2.3 Heat transfer and combustion processes

Heat transfer and combustion are critical processes in aerospace propulsion. They govern how energy is generated, transferred, and utilized within engines. Understanding these mechanisms is key to designing efficient, powerful, and environmentally friendly propulsion systems.

Heat transfer impacts engine performance, component durability, and emissions. Combustion, the heart of most propulsion systems, converts chemical energy into thermal energy. Mastering these processes enables engineers to push the boundaries of aerospace technology.

Heat Transfer Mechanisms in Propulsion

Fundamentals of Heat Transfer

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• Heat transfer is the process of thermal energy moving from a hotter object to a colder object, driven by a temperature difference between the two objects
• Conduction is the transfer of heat through direct contact between particles of matter, typically occurring in solids with a temperature gradient
  • Fourier's law describes the rate of conductive heat transfer as proportional to the temperature gradient and the material's thermal conductivity
• Convection is the transfer of heat by the movement of fluids or gases, driven by buoyancy forces arising from density differences caused by temperature variations
  • Newton's law of cooling describes the rate of convective heat transfer as proportional to the temperature difference between the surface and the fluid
  • Natural convection occurs when fluid motion is caused by buoyancy forces due to density differences (hot air rising), while forced convection occurs when an external source, such as a pump or fan, induces fluid motion (air conditioning)
• Radiation is the transfer of heat through electromagnetic waves, requiring no medium for propagation
  • The Stefan-Boltzmann law states that the rate of radiative heat transfer is proportional to the fourth power of the absolute temperature difference between the emitting and absorbing surfaces (heat from the sun)

Heat Transfer in Propulsion Systems

• In propulsion systems, heat transfer plays a crucial role in managing thermal loads, ensuring efficient combustion, and maintaining the integrity of engine components
• Conduction, convection, and radiation all contribute to the overall heat transfer within the system
  • Conductive heat transfer occurs through the walls of the combustion chamber and other engine components
  • Convective heat transfer takes place between the hot combustion gases and the surrounding surfaces, as well as within the cooling systems
  • Radiative heat transfer is significant in high-temperature regions, such as the combustion chamber and turbine sections
• Effective heat management is essential for preventing overheating, maintaining optimal operating temperatures, and ensuring the longevity of propulsion system components (turbine blades, nozzles)

Heat Transfer in Combustion Processes

Role of Heat Transfer in Combustion Efficiency

• Combustion is an exothermic chemical reaction that releases heat, which is then transferred to the working fluid (air) in propulsion systems
• Efficient heat transfer from the combustion process to the working fluid is essential for maximizing propulsion efficiency
• The rate of heat transfer in combustion chambers is influenced by factors such as the combustion temperature, the surface area of the combustion chamber, and the thermal properties of the chamber walls
  • Increasing the rate of heat transfer can lead to higher propulsion efficiency by allowing more energy to be extracted from the combustion process (higher turbine inlet temperatures)
• Flame propagation and stability are affected by heat transfer within the combustion chamber
  • Sufficient heat transfer is necessary to maintain a stable flame and prevent flame extinction or flashback (combustion instability)

Heat Transfer and Pollutant Formation

• The formation of pollutants, such as nitrogen oxides (NOx) and carbon monoxide (CO), is influenced by the temperature and residence time of the combustion products
• Controlling heat transfer can help reduce pollutant formation by maintaining optimal combustion temperatures and minimizing hot spots
  • Cooling systems, such as regenerative cooling or film cooling, are employed to manage the heat transfer from the combustion process to the engine components, preventing overheating and ensuring the structural integrity of the propulsion system (liquid rocket engines)
• Proper heat transfer management is crucial for meeting emission regulations and minimizing the environmental impact of propulsion systems (aircraft engines, automotive engines)

Fundamentals of Combustion

Stoichiometry and Air-Fuel Ratio

• Combustion is a chemical reaction between a fuel and an oxidizer (usually air) that releases heat and light
  • The three essential components for combustion are fuel, oxidizer, and an ignition source (spark, high temperature)
• Stoichiometry is the quantitative relationship between reactants and products in a chemical reaction
  • In combustion, stoichiometry determines the ideal ratio of fuel to oxidizer (air) required for complete combustion, known as the stoichiometric air-fuel ratio
• Equivalence ratio (φ) is a measure of the actual air-fuel ratio relative to the stoichiometric air-fuel ratio
  • φ < 1 indicates a lean mixture (excess air), φ = 1 indicates a stoichiometric mixture, and φ > 1 indicates a rich mixture (excess fuel)
  • Lean mixtures can result in lower combustion temperatures and reduced NOx formation, while rich mixtures can lead to incomplete combustion and increased CO and UHC emissions

Flame Propagation and Pollutant Formation

• Flame propagation refers to the spread of the combustion reaction through the fuel-oxidizer mixture
  • Laminar flame speed is the velocity at which a flame front propagates through a quiescent mixture, while turbulent flame speed is the velocity in a turbulent flow field (gas turbine combustors)
• Pollutant formation during combustion is a result of incomplete combustion, high combustion temperatures, or the presence of impurities in the fuel or air
  • The primary pollutants formed during combustion include carbon monoxide (CO), unburned hydrocarbons (UHC), nitrogen oxides (NOx), and particulate matter (PM)
• NOx formation is highly dependent on the combustion temperature and the residence time of the combustion products in high-temperature regions
  • Thermal NOx, prompt NOx, and fuel NOx are the three main mechanisms of NOx formation
  • Thermal NOx is formed at high temperatures (above 1800 K) through the oxidation of atmospheric nitrogen, prompt NOx is formed by the rapid reaction of fuel-derived hydrocarbon radicals with atmospheric nitrogen, and fuel NOx is formed by the oxidation of nitrogen-containing compounds in the fuel

Fuel and Combustion Chamber Performance

Fuel Types and Characteristics

• The choice of fuel significantly influences the performance, efficiency, and emissions of propulsion systems
• Liquid fuels, such as kerosene (jet fuel) and gasoline, are commonly used in aircraft and automotive propulsion systems due to their high energy density, ease of storage, and handling
  • However, they require atomization and vaporization for efficient combustion (fuel injectors, carburetors)
• Gaseous fuels, such as natural gas and hydrogen, have the advantage of easier mixing with air and more complete combustion
  • However, they have lower energy densities compared to liquid fuels and require specialized storage systems (pressurized tanks, cryogenic storage)
• Biofuels, such as biodiesel and ethanol, are renewable alternatives to fossil fuels
  • They can reduce net carbon dioxide emissions but may have compatibility issues with existing propulsion systems and lower energy densities compared to conventional fuels

Combustion Chamber Design and Performance

• Combustion chamber design plays a crucial role in the performance and emissions of propulsion systems
• Swirl-stabilized combustors use swirling flow to enhance mixing and create recirculation zones that stabilize the flame
  • They are commonly used in gas turbine engines for their compact size and stable combustion characteristics
• Staged combustion systems divide the combustion process into multiple stages to control the temperature and residence time of the combustion products, reducing NOx formation
  • Examples include the rich-burn, quick-mix, lean-burn (RQL) combustor and the lean direct injection (LDI) combustor
• Lean-premixed combustors operate with excess air to lower the combustion temperature and minimize NOx formation
  • However, they require careful control to prevent flame instability and lean blow-out (combustion oscillations)
• Advanced combustion chamber designs, such as trapped vortex combustors and catalytic combustors, are being developed to further improve combustion efficiency and reduce emissions
  • Trapped vortex combustors use cavities to create stable recirculation zones and enhance fuel-air mixing, while catalytic combustors use catalytic materials to promote complete combustion at lower temperatures