๐ฅAdvanced Combustion Technologies Unit 7 โ Advanced Combustion Diagnostics
Advanced combustion diagnostics is a crucial field for optimizing efficiency and reducing emissions in combustion processes. It involves measuring key parameters like temperature, pressure, and species concentrations using intrusive and non-intrusive techniques.
These diagnostic methods enable the validation of computational models and development of cleaner combustion technologies. Techniques range from optical methods like laser-induced fluorescence to laser-based systems such as particle image velocimetry, providing high temporal and spatial resolution for detailed analysis.
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