Thermodynamic Studies in Subsonic Gas Flows Using a Laser Diode Raman Spectrometer

Abstract

We present noninvasive, spatially resolved measurements on low-pressure, subsonic, laminar gas flows using Raman spectroscopy. From the Raman signal, the density and temperature of the flow can be extracted directly, with reasonable integration times (∼10 min-1 h). The use of a high-power laser diode makes it a very attractive low-cost, efficient technique. Temperatures studied range from 290 to 790 K; the spatial resolution was 2.5 × 1 mm² in the flow cross section, and 10–20 μm in the flow direction, within a flow profile of diam. ∼ 15 mm. The flow was generated by a conical double-nozzle system with CO₂ as the sample and Ar as the sheath gas. Vibrational and rotational state population and relaxation properties in the gas flow have been studied, and nonequilibrium conditions were found. A computational fluid dynamics (CFD) calculation is compared with the data; it supports the physical model suggested within the experimental errors.

Keywords

Raman spectroscopy Laser diodes Gas flows Relaxation processes

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