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

Abstract

Rarefied gas flows have a central role in microfluidic devices for many applications in various scientific fields. Local thermodynamic non-equilibrium at the wall-gas interface produces macroscopic effects, one of which is a velocity slip between the gas flow and the solid surface. Local experimental data able to shed light on this physical phenomenon are very limited in the literature. The molecular tagging velocimetry (MTV) could be a suitable technique for measuring velocity fields in gas micro flows. However, the implementation of this technique in the case of confined and rarefied gas flows is a difficult task: the reduced number of molecules in the system, which induces high diffusion, and the low concentration of the molecular tracer both drastically reduce the intensity and the duration of the exploitable signal for carrying out the velocity measures. This work demonstrates that the application of the 1D-MTV by direct phosphorescence to gas flows in the slip flow regime and in a rectangular long channel is, actually, possible. New experimental data on phosphorescence emission of acetone and diacetyl vapors at low pressures are presented. An analysis of the optimal excitation wavelength is carried out to maximize the intensity and the lifetime of the tracer emission. The experimental results demonstrate that a little concentration of about 5-10 % of acetone vapor excited at 310 nm or of diacetyl vapor excited at 410 nm in a helium mixture at pressures on the order of 1 kPa provides an intense and durable luminescent signal. In a 1-mm deep channel, a gas flow characterized by these thermodynamic conditions is in the slip flow regime. Moreover, numerical experiments based on DSMC simulations are carried out to demonstrate that an accurate measurement of the velocity profile in a laminar pressure-driven flow is possible for the rarefied conditions of interest.

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

“…The definitions of fluorescence and phosphorescence emissions are not based on the intensity and the duration of the emission but on the type of intramolecular electronic transition that makes an excited molecule come back to its ground electronic state [18]. An extensive analysis of the dependency of acetone phosphorescence intensity and lifetime on excitation wavelength and pressure is discussed in Fratantonio et al's research [15]. The laser beam diameter has been estimated from images of the fluorescence intensity distribution.…”

“…A higher number N i of averaged images can increase the quality of the resulting image by reducing the data fluctuations, and thus increases the signal-to-noise ratio (SNR). More information on the signal acquisition can be found in Fratantonio et al's research [15]. The uncertainty on the velocity measurement depends on the accuracy of the measured molecular displacement and the time separation between two images.…”

“…Micromachines 2020, 11, x 13 of 32 (e) Gaussian fitting per line: in order to track the molecular displacement at different times from the initial position, an appropriate criterion for identifying the tagged line profile is required. Previous analyses have shown that the best fitting function that represents the evolution in time of the light distribution is the Gaussian function [15]. Nevertheless, in a confined gas flow, the light distribution may be affected by the combined mechanisms of streamwise advection and spanwise molecular diffusion in the direction perpendicular to the wall, thus differing from the Gaussian distribution in a quiescent gas.…”

“…As expected, the slip velocity at the wall was very close to zero, with values ranging between 0.006 and 0.2 m/s. The diffusion coefficient employed for the velocity reconstruction has been estimated by means of the Blanc's law [15] and was set to 3.5 × 10 −5 , 3.3 × 10 −5 and 3.2 × 10 −5 m 2 /s, respectively, for the data at Δ = 10, 25, and 50 µ s. Since argon and acetone have molecular mass and kinetic diameter of the same order of magnitude, small variations of the tracer concentration in different experimental runs did not modify considerably the estimated diffusion coefficient used in the reconstruction. Moreover, preliminary tests showed that for these experimental conditions the resulting velocity profile was quite insensitive to the value of the diffusion coefficient used.…”

“…In this respect, a very important step forward was realized by experimentally characterizing the photoluminescence properties of the tracer molecules in terms of signal intensity and lifetime at low pressures. The experiments performed on acetone and diacetyl vapors demonstrated that MTV measurements in confined rarefied gas flows should be possible [15].…”