Molecular tagging velocimetry by direct phosphorescence in gas microflows: Correction of Taylor dispersion

Abstract

Molecular tagging velocimetry is a little-intrusive technique based on the properties of specific molecules able to emit luminescence once properly excited. Several variants of this technique have been successfully developed for analyzing external gas flows or internal gas flows in large systems. There is, however, very few experimental data on molecular tagging velocimetry for gas flows in mini or microsystems, and these data are strongly affected by the molecular diffusion of the tracer molecules. In the present paper, it is demonstrated that the velocity field in gas microflows cannot be directly deduced from the measured displacement field without taking into account Taylor dispersion effects. For that purpose, the benchmark case of a Poiseuille gas flow through a rectangular channel 960 µm in depth is experimentally investigated by micro molecular tagging velocimetry using acetone vapor as a molecular tracer. An appropriate reconstruction method based on the advection diffusion equation is used to process the data and to correctly extract the velocity profiles. The comparison of measured velocities with flowrate data and with theoretical velocity profiles shows a good agreement, whereas when the reconstruction method is not implemented, the extracted velocity field exhibits qualitative and quantitative anomalies, such as a non-physical slip at the walls. The robustness of the reconstruction method is demonstrated on flows with light and heavy molecular species, namely helium and argon, at atmospheric as well as at 2 lower pressure conditions, for which diffusion effects are stronger. The obtained results represent an encouraging step for future analysis of rarefied gas microflows.

Velocity Measurements in Channel Gas Flows in the Slip Regime by means of Molecular Tagging Velocimetry

“…However, the image data revealed that these effects are negligible, and the Gaussian function is still representative of the light distribution in the gas flow direction. As it has been done in previous works [12,14,20], the Gaussian fitting applied to each horizontal line of pixels at any position y = y j is…”

“…In this section, the MTV technique was applied to acetone-seeded argon flows in non-rarefied conditions. The previous works of Samouda et al [12] and Si-Hadj Mohand et al [14] already demonstrated the successful application of MTV for velocity measurements in channel gas flow in non-rarefied conditions by using as a tracer acetone vapor excited at 266 nm. Si Hadj Mohand et al were not able to apply MTV for pressures lower than about 42 kPa because of the low signal intensity due to the limited phosphorescence quantum yield of acetone excited at 266 nm.…”

“…Later on, the method was proven to be valid also in the case of physical MTV experiments in confined non-rarefied flows. The experiments were performed in a millimetric channel at atmospheric and subatmospheric pressures down to a minimum average pressure of 42 kPa [14]. At this pressure level in a 1-mm deep channel, the flow was still in the continuum regime.…”

Molecular tagging velocimetry for confined rarefied gas flows: Phosphorescence emission measurements at low pressure

“…1D-MTV by direct phosphorescence has already been applied to non-rarefied external supersonic or hypersonic turbulent gas flows [23,24], to rarefied supersonic jets [25] and to non-rarefied gas flows in millimetric channels [26,27]. However, some difficulties have prevented until now the successful application of this technique to confined rarefied gas flows.…”

“…A reconstruction method of the velocity profile from the displacement profile was developed and validated with numerical experiments. Si Hadj Mohand et al [27] successfully applied this reconstruction method on MTV data in a millimetric channel and correctly extracted the velocity profile at atmospheric and sub-atmospheric pressures down to a minimum average pressure of 50 kPa. At this pressure level in a 1-mm deep channel, the flow is still in a nonrarefied regime.…”