Coherent Anti-Stokes Raman Spectroscopic Measurement of Air Entrainment in Argon Plasma Jets
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COHERENT ANTI-STOKES RAMAN SPECTROSCOPIC MEASUREMENT OF AIR ENTRAINMENT IN ARGON PLASMA JETS J. R. Fincke, R. Rodriguez, C. G. Pentecost Idaho National Engineering Laboratory EG&G Idaho, Inc., Idaho Falls, ID
ABSTRACT
The concentration and temperature of air entrained into an argon plasma jet have been measured using coherent anti-Stokes Raman spectroscopy (CARS). The flow field is characterized by a short region of well behaved laminar flow near the nozzle exit followed by an abrupt transition to turbulence. Once the transition to turbulence occurs, air is rapidly entrained into the jet core. The location of the transition region is determined by the rapid cooling of the jet and the resulting increase in Reynolds number. INTRODUCTION In the plasma spray fabrication of coatings the composition of the surroundings into which the plasma jet flows is known to exert considerable influence on the behavior of the jet and can alter the characteristics and quality of the coatings produced [1]. For example, while the plasma spraying of oxides is successfully conducted in oxygen containing environments, tungsten carbide-cobalt, when similarily sprayed thermally decomposes via oxidation of the carbides leading to inferior, carbon deficient coatings [2,3]. The presence of oxygen is due to entrainment of the surrounding atmosphere into the plasma flow. The development of spray processes requires a more detailed understanding of entrainment mechanisms, the influence of the surrounding atmosphere on both the flow field and on the behavior of reactive particles. As an initial step in understanding this process we have begun to investigate the entrainment of the surrounding atmosphere into plasma jets using coherent anti-Stokes Raman spectroscopy (CARS). This technique is applicable to the measurement of the concentration of any Raman active species, including 02, N2 , and CO. In the work described here we have chosen N2 as the species to be probed. The CARS spectrum of N2 is well known, and the dissociation and ionization characteristics result in non-depleted populations of neutral, molecular nitrogen at temperatures in excess of 6000 K. The initial results on an argon plasma, issuing into a stagnant atmospheric pressure air environment, are presented here.
Mat. Res. Soc. Symp. Proc. Vol. 190. 01991 Materials Research Society
185
MEASUREMENT TECHNIQUE Non-intrusive optical diagnostic techniques such as CARS have become standard tools in high temperature flow-field research. CARS has the advantage of high conversion efficiency, a laser-like coherent signal beam for high collection efficiency, excellent fluorescence and luminosity discrimination, and high spatial and temporal resolution. The theory of CARS and its application as a combustion diagnostic tool has been detailed in several reviews [4-6]. A CARS signal, Figure 1, is generated when two laser beams at frequency wl, (termed the pump beams) and one laser beam at frequency w2 (termed the Stokes beam) interact through the third-order nonlinear susceptibility of the medi
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