Atomic Fluorescence Study of High Temperature Aerodynamic Levitation
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ATOMIC FLUORESCENCE STUDY OF HIGH TEMPERATURE AERODYNAMIC LEVITATION
PAUL C. NORDINE, ROBERT A. SCHIFFMAN AND D. S. SETHI* Department of Chemical Engineering, Yale University, New Haven,
CT 06520
ABSTRACT Ultraviolet laser induced atomic fluorescence has been used to characterize supersonic jet aerodynamic levitation experiments. The levitated specimen was a 0.4 cm sapphire sphere that was separately heated at temperatures up to 2327K by an infrared laser. The supersonic jet expansion and thermal gradients in the specimen wake were studied by measuring spatial variations in the concentration of atomic Hg added to the levitating argon gas stream. Further applications of atomic fluorescence in containerless experiments, such as ideal gas fluorescence thermometry and containerless process control are discussed.
INTRODUCTION Laser induced atomic fluorescence is a tool for investigating high temperature properties that is applicable to containerless systems. In this work, fluorescence measurements of atomic Hg concentrations and continuous wave CO2 laser heating of aerodynamically levitated sapphire spheres are used to study levitation jet behavior and thermal gradients in the hot specimen wake. The purposes of these experiments are to (i) develop a method to measure temperatures of non-opaque specimens or opaque materials whose emittances are not known and (ii) study vaporization processes in the absence of specimencontainer interactions. If similar experiments were carried out in a reduced gravity enviornment, where convective mass and energy transport may be eliminated, accurate investigation of transport properties and chemical reaction kinetics should also be possible. The fluorescence technique may also be used to measure various process control variables in containerless circumstances. The basic concept in the present use of laser induced fluorescence is that a laser beam, whose bandwidth includes a doppler and pressure broadened atomic absorption line will, in the absence of quenching processes, produce fluorescence whose intensity is proportional to concentration. Mercury vapor is the convenient atomic fluorescent species of choice for these first experiments. It may be easily added in appropriate concentrations to the levitating gas, yields fluorescent radiation in the ultraviolet (253.65 nm) where interference from incandescent radiation is negligible, and its ionization energy is sufficiently large that an insignificant fraction of the atomic species is ionized at the melting point of Al 203 (2327K). The experimental apparatus has a sensitivity to Hg concentration well below that which requires corrections for selfabsorption of the emitted fluorescent radiation. A disadvantage of the use of Hg in Ar is that heavy atom inertial separation effects[4] and thermal diffusion [5] greatly influence the relation between concentration and gas temperature, which would otherwise be given by the ideal gas law. Thus, we cannot deduce gas and specimen temperatures from concentration measure
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