thermal management. The blade profile is typically derived from cascade theory, where the cross-section is designed to achieve optimal pressure distribution and minimize boundary layer separation. Key considerations include the camber line, which dictates the turning angle of the flow, and the thickness distribution, which must provide sufficient internal volume for cooling passages while maintaining structural rigidity to withstand high centrifugal and gas bending loads.

Modern high-pressure turbine blades often utilize controlled diffusion airfoil designs, which delay the onset of flow separation and reduce profile losses. The profile must also account for the leading edge radius to manage stagnation point heat transfer and the trailing edge thickness, which is a compromise between aerodynamic wake losses and the requirement for cooling air discharge.

Due to turbine inlet temperatures exceeding the melting point of superalloys, advanced cooling methods are essential:

• Internal Convection Cooling: Air is bled from the compressor and passed through serpentine passages within the blade. Turbulators or ribs are often added to these passages to increase turbulence and enhance the heat transfer coefficient.
• Impingement Cooling: High-velocity jets of cooler air are directed onto the inner surface of the leading edge, where heat flux is highest, significantly increasing the local heat transfer rate.
• Film Cooling: Cooling air is ejected through small holes on the blade surface to create a thin, protective layer of cool air between the hot gas stream and the blade material. This acts as an insulating barrier.
• Transpiration Cooling: A more advanced form where air passes through a porous blade skin, creating a continuous cooling film over the entire surface.
• Thermal Barrier Coatings (TBC): While not a cooling method per se, ceramic coatings are applied to the blade surface to provide a temperature gradient, reducing the heat load on the metallic substrate and allowing for higher operating temperatures.