High-temperature hypersonic Laval nozzle for non-LTE cavity ringdown spectroscopy
Abstract
A small dimension Laval nozzle connected to a compact high enthalpy source equipped with cavity ringdown spectroscopy (CRDS) is used to produce vibrationally hot and rotationally cold high-resolution infrared spectra of polyatomic molecules in the 1.67 mu m region. The Laval nozzle was machined in isostatic graphite, which is capable of withstanding high stagnation temperatures. It is characterized by a throat diameter of 2 mm and an exit diameter of 24 mm. It was designed to operate with argon heated up to 2000 K and to produce a quasi-unidirectional flow to reduce the Doppler effect responsible for line broadening. The hypersonic flow was characterized using computational fluid dynamics simulations, Pitot measurements, and CRDS. A Mach number evolving from 10 at the nozzle exit up to 18.3 before the occurrence of a first oblique shock wave was measured. Two different gases, carbon monoxide (CO) and methane (CH4), were used as test molecules. Vibrational (T-vib) and rotational (T-rot) temperatures were extracted from the recorded infrared spectrum, leading to T-vib = 1346 +/- 52 K and T-rot = 12 +/- 1 K for CO. A rotational temperature of 30 +/- 3 K was measured for CH4, while two vibrational temperatures were necessary to reproduce the observed intensities. The population distribution between vibrational polyads was correctly described with TvibI=894 +/- 47K, while the population distribution within a given polyad (namely, the dyad or the pentad) was modeled correctly by TvibII=54 +/- 4K, testifying to a more rapid vibrational relaxation between the vibrational energy levels constituting a polyad.
Domains
Physics [physics]
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Dudas et al-2020-High-temperature hypersonic Laval nozzle.pdf (4.47 Mo)
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