8.5 Laser-based diagnostic techniques

Laser-based diagnostic techniques revolutionize how we study complex systems. These methods use lasers to probe and measure physical and chemical phenomena without disrupting the system. They enable high-resolution measurements of temperature, pressure, velocity, and species concentrations in various fields.

These techniques rely on laser light interacting with matter through processes like absorption, emission, and scattering. This interaction provides valuable information about the system being studied. Common methods include laser-induced fluorescence, Raman scattering, and particle image velocimetry, each offering unique insights into different aspects of the system.

Principles of laser-based diagnostics

• Laser-based diagnostics leverage the unique properties of lasers to probe and measure various physical and chemical phenomena in a non-intrusive manner
• These techniques enable high-resolution spatial and temporal measurements of temperature, pressure, velocity, and species concentrations in complex systems such as combustion, fluid dynamics, and materials science
• The principles of laser-based diagnostics involve the interaction of laser light with matter through processes such as absorption, emission, scattering, and ionization, which provide valuable information about the system under investigation

Laser-induced fluorescence (LIF)

Fundamentals of LIF

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• LIF is a spectroscopic technique that involves the excitation of atoms or molecules to a higher energy state using a laser and the subsequent detection of the fluorescence emission as they relax back to the ground state
• The fluorescence intensity is proportional to the concentration of the species being probed, allowing for quantitative measurements
• The wavelength of the excitation laser and the fluorescence emission are specific to the target species, enabling selective detection in complex mixtures

LIF instrumentation and setup

• A typical LIF setup consists of a tunable laser source (such as a dye laser or a tunable diode laser) to excite the target species, a collection optics system to gather the fluorescence signal, and a detector (such as a photomultiplier tube or a CCD camera) to record the signal
• The laser beam is often shaped into a sheet using cylindrical lenses to provide planar measurements, while the fluorescence signal is collected perpendicular to the laser sheet
• Spectral filters are used to isolate the fluorescence signal from the excitation laser and other background noise

Applications of LIF in combustion diagnostics

• LIF is widely used in combustion research to measure the concentration and distribution of important species such as OH, CH, and NO, which are indicators of flame structure and chemistry
• Planar LIF (PLIF) imaging provides 2D maps of species concentrations in combustion systems such as engines, gas turbines, and burners, helping to validate combustion models and optimize engine design
• LIF can also be used to measure temperature in flames by probing the temperature-dependent population distribution of molecular energy states (two-line atomic fluorescence)

Advantages and limitations of LIF

• LIF offers high sensitivity, species selectivity, and spatially resolved measurements, making it a powerful tool for non-intrusive diagnostics
• The technique can provide quantitative measurements of species concentrations with proper calibration and correction for quenching effects
• However, LIF measurements can be affected by collisional quenching, spectral interferences, and laser-induced photochemistry, which need to be carefully considered and corrected for
• The application of LIF is limited to species that exhibit strong fluorescence signals and have well-characterized spectroscopic properties

Raman scattering diagnostics

Principles of Raman scattering

• Raman scattering is an inelastic scattering process in which the incident photons exchange energy with the molecular vibrations or rotations, resulting in a shift in the scattered photon's frequency
• The Raman shift is characteristic of the specific molecular bond and provides a unique fingerprint for chemical identification
• Raman scattering is a weak process compared to Rayleigh scattering, typically requiring high laser power and sensitive detectors

Spontaneous vs stimulated Raman scattering

• Spontaneous Raman scattering occurs when the incident photons scatter independently, resulting in a weak signal that is proportional to the number of scattering molecules
• Stimulated Raman scattering (SRS) is a nonlinear process in which the incident photons stimulate the emission of additional Raman photons, leading to a coherent amplification of the Raman signal
• SRS requires high laser intensities and is often used in combination with other nonlinear techniques such as coherent anti-Stokes Raman spectroscopy (CARS)

Coherent anti-Stokes Raman spectroscopy (CARS)

• CARS is a four-wave mixing process that combines Raman scattering with anti-Stokes frequency generation to produce a coherent, laser-like signal at the anti-Stokes frequency
• CARS offers significantly higher signal strength compared to spontaneous Raman scattering and enables spatially resolved measurements with high spectral resolution
• The CARS signal is generated only when the frequency difference between the pump and Stokes lasers matches a Raman-active vibrational mode, providing chemical selectivity

Applications of Raman diagnostics in gas sensing

• Raman scattering is used for the detection and quantification of various gas species, such as CO2, CH4, and H2, in environmental monitoring, industrial process control, and combustion diagnostics
• Raman gas sensors can provide real-time, in-situ measurements of gas composition and concentration without the need for sample extraction or preparation
• Spatially resolved Raman measurements can map the distribution of gas species in complex flow fields and combustion systems

Challenges and advancements in Raman diagnostics

• The main challenge in Raman diagnostics is the weak signal strength, which requires high laser power, efficient collection optics, and sensitive detectors
• Fluorescence interference can obscure the Raman signal, especially in complex mixtures or samples with impurities, requiring the use of time-gated detection or wavelength-shifted excitation
• Advances in laser technology, such as the development of high-power, narrowband lasers and tunable filters, have greatly improved the sensitivity and selectivity of Raman diagnostics
• Coherent Raman techniques, such as CARS and SRS, have overcome some of the limitations of spontaneous Raman scattering and enabled high-resolution, high-speed measurements in challenging environments

Laser-induced breakdown spectroscopy (LIBS)

Fundamentals of LIBS

• LIBS is an atomic emission spectroscopy technique that uses a high-power laser pulse to generate a plasma on the surface of a solid, liquid, or gas sample
• The laser-induced plasma contains excited atoms and ions from the sample, which emit characteristic optical emission as they relax back to lower energy states
• The emission spectrum is collected and analyzed to determine the elemental composition and concentration of the sample

LIBS instrumentation and data analysis

• A LIBS system typically consists of a pulsed laser (such as a Nd:YAG or excimer laser), focusing optics to deliver the laser pulse to the sample, a collection optics system to gather the plasma emission, and a spectrometer to disperse and detect the emission spectrum
• The laser pulse duration and energy are critical parameters that affect the plasma formation and the signal-to-noise ratio of the LIBS measurements
• LIBS data analysis involves spectral preprocessing (background subtraction, normalization), peak identification and assignment, and quantitative analysis using calibration curves or multivariate techniques

Applications of LIBS in material analysis

• LIBS is used for the rapid, in-situ analysis of the elemental composition of various materials, such as metals, alloys, ceramics, and geological samples
• The technique is particularly useful for the analysis of heterogeneous or layered materials, as it provides depth-resolved measurements with each laser shot
• LIBS has been applied in fields such as metallurgy, mining, environmental monitoring, and forensic science for the identification and quantification of trace elements and contaminants

Advantages and limitations of LIBS

• LIBS offers fast, in-situ, and multi-elemental analysis with minimal sample preparation, making it suitable for real-time monitoring and process control
• The technique can be applied to a wide range of materials, including conductive and non-conductive samples, and can provide spatially resolved measurements with high sensitivity
• However, LIBS measurements can be affected by matrix effects, self-absorption, and spectral interferences, which require careful calibration and correction methods
• The quantitative accuracy of LIBS is often limited compared to other analytical techniques, such as ICP-OES or XRF, due to the complex nature of the laser-induced plasma and the variability of the sampling process

Particle image velocimetry (PIV)

Principles of PIV

• PIV is a non-intrusive, optical technique for measuring the velocity field in a fluid flow by tracking the motion of small tracer particles suspended in the fluid
• The tracer particles are illuminated by a pulsed laser sheet, and their positions are recorded at two closely spaced time intervals using a high-speed camera
• The displacement of the particles between the two images is determined using cross-correlation analysis, and the velocity vector field is calculated based on the known time interval and the magnification of the imaging system

PIV experimental setup and data processing

• A typical PIV setup consists of a pulsed laser (such as a dual-cavity Nd:YAG laser), a sheet-forming optics system to create a thin laser sheet, a high-speed camera (such as a CCD or CMOS camera) to record the particle images, and a synchronizer to control the timing of the laser and camera
• The tracer particles should be small enough to faithfully follow the fluid motion, yet large enough to scatter sufficient light for imaging
• PIV data processing involves image pre-processing (background subtraction, intensity normalization), image segmentation into interrogation windows, cross-correlation analysis to determine the particle displacements, and post-processing (vector validation, interpolation, and smoothing) to obtain the final velocity field

Applications of PIV in fluid dynamics

• PIV is widely used in experimental fluid dynamics to study complex flow phenomena, such as turbulence, vortex shedding, and boundary layer flows
• The technique has been applied to a variety of flow systems, including wind tunnels, water channels, and combustion chambers, to provide detailed velocity measurements and flow visualization
• PIV data can be used to validate computational fluid dynamics (CFD) models, investigate flow-structure interactions, and optimize the design of fluid machinery and devices

Challenges and advancements in PIV techniques

• One of the main challenges in PIV is the trade-off between spatial resolution and measurement accuracy, as smaller interrogation windows provide higher spatial resolution but may contain fewer particles, leading to increased measurement uncertainty
• Out-of-plane motion of the tracer particles can cause errors in the velocity measurements, requiring the use of stereoscopic or tomographic PIV techniques to resolve the three-dimensional velocity field
• Advances in laser technology, high-speed cameras, and data processing algorithms have greatly improved the temporal and spatial resolution of PIV measurements, enabling the study of fast, transient flow phenomena
• Recent developments, such as microscopic PIV (micro-PIV) and holographic PIV, have extended the application of PIV to microfluidic systems and three-dimensional flow measurements

Laser-based temperature and pressure measurements

Laser-based thermometry techniques

• Laser-based thermometry techniques use the temperature-dependent properties of atoms, molecules, or materials to measure the temperature in a non-intrusive manner
• Coherent anti-Stokes Raman spectroscopy (CARS) can be used to measure the temperature by probing the population distribution of molecular rotational and vibrational states, which is governed by the Boltzmann distribution
• Laser-induced fluorescence (LIF) thermometry relies on the temperature-dependent population of electronic energy states, which can be probed by two-line atomic fluorescence or by measuring the fluorescence lifetime
• Rayleigh scattering thermometry measures the temperature-dependent density of a gas by detecting the intensity of the elastically scattered laser light

Pressure-sensitive paint (PSP) diagnostics

• PSP is a luminescent coating that exhibits a pressure-dependent emission intensity, allowing for the non-intrusive measurement of surface pressure in aerodynamic testing
• The paint consists of luminescent molecules (such as porphyrins or ruthenium complexes) embedded in an oxygen-permeable polymer matrix
• When excited by a laser or UV light, the luminescent molecules emit light with an intensity that decreases with increasing oxygen concentration, which is related to the local air pressure through Henry's law

Applications in aerodynamics and combustion research

• Laser-based temperature and pressure measurements are essential for understanding the complex flow and heat transfer phenomena in aerodynamics and combustion systems
• PSP has been widely used in wind tunnel testing to measure the surface pressure distribution on aircraft wings, turbine blades, and other aerodynamic surfaces, providing valuable data for validating CFD models and optimizing design
• Laser-based thermometry techniques have been applied to measure the temperature distribution in combustion systems, such as engines, gas turbines, and flames, helping to improve combustion efficiency and reduce emissions

Advantages and limitations of laser-based measurements

• Laser-based temperature and pressure measurements offer non-intrusive, spatially resolved, and time-resolved data, enabling the study of dynamic and transient phenomena
• These techniques can provide high sensitivity and accuracy, with temperature resolutions of a few Kelvin and pressure resolutions of a few pascals
• However, laser-based measurements can be affected by optical access limitations, background noise, and signal attenuation in harsh environments, such as high-pressure or sooting flames
• The application of these techniques requires careful calibration, correction for environmental effects (such as pressure and gas composition), and validation against conventional measurement methods

Laser-based imaging techniques

Planar laser-induced fluorescence (PLIF)

• PLIF is an imaging technique that uses a laser sheet to excite a fluorescent species (such as OH, CH, or NO) in a plane of interest, and the resulting fluorescence is recorded using a camera perpendicular to the laser sheet
• PLIF provides spatially resolved measurements of species concentration, temperature, and velocity in combustion systems, enabling the study of flame structure, ignition, and extinction phenomena
• The technique can be extended to multiple species or multiple planes using different laser wavelengths or scanning the laser sheet, providing a comprehensive understanding of the combustion process

Laser sheet imaging

• Laser sheet imaging is a general technique that uses a laser sheet to illuminate a plane in a flow or combustion system, and the scattered or emitted light is recorded using a camera
• The technique can be used with various scattering processes, such as Mie scattering from particles, Rayleigh scattering from gas molecules, or laser-induced incandescence (LII) from soot particles, to visualize the flow structure or measure the concentration of specific species
• Laser sheet imaging has been applied to study mixing processes, jet flows, spray diagnostics, and soot formation in various industrial and environmental systems

Tomographic imaging techniques

• Tomographic imaging techniques aim to reconstruct the three-dimensional distribution of a measured quantity (such as species concentration or temperature) from a series of planar measurements at different angles or positions
• Tomographic PIV uses multiple cameras to record the particle images from different viewing angles, enabling the reconstruction of the three-dimensional velocity field in a volume
• Tomographic PLIF employs multiple laser sheets and cameras to measure the species concentration or temperature distribution in a volume, providing insights into the three-dimensional structure of flames and turbulent flows
• The reconstruction algorithms, such as algebraic reconstruction technique (ART) or multiplicative algebraic reconstruction technique (MART), use the measured projections to iteratively update the 3D distribution until convergence is reached

Applications in combustion and flow visualization

• Laser-based imaging techniques have revolutionized the understanding of combustion and flow phenomena by providing detailed, non-intrusive measurements of key quantities such as species concentration, temperature, and velocity
• PLIF imaging has been used to study the structure and dynamics of various combustion systems, including laminar and turbulent flames, internal combustion engines, and gas turbines, providing insights into flame stabilization, pollutant formation, and combustion instabilities
• Laser sheet imaging and tomographic techniques have been applied to visualize and quantify complex flow structures, such as vortices, recirculation zones, and boundary layers, in both reacting and non-reacting flows, aiding in the development and validation of computational models

Advanced laser diagnostic techniques

Femtosecond laser diagnostics

• Femtosecond laser diagnostics use ultrashort laser pulses (typically in the range of 10-15 seconds) to probe fast chemical and physical processes with high temporal resolution
• Pump-probe techniques, such as femtosecond transient absorption spectroscopy and femtosecond stimulated Raman spectroscopy, can measure the dynamics of molecular vibrations, chemical reactions, and energy transfer processes on the timescale of atomic motion
• Femtosecond laser-induced breakdown spectroscopy (fs-LIBS) uses ultrashort laser pulses to generate a plasma with minimal thermal effects, enabling the analysis of delicate or heat-sensitive materials

Nonlinear optical diagnostics

• Nonlinear optical diagnostics exploit the nonlinear response of matter to high-intensity laser fields to generate new frequencies or to probe specific molecular interactions
• Coherent anti-Stokes Raman spectroscopy (CARS) and stimulated Raman scattering (SRS) are nonlinear Raman techniques that offer high sensitivity and spectral resolution for measuring molecular vibrations and chemical composition
• Four-wave mixing (FWM) techniques, such as degenerate four-wave mixing (DFWM) and two-color four-wave mixing (TC-FWM), can measure species concentrations, temperature, and pressure with high spatial and temporal resolution
• Laser-induced grating spectroscopy (LIGS) uses the interference of two laser beams to create a transient grating in the medium, which can be probed by a third beam to measure the speed of sound, thermal diffusivity, and other transport properties

Combining multiple diagnostic techniques

• The combination of multiple laser diagnostic techniques can provide a more comprehensive understanding of complex flow and combustion phenomena by measuring complementary quantities simultaneously
• For example, the combination of PIV and PLIF can measure the velocity field and species concentration distribution simultaneously, enabling the study of turbulence-chemistry interactions and scalar transport
• The integration of laser diagnostics with other measurement