Laser-based diagnostics revolutionize combustion research by providing non-intrusive, high-resolution measurements. These techniques, including Laser-Induced Fluorescence (LIF), Particle Image Velocimetry (PIV), and Planar Laser-Induced Fluorescence (PLIF), offer unprecedented insights into combustion processes.
LIF measures species concentrations, PIV tracks fluid velocities, and PLIF combines LIF with 2D imaging. Together, these methods enable detailed analysis of flame structure, pollutant formation, and flow dynamics, advancing our understanding of complex combustion phenomena.
Laser-Induced Fluorescence (LIF)
Principles and Applications of LIF
- Laser-Induced Fluorescence employs laser light to excite molecules in a specific quantum state
- Excited molecules emit light at longer wavelengths upon returning to their ground state
- Fluorescence spectroscopy analyzes the emitted light to gather information about the sample
- Excitation wavelength must match the absorption spectrum of the target species
- Emission spectrum provides information about the molecular structure and environment
- LIF enables non-intrusive measurements in combustion systems
Species Concentration Mapping with LIF
- LIF allows for quantitative measurements of species concentrations in combustion environments
- Intensity of fluorescence signal correlates with the concentration of the target species
- Technique can detect trace amounts of molecules (parts per billion range)
- 2D and 3D concentration maps can be created by scanning the laser beam or using multiple detectors
- LIF detects important combustion species such as OH, NO, and CH radicals
- Temporal resolution of LIF measurements can reach nanosecond timescales
Challenges and Considerations in LIF
- Quenching effects can reduce fluorescence intensity in high-pressure environments
- Temperature dependence of fluorescence signal requires careful calibration
- Laser beam attenuation may occur in optically thick media
- Multiple species with overlapping spectra can complicate data interpretation
- Signal-to-noise ratio improves with increasing laser power, but may lead to saturation effects
- Advanced LIF techniques (two-photon LIF, femtosecond LIF) address some limitations
Particle Image Velocimetry (PIV)
Fundamentals of PIV Technique
- Particle Image Velocimetry measures fluid velocity fields by tracking the motion of tracer particles
- Tracer particles (typically 1-100 μm in diameter) are added to the flow and assumed to follow it faithfully
- Flow visualization achieved by illuminating particles with a laser sheet
- Two consecutive images of illuminated particles captured by a high-speed camera
- Cross-correlation algorithms determine particle displacements between image pairs
- Velocity vectors calculated from particle displacements and known time interval
PIV System Components and Setup
- Laser system (typically Nd:YAG) generates high-energy light pulses
- Optical arrangement converts laser beam into a thin sheet (cylindrical lens, beam expander)
- Seeding system introduces tracer particles (oil droplets, solid particles) into the flow
- High-resolution digital cameras (CCD or CMOS) capture particle images
- Synchronization unit coordinates laser pulses and camera exposures
- Computer system with specialized software processes and analyzes image data
Advanced PIV Techniques and Applications
- Stereoscopic PIV uses two cameras to measure all three velocity components
- Time-resolved PIV achieves high temporal resolution (kHz range) for studying unsteady flows
- Micro-PIV enables velocity measurements in microscale flows (microfluidics)
- Tomographic PIV reconstructs 3D velocity fields from multiple camera views
- PIV applied in various fields includes aerodynamics, combustion, and biomedical engineering
- Spatial resolution depends on camera resolution and particle density (typically 0.1-1 mm)
Planar Laser-Induced Fluorescence (PLIF)
PLIF Technique and Implementation
- Planar Laser-Induced Fluorescence combines principles of LIF with 2D imaging capabilities
- Laser sheet (typically 0.1-1 mm thick) excites molecules in a plane within the flow
- Fluorescence from the excited plane captured by an intensified CCD camera
- PLIF provides instantaneous 2D maps of species concentrations or temperature
- Technique allows visualization of mixing processes and reaction zones in combustion
- Spatial resolution determined by laser sheet thickness and camera pixel size (typically 0.1-1 mm)
PLIF Applications in Combustion Diagnostics
- OH-PLIF detects regions of high temperature and identifies flame fronts
- NO-PLIF measures pollutant formation in combustion processes
- Acetone-PLIF visualizes fuel distribution and mixing in non-reacting flows
- Temperature measurements possible using two-line PLIF techniques
- PLIF detects important combustion intermediates (CH, HCO) to study reaction mechanisms
- Combination of PLIF with PIV enables simultaneous measurement of velocity and scalar fields
Advanced PLIF Techniques and Considerations
- High-speed PLIF achieves kHz repetition rates for studying transient phenomena
- Multi-species PLIF simultaneously measures multiple species using different excitation wavelengths
- Quenching effects and laser sheet non-uniformity require careful calibration procedures
- Signal trapping can occur in optically thick media, affecting quantitative measurements
- Photochemical effects (photodissociation, photoionization) may interfere with measurements
- PLIF data interpretation often requires complementary numerical simulations or modeling