11 Flow Visualization in Supersonic Flows
Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 13, 2017 | http://pubs.acs.org Publication Date: September 23, 1980 | doi: 10.1021/bk-1980-0134.ch011
N. L. RAPAGNANI and STEVEN J. DAVIS Chemical Laser Branch, Air Force Weapons Laboratory, Kirtland Air Force Base, Albuquerque, NM 87117
Since the invention of flowing chemical lasers, a persistent problem has been the difficulty of understanding the mixing phenomena so that accurate modeling of these devices could be accomplished. The system efficiencies are controlled in great part by the mixing and, i f one hopes to build a bigger and better device, a degree of understanding of the mixing is certainly needed. In recent years there has been a considerable amount of effort devoted to developing new nonintrusive techniques and also applying well-established techniques to interrogate these flow fields. Some of the methods which have been applied are chemiluminescence and Schlieren photography, Coherent Anti-Stokes Raman Scattering, and Laser Doppler Velocimetry. A l l of the above techniques have been used in an attempt to understand the mixing process and construct a map of the flow field in the laser. While useful, these techniques have some inherent problems and difficulties. What was needed was a fast and efficient method for obtaining mixing efficiency on new nozzle concepts. 1
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The recent introduction of laser induced fluorescence as a diagnostic has opened up a new f i e l d as a nonintrusive flow f i e l d d i a g n o s t i c - In this paper we describe how seeding of the flow f i e l d of these devices with I 2 , which was made to fluoresce by an argon ion (Ar+) laser, gave us information on the mixing process. Theory: There i s a fortuitous match between strong I2 absorption and the 51458 line of the Ar+ laser.*" This absorption i s from the v" » 0 level of the X ! state to the v - 43 level of the B r i o state. The absorption can be made specific to one or two rotational levels of the v" s 0 level by insertion of an intracavity etalon and forcing the Ar+ laser to o s c i l l a t e on a single longitudinal mode. The absorption process then becomes very e f f i c i e n t and the resulting fluorescence becomes much brighter. The v' s 43 level of B flo state fluoresces to a multitude of v" 1
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This chapter not subject to U.S. copyright. Published 1980 Ameiican Chemical Society
Crosley; Laser Probes for Combustion Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 13, 2017 | http://pubs.acs.org Publication Date: September 23, 1980 | doi: 10.1021/bk-1980-0134.ch011
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levels giving a characteristic yellow emission. The resulting fluorescence i s extremely intense when produced this way. In a typical run, the Ar+ laser was properly tuned by maximizing the fluorescence i n a sealed glass c e l l containing I2 vapor. The laser was stable enough that further tuning was not required. There are several advantages i n using I2 as the seed for LIF studies. F i r s t , the vapor pressure of I2 i s sufficiently high that I2 vapor (not crystals) can be easily entrained i n a He gas flow and carried into the flow f i e l d as a true molecular vapor. Thus, the I2 can be injected through extremely small o r i f i c e s . Secondly, the I fluorescence u t i l i z e d i n the present work extends to much longer wavelengths than the excitation source. Consequently, the LIF i s easily isolated from any scattered laser light by insertion of a long pass f i l t e r over the viewing port. Thirdly, the radiative lifetime of the relevant excited state i n I2 i s long enough to allow excited molecules to travel a significant distance (~1 cm) i n the flow direction i f the flow i s supersonic and pressure low enough. Thus, the v i s i b l e fluorescence w i l l persist downstream from the excitation source and one can track the flow f i e l d for a single excitation point. The v i s i b l e fluorescence was used to monitor the flow f i e l d and both black and white and color photographs of the flow f i e l d were obtained. Experiment: A 2 - l i t e r stainless steel vessel, p a r t i a l l y f i l l e d with I2 crystals, was connected directly to the secondary He feed supply line of a supersonic chemical laser. A complete description i s given i n a separate paper.- The laser cavity had viewing windows on top and bottom so that the nozzle array could be viewed i n a direction perpendicular to the optical axis. The pump laser was a Spectra-Physics Model 170-03 Ar+ laser equipped with an intracavity etalon. The Ar+ beam was directed into the chemical laser cavity along the optical axis by a focusing optical train. The spot size i n the cavity was a fraction of a millimeter, although tighter focusing could have been done, thus increasing the spatial resolution. The Ar+ beam could be translated i n two dimensions, up and down the nozzle face, at a single position i n the flow direction, and also downstream from the nozzle face. Hence, the flow f i e l d could be visually mapped out. The c o l l i s i o n free lifetime of the level of the 1^ B state i s on the order of a few microseconds. The pressure i n the laser cavity was estimated to be only ~lm torr. The He pressure was less than 5 torr for a l l runs. At these conditions neither self-quenching nor electronic quenching by He w i l l significantly alter the lifetime.- A lifetime of a few micro seconds i s nearly ideal to use as a tracer i n our supersonic flow fields i n which velocities of 10 cm/sec are typical. This means that the fluorescing I2 would "light up" a region down2
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Crosley; Laser Probes for Combustion Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
11.
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stream of the excitation point of ~1 cm. It i s important to note that the region of excitation i s defined by the spot size of the laser which i s variable as described above. Collisions with He w i l l , however, cause the originally excited V =43 level to be relaxed to lower V*. This causes the v i s i b l e fluorescence to become red shifted. He gas with the I2 vapor was injected into the flow f i e l d through the secondary nozzles. A complete description of the geometry i s given i n Figure 1. The Ar+ laser beam was positioned in the flow f i e l d and photographs were taken. The majority of photographs were taken with the laser operating i n the cold flow condition i n which no F2 was added i n the combustor. Hot flow conditions were imitated using N and He gases. Hot flow experi ments where I^ reacts with F to produce IF with subsequent LIF on IF w i l l be completed i n the coming year. Results and Discussions: Typical results of our effort are summarized i n Figure 2. The right portion of this photograph shows the v i s i b l e fluores cence from I2 (B-X) as viewed perpendicular to the optical axis through the top of the cavity. The l e f t portion (2b) of the picture emphasizes the extremely narrow region ( s l i t ) that i s fluorescing i n the optical axis direction. Figure 2b was obtained by means of looking through a dichrocic mirror down the optical axis of the chemical laser, collinear to the Ar+ beam. The fluorescence downstream from the Ar+ beam i s observed to be well defined and remains approximately the width of the Artbeam. In a l l these photographs, the Ar+ laser excited a region at the nozzle exit plane, one-half way up i n the v e r t i c a l direc tion. In both figures, the Ar+ pump beam can be seen inter secting the flow f i e l d with the fluorescence reflecting off the nozzle face. The f i r s t remark one can make about Photograph 2 i s that of flow nonuniformity. This particular nozzle bank has a severe nonuniformity problem and actually some of the secondary nozzles are not flowing. By redirecting the beam higher i n the cavity, the clogged nozzles were observed to flow much better. This demonstrated the high degree of spatial resolution since the nozzle was shown to have severe nonuniformities i n flow over small dimensions,
