🔥Advanced Combustion Technologies Unit 7 – Advanced Combustion Diagnostics

Key Concepts and Principles

• Combustion diagnostics involves measuring and analyzing various parameters in combustion processes to optimize efficiency and reduce emissions
• Key parameters include temperature, pressure, species concentrations, and flow velocities
• Diagnostic techniques can be classified as intrusive (e.g., thermocouples, sampling probes) or non-intrusive (e.g., optical methods, laser-based systems)
• Temporal and spatial resolution are crucial factors in selecting appropriate diagnostic techniques
  • Temporal resolution refers to the ability to capture rapid changes in combustion processes
  • Spatial resolution determines the level of detail in measuring local variations within the combustion chamber
• Combustion diagnostics enables the validation and improvement of computational fluid dynamics (CFD) models
• Advanced diagnostics techniques contribute to the development of cleaner and more efficient combustion technologies (low-emission engines, alternative fuels)

Diagnostic Techniques Overview

• Intrusive techniques involve direct contact with the combustion environment
  • Thermocouples measure local temperatures by generating a voltage proportional to the temperature difference between two dissimilar metals
  • Sampling probes extract gas samples for analysis using gas chromatography or mass spectrometry
• Non-intrusive techniques do not disturb the combustion process and offer high temporal and spatial resolution
• Optical diagnostics methods rely on the interaction of light with the combustion environment
  • Laser-induced fluorescence (LIF) measures species concentrations and temperatures by exciting molecules with a laser and detecting the emitted fluorescence
  • Particle image velocimetry (PIV) determines flow velocities by tracking the movement of seeded particles in the flow field
• Laser-based measurement systems provide precise and non-intrusive measurements of various combustion parameters
• Spectroscopic techniques analyze the emission, absorption, or scattering of light to determine species concentrations and temperatures

Optical Diagnostics Methods

• Laser-induced fluorescence (LIF) is a widely used technique for measuring species concentrations and temperatures in combustion
  • A laser excites specific molecules (OH, CH, NO) to a higher energy state, causing them to emit fluorescence
  • The intensity of the fluorescence is proportional to the concentration of the target species
  • Two-line LIF can determine temperatures by measuring the fluorescence intensity ratio at two different wavelengths
• Rayleigh scattering measures the elastic scattering of light by gas molecules to determine temperature and density
  • The intensity of the scattered light is proportional to the gas density
  • Calibration with known temperatures and densities is required for quantitative measurements
• Raman scattering is an inelastic scattering process that provides information on molecular composition and temperature
  • The scattered light experiences a wavelength shift characteristic of the specific molecule
  • Raman scattering has a weak signal intensity compared to Rayleigh scattering, requiring high-power lasers and sensitive detectors
• Coherent anti-Stokes Raman scattering (CARS) enhances the Raman signal by using a multi-photon excitation process
  • CARS offers improved signal-to-noise ratio and spatial resolution compared to conventional Raman scattering

Laser-Based Measurement Systems

• Laser Doppler velocimetry (LDV) measures local flow velocities by analyzing the Doppler shift of laser light scattered by particles in the flow
  • Two laser beams intersect to form a measurement volume where interference fringes are created
  • Particles crossing the fringes scatter light with a Doppler shift proportional to their velocity
• Phase Doppler anemometry (PDA) is an extension of LDV that measures particle size and velocity simultaneously
  • PDA uses multiple detectors at different angles to determine the phase shift of the scattered light, which is related to the particle size
• Tunable diode laser absorption spectroscopy (TDLAS) measures species concentrations and temperatures by scanning a narrow-linewidth laser across a specific absorption line
  • The absorption of light is proportional to the species concentration according to the Beer-Lambert law
  • TDLAS offers high sensitivity and fast response times for real-time monitoring of combustion processes
• Laser-induced incandescence (LII) measures the volume fraction and size distribution of soot particles in combustion
  • A high-energy laser pulse heats the soot particles to their vaporization temperature, causing them to emit blackbody radiation
  • The temporal decay of the LII signal provides information on the soot particle size distribution

Spectroscopy in Combustion Analysis

• Emission spectroscopy analyzes the light emitted by excited species in the combustion environment
  • The wavelength and intensity of the emitted light provide information on species concentrations and temperatures
  • Chemiluminescence is a commonly used emission spectroscopy technique that measures the light emitted from chemically excited species (OH*, CH*, C2*)
• Absorption spectroscopy measures the attenuation of light as it passes through the combustion environment
  • The absorption of light at specific wavelengths is related to the concentration of absorbing species according to the Beer-Lambert law
  • Fourier-transform infrared (FTIR) spectroscopy is a widely used absorption technique that measures the absorption spectrum over a wide range of wavelengths
• Cavity ring-down spectroscopy (CRDS) is a highly sensitive absorption technique that measures the decay rate of light in an optical cavity
  • The decay rate is related to the concentration of absorbing species in the cavity
  • CRDS offers high sensitivity and long effective path lengths for measuring trace species concentrations
• Laser-induced breakdown spectroscopy (LIBS) uses a high-energy laser pulse to create a plasma from the sample material
  • The emission spectrum of the plasma provides information on the elemental composition of the sample
  • LIBS enables in-situ and real-time analysis of solid, liquid, and gaseous samples

Data Acquisition and Processing

• Data acquisition systems convert analog signals from diagnostic instruments into digital data for further processing and analysis
  • High-speed analog-to-digital converters (ADCs) are used to capture fast-changing signals with high temporal resolution
  • Simultaneous sampling of multiple channels is essential for correlating measurements from different diagnostic techniques
• Signal conditioning techniques improve the quality of the acquired data
  • Amplification increases the signal-to-noise ratio for weak signals
  • Filtering removes unwanted noise and interference from the signal
  • Averaging multiple measurements reduces random noise and improves the signal-to-noise ratio
• Data processing algorithms extract relevant information from the acquired data
  • Fourier analysis converts time-domain signals into frequency-domain spectra, revealing periodic components and noise sources
  • Correlation techniques identify relationships between different measured quantities (pressure, heat release rate)
• Visualization tools present the processed data in a meaningful and intuitive format
  • False-color images display the spatial distribution of measured quantities (temperature, species concentrations)
  • Line plots show the temporal evolution of combustion parameters at specific locations
  • 3D renderings combine data from multiple diagnostic techniques to provide a comprehensive view of the combustion process

Advanced Imaging Technologies

• High-speed cameras capture fast-moving phenomena in combustion processes
  • Frame rates up to millions of frames per second enable the visualization of flame propagation, turbulence, and instabilities
  • Intensified cameras amplify weak light signals, allowing for the imaging of low-light events (chemiluminescence, fluorescence)
• Planar laser-induced fluorescence (PLIF) provides two-dimensional measurements of species concentrations and temperatures
  • A laser sheet excites the target species in a plane, and the resulting fluorescence is imaged onto a camera
  • PLIF enables the visualization of the spatial distribution and temporal evolution of species in turbulent flames
• Tomographic techniques reconstruct three-dimensional fields from multiple two-dimensional measurements
  • Tomographic PIV uses multiple cameras to capture the flow field from different angles, allowing for the reconstruction of the 3D velocity field
  • Tomographic PLIF extends the concept to the measurement of 3D species concentration and temperature fields
• Volumetric laser-induced fluorescence (VLIF) directly measures three-dimensional species concentration and temperature fields
  • Multiple laser sheets are used to excite the target species in a volume, and the fluorescence is captured by multiple cameras
  • VLIF provides instantaneous 3D measurements, enabling the study of complex turbulent flame structures

Practical Applications and Case Studies

• Gas turbine combustion diagnostics
  • Monitoring of flame stability, emissions, and combustion dynamics in gas turbine engines
  • Optimization of fuel injection systems and combustor geometries for improved efficiency and reduced emissions
• Internal combustion engine diagnostics
  • In-cylinder measurements of temperature, pressure, and species concentrations to validate and improve engine simulation models
  • Study of knock, misfire, and cycle-to-cycle variations in spark-ignition engines
  • Investigation of fuel spray characteristics and mixing processes in compression-ignition engines
• Furnace and boiler diagnostics
  • Monitoring of flame shape, stability, and emissions in industrial furnaces and boilers
  • Optimization of burner designs and operating conditions for improved efficiency and reduced pollutant formation
• Rocket engine combustion diagnostics
  • Characterization of the combustion environment in rocket engines under extreme conditions (high pressure, high temperature)
  • Study of flame holding mechanisms, combustion instabilities, and thermal loads on engine components
• Fundamental combustion research
  • Investigation of laminar and turbulent flame structures, ignition processes, and extinction phenomena
  • Validation of chemical kinetic models and turbulence-chemistry interaction models
  • Development of advanced combustion concepts (flameless combustion, high-pressure combustion) for clean and efficient energy conversion