Measurement of bulk viscosity of nitrogen based on spontaneous Rayleigh-Brillouin scattering

Abstract

Bulk viscosity is an important parameter to understand gas viscosity in micro perspective. The traditional ultrasound absorbtion method with acoustic frequencies in a megahertz range cannot be directly applied to high frequencies field, where acoustic waves are in the gigahertz domain. However, gas bulk viscosity at high frequency can be measured by spontaneous Rayleigh-Brillouin scattering (SRBS) and coherent Rayleigh-Brillouin scattering (CRBS). Recent researches show that the bulk viscosity of nitrogen measured by CRBS at a wavelength of 532 nm is obviously different from the values from SRBS in the near-ultraviolet region. In order to obtain accurate bulk viscosity of nitrogen at the wavelength of 532 nm, the SRBS spectra of nitrogen excited by a 532 nm laser are measured in a pressure range from 1 bar to 9 bar at the constant room temperature. The measured SRBS spectrum at the pressure of 7 bar is compared with the theoretical spectrum to obtain optimal scattering angle by using the principle of minimum value of χ2. The theoretical spectrum is calculated by convolving the Tenti S6 model with the instrument transmission function of measurement system. Given that the effect of pressure on the bulk viscosity is negligible, the bulk viscosity value (1.46±0.14)×10-5 kg·m-1-1 of nitrogen at a temperature of 299 K is acquired by averaging the values of bulk viscosity under different pressures (4-9 bar), each value is obtained by comparing the measured spectra at different pressures with the theoretical spectra by using the optimal scattering angle and the principle of minimum value of χ2. The values of bulk viscosity of nitrogen over the pressure of 1-3 bar are not considered because of its big deviation compared with the values under higher pressures (4-9 bar). The results show that the average value of bulk viscosity obtained in our experiment is close to that from the theoretical calculation and SRBS experiments reported in the literature but different obviously from the bulk viscosity obtained by CRBS. In order to testify the bulk viscosity of nitrogen measured in our experiment, it is used to retrieve temperature of nitrogen under pressure ranging from 1 bar to 9 bar. The results show that the absolute error between the retrieved temperature and the reference temperature under different pressures are all below 2.50 K and the difference between the average temperature and the reference temperature is less than 0.15 K. This demonstrates that the measured bulk viscosity of nitrogen in our experiment is accurate and reliable for the gas parameters retrieved by SRBS.