🔥Advanced Combustion Technologies Unit 10 – Efficient Combustion Device Design

Fundamentals of Combustion

• Combustion is an exothermic chemical reaction between a fuel and an oxidizer that releases heat and light
• Requires three essential components: fuel, oxidizer (usually oxygen), and an ignition source (heat)
• Combustion process converts chemical energy stored in the fuel into thermal energy (heat)
• Products of complete combustion include carbon dioxide (CO2), water vapor (H2O), and heat
• Incomplete combustion occurs when there is insufficient oxygen, leading to the formation of carbon monoxide (CO) and unburned hydrocarbons (UHCs)
  • Incomplete combustion reduces efficiency and increases harmful emissions
• Stoichiometric combustion refers to the ideal ratio of fuel to oxidizer, resulting in complete combustion without excess reactants
• Equivalence ratio ($\phi$) represents the actual fuel-to-oxidizer ratio compared to the stoichiometric ratio
  • $\phi < 1$ indicates lean combustion (excess air)
  • $\phi > 1$ indicates rich combustion (excess fuel)

Types of Combustion Devices

• Internal combustion engines (ICEs) are widely used in transportation (automobiles, motorcycles)
  • Utilize reciprocating pistons to convert the heat and pressure from combustion into mechanical work
• Gas turbines are used for power generation and aircraft propulsion
  • Operate on the Brayton cycle, comprising compression, combustion, and expansion stages
• Boilers and furnaces are used for heating and steam generation in industrial processes and power plants
  • Burn fuel to heat water or generate steam for various applications
• Fluidized bed combustors employ a bed of solid particles suspended by an upward flow of air, enhancing mixing and heat transfer
• Rocket engines utilize combustion to generate high-velocity exhaust gases for propulsion (spacecraft, missiles)
• Afterburners are used in jet engines to increase thrust by burning additional fuel in the exhaust stream
• Catalytic combustors use catalysts to promote combustion at lower temperatures, reducing emissions

Efficiency Metrics and Performance Indicators

• Thermal efficiency measures the proportion of chemical energy in the fuel converted into useful work or heat
  • Calculated as the ratio of useful output energy to input energy from the fuel
• Combustion efficiency indicates the completeness of the combustion process
  • Determined by the ratio of actual heat released to the theoretical maximum heat release
• Specific fuel consumption (SFC) represents the fuel flow rate per unit power output
  • Lower SFC values indicate better fuel efficiency
• Emissions levels, such as CO, NOx, and particulate matter (PM), are crucial performance indicators
  • Regulated by environmental standards and legislation (Euro norms, EPA regulations)
• Power output and power density are important metrics for evaluating the performance of combustion devices
  • Power density refers to the power output per unit volume of the device
• Reliability, durability, and maintenance requirements are essential considerations for long-term performance
• Operational flexibility, such as the ability to handle different fuels or load variations, is desirable in many applications

Design Principles for Efficient Combustion

• Optimize air-fuel mixing to ensure complete combustion and minimize emissions
  • Employ techniques like fuel atomization, swirl, and turbulence to enhance mixing
• Maintain appropriate combustion temperature and residence time for efficient burning
  • Too low temperatures lead to incomplete combustion, while too high temperatures increase NOx formation
• Utilize staged combustion to control temperature and emissions
  • Primary combustion zone operates at fuel-rich conditions, followed by secondary air injection for complete burnout
• Implement lean combustion strategies to reduce flame temperature and NOx emissions
  • Requires precise control of air-fuel ratio and combustion stability
• Employ exhaust gas recirculation (EGR) to reduce peak combustion temperatures and NOx formation
  • Recirculates a portion of the exhaust gases back into the combustion chamber
• Optimize combustion chamber geometry to promote efficient mixing and flame stability
  • Considerations include chamber shape, dimensions, and placement of fuel and air inlets
• Utilize advanced ignition systems, such as laser ignition or plasma ignition, for improved combustion initiation and stability
• Implement active control systems to monitor and adjust combustion parameters in real-time
  • Closed-loop control based on sensors (pressure, temperature, emissions) and actuators (fuel injectors, air valves)

Materials and Component Selection

• Combustion chamber materials must withstand high temperatures and pressures
  • Common materials include high-temperature alloys (Inconel, Hastelloy), ceramics, and thermal barrier coatings (TBCs)
• Fuel injectors should provide efficient atomization and distribution of fuel
  • Design considerations include nozzle geometry, spray angle, and flow rate
• Ignition systems, such as spark plugs or glow plugs, must reliably initiate combustion
  • Materials should resist erosion, corrosion, and thermal shock
• Turbine blades in gas turbines require high-temperature strength and creep resistance
  • Single crystal superalloys and ceramic matrix composites (CMCs) are commonly used
• Heat exchangers, such as recuperators or regenerators, improve efficiency by preheating the combustion air
  • Materials with high thermal conductivity and resistance to corrosion are preferred (stainless steel, ceramics)
• Catalysts, such as noble metals (platinum, palladium) or metal oxides, promote low-temperature combustion and emissions reduction
  • Catalyst support materials should provide high surface area and thermal stability (alumina, ceria)
• Sensors for monitoring combustion parameters (pressure, temperature, emissions) must be durable and accurate
  • Examples include thermocouples, pressure transducers, and gas analyzers
• Insulation materials, such as ceramic fibers or microporous insulation, minimize heat losses and improve efficiency

Emission Control Strategies

• Primary emission control measures focus on preventing the formation of pollutants during combustion
  • Includes optimizing air-fuel ratio, combustion temperature, and residence time
• Secondary emission control measures treat the exhaust gases after combustion
  • Includes catalytic converters, particulate filters, and selective catalytic reduction (SCR) systems
• Three-way catalytic converters simultaneously reduce CO, HC, and NOx emissions in gasoline engines
  • Requires precise control of air-fuel ratio around stoichiometric conditions
• Diesel particulate filters (DPFs) capture and periodically burn off soot particles from diesel engine exhaust
• SCR systems inject urea solution into the exhaust stream, which decomposes to ammonia and reacts with NOx over a catalyst
  • Commonly used in diesel engines and large stationary combustion sources
• Lean NOx traps (LNTs) adsorb NOx during lean combustion and release it for reduction during brief rich conditions
• Oxidation catalysts convert CO and unburned hydrocarbons into CO2 and water
• Exhaust gas recirculation (EGR) reduces NOx formation by lowering peak combustion temperatures
• Water injection or emulsified fuels can also help reduce NOx emissions by lowering flame temperatures

Advanced Modeling and Simulation Techniques

• Computational Fluid Dynamics (CFD) simulations predict fluid flow, heat transfer, and chemical reactions in combustion devices
  • Solve governing equations (Navier-Stokes, energy, species transport) using numerical methods (finite volume, finite element)
• Chemical kinetics modeling predicts the rates of chemical reactions and species formation during combustion
  • Detailed kinetic mechanisms include elementary reactions and species (GRI-Mech, USC Mech)
  • Reduced mechanisms simplify the chemistry while maintaining accuracy for specific conditions
• Turbulence modeling captures the effects of turbulent flow on combustion processes
  • Reynolds-Averaged Navier-Stokes (RANS) models (k-epsilon, k-omega) provide time-averaged solutions
  • Large Eddy Simulation (LES) resolves large-scale turbulent structures and models small-scale eddies
• Spray and atomization modeling predicts the breakup and dispersion of liquid fuel droplets
  • Lagrangian particle tracking follows individual droplets, while Eulerian methods treat the spray as a continuum
• Conjugate heat transfer modeling couples fluid flow and solid heat conduction to predict temperature distributions
  • Important for designing cooling systems and predicting thermal stresses
• Emissions modeling predicts the formation and destruction of pollutants, such as NOx, CO, and soot
  • Involves detailed chemical kinetics and turbulence-chemistry interaction models
• Multidimensional modeling captures spatial variations in combustion processes
  • One-dimensional (1D) models are used for system-level analysis and optimization
  • Three-dimensional (3D) models provide detailed insights into local phenomena

Optimization and Testing Methods

• Parametric studies vary design parameters systematically to identify their impact on performance
  • Parameters can include geometry, operating conditions, and fuel properties
• Design of Experiments (DOE) techniques efficiently explore the design space and identify optimal configurations
  • Factorial designs, response surface methods, and Taguchi methods are commonly used
• Optimization algorithms, such as genetic algorithms or gradient-based methods, search for optimal designs based on defined objectives and constraints
  • Objectives can include efficiency, emissions, power output, or cost
• Sensitivity analysis determines the influence of input parameters on the output variables
  • Helps identify critical design parameters and guides optimization efforts
• Experimental testing validates computational models and assesses the performance of combustion devices
  • Bench-scale testing evaluates individual components or subsystems
  • Engine dynamometer testing measures power output, fuel consumption, and emissions under controlled conditions
• Optical diagnostics, such as laser-induced fluorescence (LIF) or particle image velocimetry (PIV), provide non-intrusive measurements of combustion processes
  • Helps validate models and gain insights into flame structure, temperature, and species concentrations
• Emissions testing, using gas analyzers and particulate measurement systems, ensures compliance with regulations
• Durability and reliability testing assess the long-term performance and failure modes of combustion devices
  • Accelerated life testing and thermal cycling simulate real-world operating conditions
• Field testing and monitoring provide data on the performance and emissions of combustion devices in actual applications
  • Helps identify areas for improvement and optimize maintenance schedules