The characteristics of coherent structures in the rapid expansion flow of the supercritical carbon dioxide

Abstract

By the use of a tiny pressure-transferring probe and capacitor-type sensor, the dynamic pressure signals in the jet flow of the rapid expansion of supercritical carbon dioxide are measured, which distinctively shows the characteristics of quasi-periodical coherent structures. After Fast Fourier Transform (FFT) conversion of the time series signals of the dynamic pressure, there exist three dominant frequency bands in the power spectrum, which correlate with the scale of the dominant eddies in the turbulent field. The dominant frequencies change little with the distance from the nozzle exit or the pre-expansion pressure, while the power density of the dominant frequencies, which correlates with the energy of the dominant eddies in the turbulence, attenuates along the axial direction and with the decrease of the pre-expansion pressure. Through analysis, it is inferred that the nozzle structure and initial conditions remarkably affect the coherent structures in the expansion flow which should be the important factors in the particle nucleation and its growth process in ultra-fine particle preparation by rapid expansion of supercritical fluid solution (RESS).