Fast optical tomography for quantitative visualization of turbulent structure in reacting flows

Most attempts to visualize individual species in a turbulent flow have relied 011 plaiiar laser induced flurescenre (PLIF). The data generated by this technique are not easily interpreted i l l complex environments unless the pressure is low enougli so that the collisional quenching rate is small compared to the radiative emission rate from the laser excited state. Optical compnterized axial tomography (OCAT) is capable of generating images of species specific concentrations and of temperature and is immunc to t,he quenching problem. Previous implementations of OC:\T Iiave not been fast enough to freeze turbulent structures. A new design of a fan beam tomography instrument. is introduced that is capable of producing a high resolution image of both temperature and species concentration in less than one microsecond. Results of analytical and experimental studies designed to test the feasibi1it.y of this concept and ibs application to an atmosplieric pressure flow tube are presented. *. I n t r o d u c t i o n The objective of this work is to develop a fast, two-dimensional flow visualization diagnostic to investigate nonreacting and reacting turbulent flows at atm* spheric pressure or above. The system will have sufficient temporal and spatial resolution both to freeze and to identify turbulent structures while providing quantitative measurements of concentration or temperature or both. The approach is to use multiangular absorption at optical wavelengths and tomographic computer t This work was sponsored by H Q AFESC/RDVS, Tyndall AFR, FL and The Aerospace Corp. Department of Defense review of this material does not imply DoD endorsement of factual accuracy or opinion. Copyright 0 American Institute of Aeronautics and Astronautics, Inc., 1990. All rights reserved. v 1 algorithms to reconstruct a cross section of the flow. Most efforts directed toward the development of a flow visualization diagnostic are based on laser-induced fluorescence. A general disadvantage of this technique is the rapid quenching of excited states resulting in qualitative data only. Other two-dimensional techniques under development (Raman and Rayleigh scattering) do not have this disadvantage, but produce only weak signals and are applicable only t o the most benign laboratory flows. Optical tomography was first suggested for the study of reacting flows i n 1980 by Goulard and Emmerman'. However, experimental implementation of the technique has been slow given theimportance of the problems that optical. tomograpy therefore, cylindrical symmetry was assumed to perform the reconstruction. Absorption experiments also have been undertaken at Stanford University where a fan beam geometry and rotating mirror were used t o create reconstructions of cold flowing Iz at 90 to 360 angles on 90 detectors using a cw AI+ I a s e ~ . ~ ~ This procedure allowed data to be acquired for reconstruction of concentrations in 100 ms. The data aquisition times of this method can he reduced by an order of magnitude. Synder and Hesselink7 demonstrated a novel configuration using holographic optical elements and a rotating mirror. In this proof-ofprinciple work, only one angle was measured, and the solid sample was spun t o create the multiple projection angles. Variations in the index of refraction can be used to measure density, and, indirectly, temperature fields using beam deflection tomography"" or interferometric t o m ~ g r a p h y . ' ~ ' ~ All the work cited in Refs. 8-14 used rotating mechanical elements and cw lasers, thus restricting the measurements to the millisecond time scale. This time scale is too long t o freeze turbulent cells in unsteady flows. Recently, a fast (300 ps) interferometric tomographic image was recorded on a cylindrically shaped piece of film." This demonstration was the first instantaneous optical tomographic reconstruction of a nonstationary fluid flow. Several implementations of optical emission tomography have been d e m ~ n s t r a t e d . ' ~ ~ " Reviews of optical tomography for applications to fluid flows are available.'a~'9 The instrument design presented here differs substantially from instri1ment.s used for previous experimental demonstrations of optical absorpt,ion tomography. It is capable of producing a high resolution image (100 angles, 100 elementsfangle) in a considerably shorter time interval t h a n the fastest instruments demonst.rated t o date (IO-'s versus IO-* s). This increase in performance is attainable because this configuration uses pulsed rat,lier t h a n C\7' radiation and has no moving parts. Furthermore, [,his design is easily scalable through its unique tiine iiiult iplcsing design to higher resolution, multi-spccies detect ion, greater dynamic range, or to a configuration tliat. allows the teinporal development of turbulent structure to be viewed through a series of fast snapshots.