Laser Rayleigh scattering
TL;DR: In this paper, the spectral properties of Rayleigh scattering are discussed and a review of the new advances in flow field imaging that have been achieved using the new filter approaches is presented.
Abstract: Rayleigh scattering is a powerful diagnostic tool for the study of gases and is particularly useful for aiding in the understanding of complex flow fields and combustion phenomena. Although the mechanism associated with the scattering, induced electric dipole radiation, is conceptually straightforward, the features of the scattering are complex because of the anisotropy of molecules, collective scattering from many molecules and inelastic scattering associated with rotational and vibrational transitions. These effects cause the scattered signal to be depolarized and to have spectral features that reflect the pressure, temperature and internal energy states of the gas. The very small scattering cross section makes molecular Rayleigh scattering particularly susceptible to background interference. Scattering from very small particles also falls into the Rayleigh range and may dominate the scattering from molecules if the particle density is high. This particle scattering can be used to enhance flow visualization and velocity measurements, or it may be removed by spectral filtering. New approaches to spectral filtering are now being applied to both Rayleigh molecular scattering and Rayleigh particle scattering to extract quantitative information about complex gas flow fields. This paper outlines the classical properties of Rayleigh scattering and reviews some of the new advances in flow field imaging that have been achieved using the new filter approaches.
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TL;DR: A comprehensive overview of the progress and the gap in the knowledge of plasma assisted combustion in applications, chemistry, ignition and flame dynamics, experimental methods, diagnostics, kinetic modeling, and discharge control is provided in this paper.
812 citations
TL;DR: In this paper, a detailed analysis of the gas temperature determination from rotational spectra is performed, and a large range of conditions for which non-equilibrium occurs are identified.
Abstract: The gas temperature in non-equilibrium plasmas is often obtained from the plasma-induced emission by measuring the rotational temperature of a diatomic molecule in its excited state. This is motivated by both tradition and the availability of low budget spectrometers. However, non-thermal plasmas do not automatically guarantee that the rotational distribution in the monitored vibrational level of the diatomic molecule is in equilibrium with the translational (gas) temperature. Often non-Boltzmann rotational molecular spectra are found in non-equilibrium plasmas. The deduction of a gas temperature from these non-thermal distributions must be done with care as clearly the equilibrium between translational and rotational degrees of freedom cannot be achieved. In this contribution different methods and approaches to determine the gas temperature are evaluated and discussed. A detailed analysis of the gas temperature determination from rotational spectra is performed. The physical and chemical background of non-equilibrium rotational population distributions in molecular spectra is discussed and a large range of conditions for which non-equilibrium occurs are identified. Fitting procedures which are used to fit (non-equilibrium) rotational distributions are analyzed in detail. Lastly, recommendations concerning the conditions for which the gas temperatures can be obtained from diatomic spectra are formulated.
366 citations
TL;DR: In this paper, cavity ring-down spectroscopy extinction measurements have been performed in various gases straightforwardly resulting in cross sections for Rayleigh scattering, for Ar and N 2 measurements are performed in the range 470-490nm, while for CO 2 cross sections are determined in the wider range 470−570nm.
Abstract: Using the laser-based technique of cavity ring-down spectroscopy extinction measurements have been performed in various gases straightforwardly resulting in cross sections for Rayleigh scattering. For Ar and N 2 measurements are performed in the range 470–490 nm, while for CO 2 cross sections are determined in the wider range 470–570 nm. In addition to these gases also for N 2 O, CH 4 , CO, and SF 6 the scattering cross section is determined at 532 nm, a wavelength of importance for lidar applications and combustion laser diagnostics. In O 2 the cross section at 532 nm is found to depend on pressure due to collision-induced light absorption. The obtained cross sections validate the cross sections for Rayleigh scattering as derived from refractive indices and depolarization ratios through Rayleigh's theory at the few %-level, although somewhat larger discrepancies are found for CO, N 2 O and CH 4 .
330 citations
Cites background from "Laser Rayleigh scattering"
...The depolarization ratios can be expressed in terms of elements of the polarizability tensor along the principal axes of the molecule, aii [2,7,13]: rn 1⁄4 2 P ia 2 ii 2 P iojaiiajj 4 P ia 2 ii þ P iojaiiajj ; rp 1⁄4 P ia 2 ii P iojaiiajj 3 P ia 2 ii þ 2 P iojaiiajj : (5)...
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...Single or multiple scattering in the atmospheres of the Earth and planets [2,6], as well as laser-induced Rayleigh scattering for diagnosing complex flow fields, combustion processes and LIDAR applications [7] all rely on values for the refractive indices....
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...where also the local field correction and the King correction factor for the depolarization [7,13] is included....
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TL;DR: In this article, the authors present a survey of spray measurement techniques and issues in spray physics and spray engineering, including the need for simultaneous diagnostic measurements under application-relevant conditions, and the effective comparison of spray measurements and numerical simulations.
Abstract: Sprays are among the most intellectually challenging and practically important topics in fluid mechanics. This paper reviews needs, milestones, challenges, and a broad array of techniques for spray measurement. In addition, tabular summaries provide cross-referenced entry points to the vast literature by organizing over 300 citations according to key spray phenomena, physical parameters and measurement techniques for each of the principal spray regions (nozzle internal flow, near-field spray-formation region, far-field developed spray, and spray-wall interaction). The article closes with perspectives on some current issues in spray research, including the cost and complexity of apparatus for spray physics and spray engineering, the need for simultaneous diagnostic measurements under application-relevant conditions, and the effective comparison of spray measurements and numerical simulations.
220 citations
TL;DR: In this paper, a review summarizes the state of the art of plasma diagnostics on atmospheric pressure plasmas formed at characteristic length scales of approximately 1 mm or smaller and identifies challenges and prospects.
Abstract: This review summarizes the state of the art of plasma diagnostics on atmospheric pressure plasmas formed at characteristic length scales of approximately 1 mm or smaller and identifies challenges and prospects Both plasmas generated in confined geometries, so-called microplasmas, as well as discharge filaments occurring in commonly filamentary plasmas, eg microdischarges in dielectric barrier discharges are covered In spite of the differences between microplasmas which often obtain a quasi steady-state and single microdischarges or filaments which are self-limited in space and time and thus intrinsically transient, both face very similar diagnostic challenges of which two are immediately apparent: the high collisionality which requires adaptations of standard plasmas diagnostics often developed for low-pressure plasmas, and the requirements on high spatial resolution due to the strong gradients in plasma properties The complexity of the plasma generation and the physical and chemical properties of the above-mentioned plasmas requires the knowledge of an extensive series of different parameters to obtain a full characterization As the results of the diagnostics are not always unambiguous and require a detailed understanding of plasma physics and chemistry, a summary of the main properties and pecularities of high-pressure plasmas is included in this review
212 citations
References
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TL;DR: Light scattering by small particles as mentioned in this paper, Light scattering by Small Particle Scattering (LPS), Light scattering with small particles (LSC), Light Scattering by Small Parts (LSP),
Abstract: Light scattering by small particles , Light scattering by small particles , مرکز فناوری اطلاعات و اطلاع رسانی کشاورزی
9,737 citations
Book•
01 Dec 1981
TL;DR: Light scattering by small particles as mentioned in this paper, Light scattering by Small Particle Scattering (LPS), Light scattering with small particles (LSC), Light Scattering by Small Parts (LSP),
Abstract: Light scattering by small particles , Light scattering by small particles , مرکز فناوری اطلاعات و اطلاع رسانی کشاورزی
6,623 citations
01 Jan 2003
TL;DR: In this article, the basic physics of elastic light scattering from small particles is studied for the simple case of a homogeneous and isotropic sphere, where the particle velocity and its properties are analyzed.
Abstract: In the laser Doppler and phase Doppler techniques a part of the incident laser light is imaged by the particles onto the detectors. It is this scattered light which carries information about the particle velocity and its properties and thus, the light scattered from small particles plays a central role in the basic physics of these measurement techniques. In recognition of this, the following chapter is devoted to the fundamentals of elastic light scattering from small particles. The simplest case of a homogeneous and isotropic sphere is considered.
2,499 citations
TL;DR: The dispersion of the depolarization factor is shown to affect the Rayleigh phase function slightly, by approximately 1% in the forward, backscattered, and 90° scattering-angle directions.
Abstract: Rayleigh-scattering cross sections and volume-scattering coefficients are computed for standard air; they incorporate the variation of the depolarization factor with wavelength. Rayleigh optical depths are then calculated for the 1962 U.S. Standard Atmosphere and for five supplementary models. Analytic formulas are derived for each of the parameters listed. The new optical depths can be 1.3% lower to 3% higher at midvisible wavelengths and up to 10% higher in the UV region compared with previous calculations, in which a constant or incorrect depolarization factor was used. The dispersion of the depolarization factor is also shown to affect the Rayleigh phase function slightly, by approximately 1% in the forward, backscattered, and 90° scattering-angle directions.
669 citations