ARTICLE IN PRESS Progress in Aerospace Sciences 41 (2005) 93–142 www.elsevier.com/locate/paerosci Property measurement utilizing atomic/molecular filter-based diagnostics M. Boguszko, G.S. Elliottà Department of Aerospace Engineering, University of Illinois Urbana Champaign, 306 Tabolt Laboratory, 104 South Wright Street, Urbana, IL 61801-2935, USA Abstract A variety of atomic/molecular filter diagnostic techniques have been under development for qualitative and quantitative flow diagnostic tools since their introduction in the early 1990s. This class of techniques utilizes an atomic or molecular filter, which is basically a glass cell containing selected vapor-phase species (e.g., I2, Hg, K, Rb). In filtered Rayleigh scattering (FRS), and techniques derived from it, the atomic/molecular filter is placed in front of the detector to modify the frequency spectrum of radiation scattered by flow-field constituents (i.e., molecules/atoms and/or particles) when they are illuminated by a narrow linewidth laser. The light transmitted through the filter is then focused on a detector, typically a CCD camera or photomultiplier tube. The atomic/molecular filter can be used simply to suppress background surface/particle scattering, and thereby enhance flow visualizations, or to make quantitative measurements of thermodynamic properties. FRS techniques have been developed to measure individual flow properties, such as velocity (when the scattered light is from particles) or temperature (when the scattered light is from molecules), and measure multiple flow properties simultaneously such as pressure, density, temperature, and velocity. This manuscript summarizes the background needed to understand FRS techniques, and gives example measurements that have been used to develop FRS, demonstrate its capabilities, and investigate flow fields (both non-reacting and combustion) of research interest utilizing the unique capabilities of FRS. In addition, FRS has been used in conjunction with other diagnostics to improve the technique or measure properties simultaneously such as temperature and velocity (measured with PIV), or temperature and species concentration (measured by Raman scattering or laser-induced fluorescence). Also, a brief discussion is given of similar techniques being developed which utilize atomic/molecular filters and Thomson scattering from electrons to measure the electron number density and electron temperatures in plasmas. r 2005 Elsevier Ltd. All rights reserved. Contents 1. Introduction . 95 1.1. Motivation . 95 1.2. Background . 95 1.3. General description of molecular/atomic filter-based techniques for property measurement . 96 ÃCorresponding author. Tel.: +1 217 265 9211; fax: +1 217 265 0720. E-mail address: [email protected] (G.S. Elliott). 0376-0421/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.paerosci.2005.03.001 ARTICLE IN PRESS 94 M. Boguszko, G.S. Elliott / Progress in Aerospace Sciences 41 (2005) 93–142 Nomenclature Z fluid shear viscosity y scattering angle w.r.t. the laser propaga- a polarizability tion direction B background calibration coefficient j scattering wave vector c speed of light l laser wavelength cp specific heat at constant pressure r degree of polarization cv specific heat at constant volume s Rayleigh scattering cross section C dark current and offset constant ds=dO differential scattering cross section E radiant energy n optical frequency (GHz) gðy; T; nÞ scattering spectral distribution based on n0 laser central optical frequency Gaussian model n¯ frequency wave number (in cmÀ1) gðn¯Þ absorption line shape (assumed Gaussian) n¯j frequency line center of absorption transi- h Planck’s constant tion j (in cmÀ1) I transmitted radiant intensity through the f angle relative to the incident laser polar- filter ization I0 incident radiant intensity to the filter cell c angle between incident and detected polar- k Boltzmann’s constant ization vectors k^ l laser propagation direction unit vector k^ s scattering direction unit vector Subscripts K energy-to-grayscale conversion constant l filter cell optical path length cam camera filter cell L optical path imaged at detection element e electron m molecular mass i incident quantity n index of refraction iso isotropic N fluid number density j running index for multiple quantities Npe number of photo-electrons f filtered p pressure p polarization-sensitive quantity r(y) Rayleigh scattering spectral distribution pe photo-induced electrons (Cabannes line) photon relative to a photon RðfÞ Rayleigh scattering calibration coefficient ref reference condition S grayscale value (counts) s scattered quantity tðnÞ filter absorption function stp standard temperature and pressure T temperature u unfiltered quantity u, v, w Cartesian velocity components 0 polarization-insensitive quantity uk velocity component along j vector 1 undisturbed flow quantity V velocity vector x non-dimensional frequency Superscripts Xj mole fraction of gas species j y order parameter c spectral central line alone M total number of absorption transitions Greek letters Acronyms aj scattering angle with respect to horizontal plane ASE amplified spontaneous emission b background scattering cross section BBO beta barium borate crystal g anisotropy BUT buildup time Gj integrated absorption coefficient for ab- CARS coherent anti-stokes Raman scattering sorption transition j CCD charged coupled device DnD optical frequency Doppler shift CW constant wave DnT FWHM of the Rayleigh scattering spec- CMOS complimentary metal oxide semiconductor trum DC direct current Dn¯j FWHM of absorption line j DGV Doppler global velocimetry DO scattering solid angle FARRS filtered angularly resolved Rayleigh scat- optical efficiency constant tering z frequency function FRS filtered Rayleigh scattering ARTICLE IN PRESS M. Boguszko, G.S. Elliott / Progress in Aerospace Sciences 41 (2005) 93–142 95 FM-FRS frequency-modulated FRS PD photodiode FWHM frequency width at half-maximum PDV planar Doppler velocimetry HSRL spectral high-resolution LIDAR PIV particle image velocimetry ICCD intensified CCD PLIF planar laser-induced fluorescence KTP potassium titanyl phosphate crystal PMT photomultiplier tube LIDAR light detection and ranging QE quantum efficiency MFRS modulated FRS RF radio frequency Nd:YAG neodymium-doped yttrium aluminum gar- SBS stimulated Brillouin scattering net crystal STP standard temperature and pressure Nd:YVO4 neodymium-doped orthovanadate crystal 2. Rayleigh scattering from atomic and molecular species . 97 2.1. Intensity characteristics. 97 2.2. Spectral characteristics . 100 3. Atomic/molecular absorption filter . 102 4. The FRS signal . 103 5. Equipment . 105 5.1. Typical atomic/molecular filter . 105 5.2. Illuminating lasers . 106 5.3. Laser frequency monitoring. 109 6. FRS flow visualization . 111 7. Single property measurement . 115 7.1. FRS velocimetry . 116 7.2. Frequency-modulated filtered Rayleigh scattering . 120 7.3. FRS thermometry . 122 8. Multiple property measurements . 127 8.1. Average measurements (FRS frequency scanning technique) . 127 8.2. Instantaneous measurements . 131 9. Combined techniques and future trends . 134 10. Conclusion . 138 Acknowledgements . 139 References . 139 1. Introduction and in more than one spatial dimension. The perfect technique might be thought of as one that allows the 1.1. Motivation measurement of all the properties, everywhere, at all times. For example, properties in a compressible flow Recent advances in sensor and laser technologies have may vary significantly throughout the flow field (i.e., led many flow diagnostics previously utilized only in the through shock and expansion waves) and compressible development laboratory to gain widespread use as ‘‘off- turbulence quantities such as Reynolds stresses have the-shelf’’ diagnostics. Techniques such as particle image terms that involve multiple fluctuating variables that velocimetry (PIV) and spectroscopy are now more must be measured simultaneously and independently. widely available due to advances in camera, laser, The number of properties we desire to measure becomes computer, and sensor technologies and are common- even larger and more complex as we consider reacting place due to reduced costs and software integration flows and turbulent flames. Although as the subject of making them more ‘‘user friendly’’. This has led to a the current review article, atomic/molecular filter-based more complete understanding of many thermal/fluid techniques do not reach all these goals, they do provide systems and the ability to verify computer models of a unique means of measuring flow properties that few complex flows. As these techniques are more universally other techniques can achieve as effectively. applied to problems of research interest, there is still a need, however, to develop techniques that measure 1.2. Background properties non-intrusively without introducing artificial particles or substances into the flow being measured. In As an introduction to molecular/atomic filter-based addition, it is desirable to enhance current capabilities so techniques we consider first how these techniques came that multiple properties can be measured simultaneously to be utilized by the scientific research community. In ARTICLE IN PRESS 96 M. Boguszko, G.S. Elliott / Progress in Aerospace Sciences 41 (2005) 93–142 1983, Shimizu et al. [1] proposed the use of atomic and a sheet of laser light from a frequency-doubled injection- molecular vapor filters for high spectral resolution seeded Nd:YAG laser and modified the spectrum of the LIDAR (HSRL).