High-temperature hypersonic Laval nozzle for non-LTE cavity ringdown spectroscopy

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 µm 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 T_vib ^I=894±47 K, while the population distribution within a given polyad (namely, the dyad or the pentad) was modeled correctly by T_vib ^II=54±4 K, testifying to a more rapid vibrational relaxation between the vibrational energy levels constituting a polyad.