unit 7 review
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.
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