3.3 Combustion chamber design and fuel systems
6 min read•july 31, 2024
Gas turbine engines rely on combustion chambers to ignite fuel-air mixtures, generating high-temperature gases that power the turbine. These chambers must balance efficient combustion, pressure management, and durability while operating under extreme conditions.
Fuel systems in gas turbines are crucial for atomizing and injecting fuel into the combustion chamber. Various injector types and strategies are used to optimize fuel delivery, impacting engine performance, efficiency, and emissions across different operating conditions.
Combustion Chambers in Gas Turbines
Purpose and Design Requirements
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- The combustion chamber in a gas turbine engine ignites and burns the fuel-air mixture to generate high-temperature, high-pressure gases that drive the turbine
- Key design requirements for combustion chambers include:
- Achieving efficient combustion
- Minimizing pressure losses
- Providing stable combustion over a wide range of operating conditions
- Ensuring durability and reliability
- Combustion chambers must accommodate high temperatures and pressures generated during combustion, which can exceed 2000°C and 40 atmospheres, respectively
- The geometry of the combustion chamber plays a critical role in achieving optimal combustion performance, including:
- Shape
- Size
- Arrangement of fuel injectors and air inlets
- Combustion chambers are typically designed with:
- A primary zone for fuel injection and ignition
- A secondary zone for mixing and dilution
- A tertiary zone for final mixing and temperature control
- The combustion chamber liner, exposed to the highest temperatures, must be designed with advanced cooling techniques to prevent thermal damage and ensure durability, such as:
- Film cooling
- Effusion cooling
Combustion Chamber Zones and Cooling
- The primary zone of the combustion chamber is where fuel injection and ignition occur, creating a stable flame front
- In the secondary zone, additional air is introduced to promote mixing and dilution of the combustion gases, reducing the peak flame temperature
- The tertiary zone provides final mixing and temperature control before the gases enter the turbine section
- Combustion chamber liners are exposed to extreme thermal stresses due to the high-temperature combustion gases
- Film cooling involves injecting a thin layer of cooler air along the liner surface to create a protective boundary layer between the hot gases and the liner material
- Effusion cooling uses an array of small holes in the liner to allow cooler air to seep through and form a protective film on the liner surface
- Other cooling techniques include impingement cooling, where jets of cool air are directed at the back side of the liner to remove heat, and thermal barrier coatings (TBCs), which are ceramic materials applied to the liner surface to reduce heat transfer
Fuel Atomization and Combustion
Fuel Atomization Principles and Techniques
- Fuel atomization breaks down liquid fuel into fine droplets to increase its surface area and facilitate rapid mixing with the combustion air
- Effective fuel atomization is crucial for achieving efficient combustion and minimizing emissions by promoting the formation of a homogeneous fuel-air mixture
- Fuel atomization is typically achieved using:
- Pressure-swirl atomizers, which rely on high-pressure fuel injection to create a swirling, conical spray pattern
- Air-blast atomizers, which use high-velocity air streams to shear the fuel into droplets
- The quality of fuel atomization is characterized by factors such as droplet size distribution, spray angle, and penetration depth
- Smaller droplet sizes generally lead to faster evaporation and mixing rates, improving combustion efficiency and reducing emissions
Mixing and Combustion Processes
- Mixing of the atomized fuel with the combustion air is essential for creating a flammable mixture and ensuring complete combustion
- Mixing is influenced by factors such as:
- The swirl angle of the combustion air, which creates a vortex that promotes mixing
- The placement of fuel injectors relative to the air inlets
- The geometry of the combustion chamber, which can create recirculation zones that enhance mixing
- Combustion in gas turbine engines occurs in a turbulent, diffusion-controlled flame, where the fuel and air mix and burn simultaneously
- The combustion process is governed by chemical kinetics, with the rate of combustion determined by factors such as:
- The local fuel-air ratio
- Temperature
- Pressure
- Turbulent mixing plays a critical role in the combustion process by increasing the surface area between the fuel and air, promoting faster reaction rates
Fuel Injection Systems for Gas Turbines
Types of Fuel Injectors
- Fuel injection systems in gas turbine engines deliver the fuel to the combustion chamber and ensure proper atomization and mixing with the combustion air
- Pressure atomizing fuel injectors use high-pressure fuel to create a fine spray of droplets, relying on the pressure difference between the fuel and the combustion chamber
- Air-blast atomizing fuel injectors use high-velocity air streams to shear the fuel into droplets, providing better atomization at lower fuel pressures
- Dual-orifice fuel injectors combine pressure atomization and air-blast atomization, using:
- A primary orifice for pressure atomization at low power settings
- A secondary orifice for air-blast atomization at high power settings
- Staged fuel injection systems employ multiple fuel injectors or zones to optimize combustion performance and emissions across different operating conditions, such as:
- Pilot-main systems, which use a small pilot injector for low-power operation and a larger main injector for high-power operation
- Rich-quench-lean (RQL) systems, which create a fuel-rich primary zone followed by a quick mixing zone and a fuel-lean secondary zone
Impact on Engine Performance
- The choice of fuel injection system can impact various aspects of engine performance, including:
- Combustion efficiency
- Emissions
- Fuel consumption
- Throttle response
- Pressure atomizing injectors provide good atomization at high power settings but may suffer from poor atomization at low power settings, leading to increased emissions
- Air-blast atomizing injectors offer better atomization at low power settings and can reduce emissions, but they require a source of high-pressure air
- Dual-orifice injectors combine the benefits of pressure and air-blast atomization, providing good performance across a wide range of operating conditions
- Staged fuel injection systems can optimize combustion performance and emissions by tailoring the fuel-air mixture to the specific operating conditions, such as:
- Using a pilot flame for low-power stability and a main injector for high-power operation
- Creating fuel-rich and fuel-lean zones to minimize NOx formation while maintaining combustion efficiency
Emissions from Gas Turbine Combustion
Types and Formation Mechanisms
- Gas turbine combustion systems produce various emissions, including:
- Carbon dioxide (CO2)
- Water vapor (H2O)
- Nitrogen oxides (NOx)
- Carbon monoxide (CO)
- Unburned hydrocarbons (UHC)
- Particulate matter (PM)
- NOx emissions are a primary concern in gas turbine engines due to their contribution to air pollution and their role in the formation of photochemical smog and acid rain
- NOx formation in gas turbine combustion is primarily driven by thermal NOx mechanisms, which are highly dependent on the peak flame temperature and residence time in the high-temperature regions
- CO and UHC emissions are typically the result of incomplete combustion, which can occur due to:
- Poor fuel atomization
- Inadequate mixing
- Local flame extinction
- PM emissions from gas turbine engines are relatively low compared to other combustion sources but can still contribute to air quality issues and have adverse health effects
Emission Reduction Strategies and Regulations
- Strategies for reducing emissions from gas turbine combustion systems include:
- Lean , which operates with a fuel-lean mixture to reduce peak flame temperatures and NOx formation
- Water or steam injection, which lowers the flame temperature and suppresses NOx formation
- Selective catalytic reduction (SCR), which uses a catalyst to convert NOx into nitrogen and water
- The use of alternative fuels, such as natural gas or , which can reduce CO2 and PM emissions
- Stringent emissions regulations drive the development of cleaner combustion technologies for gas turbine engines, with standards set by organizations such as:
- The International Civil Aviation Organization (ICAO) for aircraft engines
- The U.S. Environmental Protection Agency (EPA) for stationary gas turbines
- These regulations typically set limits on NOx, CO, UHC, and PM emissions based on the engine power output and the specific application (e.g., aircraft, power generation, or industrial use)
- Manufacturers of gas turbine engines must continuously develop and implement advanced combustion technologies and emission control systems to meet these evolving regulations and mitigate the environmental impact of gas turbine operations
Key Terms to Review (18)
Aerodynamic shaping: Aerodynamic shaping refers to the design process of modifying the external shape of an object to reduce drag and improve its performance in a fluid flow, such as air. This involves creating smooth contours and optimizing geometries to enhance airflow characteristics, which is crucial for achieving efficient combustion and maximizing thrust in propulsion systems. Effective aerodynamic shaping can lead to improved fuel efficiency and performance across various applications, including aircraft and rocket engines.
Air-fuel ratio: The air-fuel ratio (AFR) is a measure of the amount of air in relation to the amount of fuel present in a combustion process. It plays a crucial role in optimizing engine performance and emissions by determining how efficiently fuel is burned. Achieving the right AFR is essential for effective combustion, influencing factors like power output, fuel efficiency, and emissions in various engine designs and fuel systems.
Brayton Cycle: The Brayton cycle is a thermodynamic cycle that describes the functioning of gas turbine engines, where air is compressed, mixed with fuel, ignited, and then expanded to produce work. This cycle is fundamental to understanding propulsion systems as it illustrates how energy conversion occurs through various processes like compression, combustion, and expansion.
Combined cycle: A combined cycle refers to a power generation process that utilizes both gas and steam turbines to generate electricity more efficiently than traditional methods. This system takes advantage of the waste heat produced by gas turbines to drive steam turbines, effectively maximizing energy extraction from the fuel used. By integrating these two cycles, combined cycle plants achieve higher thermal efficiencies and lower emissions, making them a preferred choice in modern power generation.
Combustion stability: Combustion stability refers to the ability of a combustion system to maintain a consistent and efficient burning process without experiencing harmful fluctuations or disturbances. Achieving combustion stability is crucial for ensuring optimal performance, minimizing emissions, and preventing issues such as blowout or combustion instability, which can lead to engine damage and reduced efficiency. Factors such as fuel-air mixture, combustion chamber design, and operating conditions significantly influence combustion stability.
Combustion temperature: Combustion temperature refers to the temperature at which a fuel and oxidizer ignite and sustain a combustion reaction. This temperature is critical in determining the efficiency of the combustion process, as it influences the formation of pollutants, the overall energy output, and the design parameters of combustion chambers and fuel systems.
Diffusion Flame: A diffusion flame is a type of combustion where fuel and oxidizer are not mixed prior to ignition, leading to the flame being established in a region where the fuel vapor diffuses into the air, allowing combustion to occur. This process is essential in various propulsion systems and is characterized by its unique temperature and chemical composition distribution. Understanding diffusion flames helps in optimizing combustion chamber designs and fuel systems for efficiency and stability.
Flame Holder: A flame holder is a device used in combustion systems, particularly in gas turbines and scramjets, to stabilize the flame within the combustion chamber. By creating turbulence or altering the flow characteristics of the air-fuel mixture, flame holders ensure continuous ignition and effective combustion. This stability is crucial for efficient fuel utilization and maintaining optimal performance in propulsion systems.
Fuel Injector: A fuel injector is a device that atomizes and delivers fuel into the combustion chamber of an engine, ensuring an efficient mix with air for combustion. This precision component plays a crucial role in optimizing engine performance and emissions by controlling the timing and amount of fuel injected, making it integral to the design and function of combustion systems.
Fuel Pump: A fuel pump is a mechanical device designed to move fuel from the storage tank to the combustion chamber in propulsion systems. It plays a crucial role in ensuring the right amount of fuel reaches the engine, which is vital for optimal performance and efficiency. The fuel pump must deliver fuel at a specific pressure and flow rate to maintain proper combustion and support the overall operation of rocket or jet engines.
Hydrogen: Hydrogen is the simplest and most abundant element in the universe, consisting of one proton and one electron. Its role in propulsion systems is significant due to its high energy content and efficiency when used as a fuel. In combustion chambers, hydrogen can produce a clean and efficient burn, contributing to the overall effectiveness of fuel systems and thrust generation in aerospace applications.
Kerosene: Kerosene is a flammable liquid hydrocarbon mixture derived from petroleum, commonly used as a fuel in jet engines and other combustion systems. Its properties, such as high energy density and good combustion characteristics, make it an essential component in various propulsion technologies and fuel systems, contributing significantly to thrust generation and efficiency.
Mixing Chamber: A mixing chamber is a component in combustion systems designed to blend fuel and oxidizer before combustion takes place. This process is essential for achieving efficient combustion, as it ensures a uniform mixture that promotes optimal performance and reduces emissions. The design of the mixing chamber directly influences the combustion characteristics and overall efficiency of propulsion systems.
Premixed combustion: Premixed combustion is a type of combustion process where fuel and oxidizer are mixed before entering the combustion chamber, allowing for a more uniform and efficient burning. This method enhances the stability of the flame and can lead to higher thermal efficiencies, which is crucial in the design of propulsion systems and combustion chambers. It also influences the heat transfer characteristics and performance of liquid propellant feed systems, making it an essential aspect in aerospace propulsion technologies.
Robert Goddard: Robert Goddard was an American engineer and inventor known as the father of modern rocketry. He made significant contributions to the field by successfully launching the world's first liquid-fueled rocket in 1926, paving the way for future developments in aerospace propulsion. His innovative ideas and experiments led to advancements in combustion chamber design, rocket cooling techniques, and even laid the groundwork for concepts used in electrostatic propulsion systems.
Sir Frank Whittle: Sir Frank Whittle was a British engineer and inventor credited with developing the first practical jet engine, revolutionizing aircraft propulsion. His pioneering work in turbojet technology laid the foundation for modern aviation, influencing the design of combustion chambers and fuel systems used in jet engines today. Whittle's contributions have greatly enhanced the efficiency and performance of powered flight, making him a key figure in aerospace history.
Thermal Management: Thermal management refers to the techniques and processes used to control temperature within a system, ensuring optimal performance and preventing damage from excessive heat. In the context of propulsion technologies, effective thermal management is essential for maintaining component integrity and efficiency during operation, especially in high-temperature environments.
Thrust Specific Fuel Consumption: Thrust specific fuel consumption (TSFC) is a measure of the efficiency of an engine, defined as the amount of fuel consumed per unit of thrust produced, typically expressed in pounds of fuel per hour per pound of thrust (lb/hp). This metric is crucial for assessing propulsion system performance, optimizing cycle analysis, and influencing combustion chamber design and fuel system choices in gas turbine engines.