9.3 Turbine-based combined cycle engines

Turbine-based combined cycle engines blend gas turbines with secondary power systems, boosting efficiency and performance. They recover waste heat from gas turbine exhaust to generate extra power, making them a game-changer for aerospace propulsion.

These engines offer higher efficiency and flexibility than standalone gas turbines. They're great for long-range flights and diverse missions, but come with added weight and complexity. It's all about balancing power, efficiency, and practicality in aircraft design.

Turbine-based Combined Cycle Engines

Operating Principles

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• Turbine-based combined cycle engines integrate a gas turbine engine with a secondary power generation system (steam turbine or closed-cycle heat engine) improves overall efficiency and performance
• The gas turbine engine (compressor, combustion chamber, and turbine) serves as the primary power source generates high-temperature exhaust gases
• The exhaust gases from the gas turbine generate steam or heat a working fluid in a secondary power cycle drives an additional turbine produces supplementary power

Key Components

• Key components of a turbine-based combined cycle engine include the gas turbine, heat recovery steam generator (HRSG), steam turbine or closed-cycle heat engine, condenser, and associated piping and control systems
• The HRSG captures the waste heat from the gas turbine exhaust generates steam for the secondary power cycle optimizes the utilization of thermal energy
• The steam turbine or closed-cycle heat engine expands the steam or heated working fluid generates additional mechanical power drives a generator or propels the aircraft

Performance of Combined Cycle Engines

Efficiency and Power Output

• Turbine-based combined cycle engines offer higher thermal efficiency compared to standalone gas turbine engines recover and utilize waste heat from the gas turbine exhaust
• The overall efficiency of a combined cycle engine can reach up to 60% or higher depends on the specific design and operating conditions
• The power output of a combined cycle engine can be modulated by adjusting the gas turbine operating point and the steam or closed-cycle heat engine parameters provides operational flexibility

Operational Flexibility

• The use of a secondary power cycle allows for independent optimization of the gas turbine and the steam or closed-cycle heat engine enables better matching of the engine performance to the specific mission requirements
• Combined cycle engines can operate efficiently over a wide range of power settings (part-load to full-load conditions) makes them suitable for various flight profiles and missions
• The integration of a secondary power cycle introduces additional complexity and weight to the engine system must be carefully considered in the overall aircraft design and performance analysis

Design Trade-offs for Combined Cycle Engines

Efficiency and Technical Challenges

• The design of a turbine-based combined cycle engine involves trade-offs between efficiency, power output, weight, complexity, and cost
• Increasing the gas turbine operating temperature and pressure ratio can improve the overall efficiency of the combined cycle engine may also increase the technical challenges and material requirements
• The selection of the secondary power cycle (steam turbine or closed-cycle heat engine) depends on factors such as the available heat source temperature, desired power output, and system complexity
• Optimization of the heat recovery system (HRSG design and pinch point temperature difference) is crucial for maximizing the utilization of waste heat improving overall efficiency

Weight Reduction and Optimization

• The integration of advanced materials (ceramic matrix composites (CMCs) or advanced alloys) can enable higher operating temperatures improve the efficiency and power density of the combined cycle engine
• Strategies for reducing the weight and size of the combined cycle engine components (compact heat exchangers or advanced turbine blade designs) can enhance the overall system performance
• Numerical simulation and optimization techniques (multidisciplinary design optimization (MDO)) can be employed to explore the design space identify optimal configurations for specific mission requirements

Combined Cycle Engines in Aerospace

Potential Applications

• Turbine-based combined cycle engines offer the potential for improved fuel efficiency and reduced emissions compared to conventional gas turbine engines makes them attractive for long-range and high-endurance aircraft applications
• The higher efficiency of combined cycle engines can lead to reduced fuel consumption and extended range capabilities beneficial for commercial aviation and military transport missions
• The operational flexibility of combined cycle engines (ability to efficiently operate at various power settings) makes them suitable for aircraft with diverse mission profiles (long-range cruise and high-speed dash)
• Combined cycle engines can be integrated into hybrid-electric propulsion systems the gas turbine serves as a primary power source and the secondary power cycle generates electricity for electric propulsion or onboard systems

Challenges and Advancements

• The waste heat recovery capability of combined cycle engines can be utilized for thermal management of aircraft systems (cabin air conditioning or anti-icing) reduces the overall energy consumption
• The application of combined cycle engines in aerospace propulsion requires careful consideration of the additional weight, complexity, and integration challenges associated with the secondary power cycle components
• Successful implementation of turbine-based combined cycle engines in aircraft propulsion systems requires advancements in materials, manufacturing techniques, and control strategies ensures reliable and efficient operation in the demanding aerospace environment