Rayleigh-Brillouin scattering in SF6 in the kinetic regime

Abstract

Rayleigh-Brillouin spectral profiles are measured with a laser-based scatterometry setup for a 90 degrees scattering angle at a high signal-to-noise ratio (r.m.s. noise below 0.15 % w.r.t. peak intensity) in sulphur-hexafluoride gas for pressures in the range 0.2 -5 bar and for a wavelength of λ = 403.0 nm. The high quality data are compared to a number of light scattering models in order to address the effects of rotational and vibrational relaxation. While the vibrational relaxation rate is so slow that vibration degrees of freedom remain frozen, rotations relax on time scales comparable to those of the density fluctuations. Therefore, the heat capacity, the thermal conductivity and the bulk viscosity are all frequency-dependent transport coefficients. This is relevant for the Tenti model that depends on the values chosen for these transport coefficients. This is not the case for the other two models considered: a kinetic model based on rough-sphere interactions, and a model based on fluctuating hydrodynamics. The deviations with the experiment are similar between the three different models, except for the hydrodynamic model at pressures p 2 bar. As all models are in line with the ideal gas law, we hypothesize the presence of real gas effects in the measured spectra.

Rayleigh–Brillouin light scattering spectroscopy of nitrous oxide (N2O)

“…While the spectral intensities scale with the familiar factor (n − 1) 2 in the Rayleigh scattering cross section, with n the index of refraction [26], the profiles are plotted on a normalized scale of unit area. In the following model calculations of the spectral profile are carried out, in the context of (i) the Tenti S6 model [17,18], (ii) the Grad's 6moment kinetic model optimizing a single relaxation time constant [19], (iii) a hydrodynamic model as developed in the past for methane gas [22], and (iv) a 'rough-spheres' model as recently developed for polyatomic molecules [25] and successfully applied to describe RB scattering in SF 6 gas [24]. Before making the comparison of the model data with the experi-…”

“…A rough-sphere model, proposed by Marques [25], was recently applied to describe the RB light scattering spectra in SF 6 gas [24], in which the molecules exhibit the structure of a regular octahedron with a sulfur atom in the center and six fluorine atoms at vertexes, hence taking the form of a spherical molecule. In such model a dimensionless moment of inertia κ is an important and uniquely adjustable parameter.…”

Extraction of the translational Eucken factor from light scattering by molecular gas

“…In recent RBS experiments (Wang et al 2017(Wang et al , 2018(Wang et al , 2019, the uncertainties in number density, temperature, scattering angle and wavelength are approximately 0.25 %, 0.03 %, 0.3 % and 0.25 %, respectively. Therefore, the induced uncertainty in Z and f tr is less than 2 %.…”

“…The experimental data of Wang et al (2017) for SF 6 molecules recorded at pressures 0.754, 1.002, 2.002, 3, 4 and 5 bar are compared with the Wu et al model, where the laser wavelength is 403 nm and the scattering angle is 89.6 • , so the effective wavelength is L = 286 nm. With the viscosity data given by Quinones-Cisneros, Huber & Deiters (2012) we find that the shear viscosity is μ s = 1.52 × 10 −5 kg m −1 s −1 and the viscosity index is ω = 0.885 at temperature 298 K, while the heat conductivity is κ = 1.30 × 10 −2 W m −1 K −1 .…”

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Figure 10.Same as figure 9, except the experimental data of measured by Wang et al. (2017) is used here. The parameters corresponding to the pressures 0.754, 1.002, 2.002, 3, 4 and 5 bar are , 1.64, 3.26, 4.88, 6.49 and 8.14, respectively.

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