Nonintrusive laser probing of unsteady combustion processes provides the capability for the remote, in—situ, spatially and temporally precise measurement of, among other parameters, gas temperature and species concentrations. The predominant spatially—precise laser diagnostics for temperature and species include spontaneous Raman scattering (RS) and coherent anti-Stokes Raman spectroscopy (CARS) for measurements of major, i.e., ≥~ 1%, constituents, and laser—induced fluorescence spectroscopy (LIFS) and degenerate four wave mixing (DFWM) for minor, ppm level, species. Rayleigh scattering can provide total density measurements. These techniques are reviewed, including two-dimensional implementations, their state of maturity assessed, and major remaining research issues identified.

Laser Diagnostics for Temperature and Species in Unsteady Combustion

An overview is given of recent developments in laser diagnostic methods for the quantitative measurement of trace species concentrations and, in conjunction, of temperature in combustion systems. After a short introduction illustrating some experiments from the pre-laser era, the article presents typical applications and discusses advantages and limitations of laser techniques including laser absorption, linear, saturated, predissociative and multi-photon-excited laser-induced fluorescence (LIF), resonance-enhanced multiphoton ionization (REMPI), electronically resonant coherent anti-Stokes Raman scattering (resonance CARS), degenerate four-wave mixing (DFWM) and amplified spontaneous emission (ASE). Recent trends including two-dimensional imaging, multi-species detection and high-pressure applications will also be discussed. Throughout the article, an attempt is made to present typical results from a large portion of the relevant technical literature. A concluding section gives a short summary of the current status and comments on the perspectives for further research.

Laser Techniques for the Quantitative Detection of Reactive Intermediates

Laser techniques for the quantitative detection of reactive intermediates in combustion systems

An overview is provided of the planar laser-induced fluorescence (PLIF) method, which currently allows simultaneous combustion measurements at more than 105 flowfield points Important advantages of the method include its relatively high signal strength, ease of interpretation, and applicability for determining several flowfield variables (including concentration, temperature, velocity, pressure and density) Example results are shown for a turbulent non-premixed flame, a spray flame, a rod-stabilized premixed flame, and a diffusion flame from a fuel jet in cross-flow

Planar laser-fluorescence imaging of combustion gases

Publisher Summary This chapter describes the techniques and applications of computer-generated holograms. “Computer-generated holograms,” “synthetic holograms,” and “computer holograms” are terms used to refer to a class of holograms that are produced as graphical output from a digital computer. Given a mathematical description of a wavefront or an object represented by an array of points, the computer can calculate the amplitude transmittance of the hologram and display the result on a CRT or plot it on paper. Using photoreduced copy of the graphical output from a computer as holograms is only one of the many things that distinguish computer generated holograms from conventional ones. The computer-generated holograms have their greatest potential in the area of interferometry. They have been shown to be useful in supplementing existing methods of optical testing. Laser beam scanning is another promising area for computer-generated holograms. Holographic grating scanners are in many ways better than other mechanical mirror scanners.

III Computer-Generated Holograms: Techniques and Applications

We report the first two-photon-excited hydrogen-atom fluorescence measurements in flames made to our knowledge. The n = 3 level of the H atom was excited by 205.1-nm radiation generated by Raman shifting a 224-nm beam produced by frequency mixing. Fluorescence was observed at 656.3 nm as a result of radiative decay from n = 3 to n = 2, the Balmer-α transition. A novel technique, photoionization-controlled loss spectroscopy, is proposed to eliminate the quenching dependence of the fluorescence signal.

Two-photon-excited fluorescence measurement of hydrogen atoms in flames

Laser-saturated fluorescence measurements of OH concentration in flames

The balanced cross-rate model is proposed to analyze laser-induced molecular fluorescence signals when the laser pulse length is of the order of nanoseconds. Nanosecond pulse length lasers, specifically Q-switched Nd:YAG-pumped dye lasers, are attractive for saturated molecular fluorescence spectroscopy because of their high peak power and because their short pulse length minimizes the risk of laser-induced chemistry. In the balanced cross-rate model, single upper and lower rotational levels are assumed to be directly coupled by the laser radiation. Because the laser-induced processes which couple these levels are so fast at saturation intensities, a steady state is established between the two levels within picoseconds. Provided that the total population of the two laser-coupled rotational levels is constant during the laser pulse, the total molecular population can be calculated from the observed upper rotational level population using a two-level saturation model and Boltzmann statistics. Numerical simulation of the laser excitation dynamics of OH in an atmospheric pressure H2/O2/N2 flame indicates that the balanced cross-rate model will give accurate results provided that the rotational relaxation rates in the upper and lower sets of rotational levels are approximately equal.

Balanced cross-rate model for saturated molecular fluorescence in flames using a nanosecond pulse length laser

OH concentration measurements have been performed in rich, atmospheric pressure CH4/O2/N2 flames using laser-saturated fluorescence. Both sooting and non-sooting conditions have been investigated. OH is thought to be an important species in soot chemistry because of its role in the oxidation of soot and soot precursors. In the sooting flame, the centerline axial OH concentration drops sharply 3-4 mm above the burner surface. Visible soot emission begins only after the OH concentration decreases significantly from its peak value in the flame zone. Rotational Raman scattering was identified as a potentially significant interference in the fluorescence measurements at the low OH concentration levels (down to 5 ppm) typical of the rich flames. Laser-saturated fluorescence and optical absorption measurements of OH concentration were also performed in a lean, atmospheric pressure CH4/O2/N2 flame. A combination fluorescence-absorption calibration in the lean flame was used to determine absolute OH numbe...

Laser-Saturated Fluorescence Measurements of OH in Atmospheric Pressure CH4/O2/N2 Flames Under Sooting and Non-Sooting Conditions

A technique is proposed and demonstrated for measuring combustion temperatures using two-line laser-saturated fluorescence The rotational temperature of OH is determined by saturating two different rotational transitions in the (0,0) band of the A2∑+–X2Π electronic system and detecting fluorescence emission which originates from the laser-pumped upper rotational levels Temperature is calculated from the ratio of the fluorescence intensities for the two different excitation–emission pairs The method is demonstrated by measuring temperature profiles in subatmospheric H2/O2/Ar flat flames Temperatures measured by two-line saturated fluorescence are compared with temperatures measured by coated thermocouples and OH absorption and with predictions from an elementary chemical kinetics code The temperatures measured by the two-line fluorescence technique are accurate to 3–5% and exhibit low random error

Donald W. Sweeney

Papers

Two-photon-excited fluorescence measurement of hydrogen atoms in flames

Laser-saturated fluorescence measurements of OH concentration in flames

Balanced cross-rate model for saturated molecular fluorescence in flames using a nanosecond pulse length laser

Laser-Saturated Fluorescence Measurements of OH in Atmospheric Pressure CH4/O2/N2 Flames Under Sooting and Non-Sooting Conditions

Temperature measurement by two-line laser-saturated OH fluorescence in flames.