Introduction to laser-based combustion diagnostics (Lecture 1b) Lecture prepared for course in laser-based combustion diagnostics by Per-Erik Bengtsson and Joakim Bood Division of Combustion Physics at the Department of Physics ~ 35 employees ~ 20 PhD-students 2-5 diploma workers Research in laser diagnostics chemical kinetics turbulent flow Strong networks and colaborations other departments at LTH other universities industry international universities and research institutes Courses Laser-based combustion diagnostics, 7.5 ECTS, VT-1 Fundamental Combustion, 7.5 ECTS, VT-2 Molecular Physics, 7.5 ECTS, HT-2 1
Combustion was early man s first practical energy source Early man s use of combustion: Warmth Light Cooking Work metals Joakim Bood Future global energy need surprise geothermal Global energy use Organic and fossil sources New and renewable sources sun new biomass wind nuclear power hydropower gas oil Year coal traditional biomass 2
Combustion is a complex phenomenon Processes / Issues Chemical kinetics Flow processes Physical processes (Diffusion, Heat conduction, Radiation) Thermodynamics Different phases (Vapor, Droplets, Particles) Practical fuels Photo: Tools Experimental techniques Theory and Modeling An example of combustion in a non-premixed flame Motivation for combustion research By an improved fundamental understanding of combustion processes there will be a potential to improve efficiency lower fuel consumption ( less CO 2 ) reduce emissions NO x, SO x, particles develop combustion on alternative fuels and new technology hydrogen combustion, fuel cells improve safety suppress fire initiation and spread & Joakim Bood 3
Combustion situations/applications For specified tasks Candle light Welding flame Bunsen burner Log fire Liquid gas stove Unwanted events Fire Explosion Detonation For efficiency and low emissions Furnace Fluidized bed Diesel engine Gasoline engine Rocket engine Jet engine Gas turbine & Joakim Bood Areas of importance for laser diagnostics of combustion Laser Detector Combustion Lasers and Detectors Optics Molecular Spectroscopy What is combustion? What information do modelers need? What characteristics are important? Which components are used? How can interaction of light and matter be used to interpret for example temperatures and species concentrations? 4
Lasers characteristics Laser wavelength Tunability Repetition rate Pulse energy Pulse duration Beam quality Polarization Frequency stability Divergence Robustness Ease of operation Cost & Joakim Bood Molecular Physics Energy level diagram Diatomic molecule E Population distribution for N 2 0,09 Relative population 0,08 0,07 0,06 0,05 0,04 0,03 0,02 0,01 T=300 K T=1000 K T=1700 K Internuclear distance 0 0 3 6 9 12 15 18 21 24 27 30 33 36 39 Rotational quantum number (J) 5
Which techniques can be used? Laser techniques Rayleigh scattering Mie scattering Raman scattering Absorption Laser-induced incandescence Laser-induced fluorescence Laser Doppler velocimetry Particle-image velocimetry Coherent anti-stokes Raman Spectroscopy Laser-induced phosphorescence (thermographic phosphors) Can be used for measurements of Temperature in non-sooting flames, soot Droplets, Soot Temperature, Major species concentrations Species concentrations Soot volume fractions Minor species concentrations Velocities Velocities Temperatures, Major species concentrations Surface temperature and many more, for example Degenerate Four-wave mixing Polarization spectroscopy Cavity-ring down spectroscopy Laser-induced grating spectroscopy etc. & Joakim Bood Merits of laser diagnostic techniques Non-intrusive A probe probe disturbs the the combustion High spatial resolution Diam. Diam. 100 100 μm, μm, length length 100 100 μm μm High temporal resolution Pulse Pulse duration: 10 10 ns ns Species specific Absorption at at specific specific wavelength Signal Signal at at specific wavelength In-situ measurements Measurements are are performed where where the the combustion occurs. occurs. Remote measurements No No upper temperature limit Non-equilibrium can be be probed Dye laser Nd:YAG laser Lens Typical experimental setup Filter Polariser Detector 6
What should be measured? Parameters Concentrations of species Temperature Velocities Particles (sizes, concentrations, volume fractions) Droplets (sizes, concentrations, volume fractions) Fuel/air ratios Pressures Demands Presence of species Qualitative/Quantitative Temporal resolution Spatial resolution (point, 1-D, 2-D, 3-D) Accuracy Precision Photo: Diffusion flame on Wolfhard-Parker burner What could limit us? Noise in laser radiation Detection noise Averaging of information from measurements Laser-induced disturbances Background emission from flame Lack of molecular data Insufficient theoretical models 7
Point measurements Lens Lens Detector One-dimensional measurements Imaging lens 1-D detector 8
Two-dimensional measurements Laser beam Flame Cylindrical lens Lens 2D-detector Distribution of OH in a turbulent flame 9
Limitations of laser diagnostic techniques Optical access is is needed Line-of-sight access access Line-of-sight access access + orthogonal window Complex experiments Operator skill is is needed Interpretation may be be advanced Limited number of of species A given given set-up set-up may may be be used used for for a few few species Best for for small molecules Spectra specific for for few-atom molecules only only High cost Should Should be be used used only only when when simpler simpler methods give give too too limited limited information Dye laser Nd:YAG laser Lens Typical experimental setup Filter Polariser Detector Flame development detection in engine Laser-induced fluorescence of CH 2 O can be used to identify hot spots in studies of engine knock. Pressure curve, 9 CAD BTDC Pressure curve, 9 CAD ATDC α Images are from Bäuerle et al.,proc. Combust. Inst. (1994) 10
CARS measurements in a spark-ignition engine Photo: In CARS (Coherent anti-stokes Raman spectroscopy, three beams are focussed to a common intersection point. These beams interact with the molecules in the intersection point and resulting in a signal that is spectrally resolved. The temperature is evaluated from the spectrum. Pressure: 1.2 MPa Evaluated temperature: 790 K Photo: 11