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

Abstract

Direct measurements of the slip velocity in rarefied gas flows produced by local thermodynamic non-equilibrium at the wall represent crucial information for the validation of existing theoretical and numerical models. In this work, molecular tagging velocimetry (MTV) by direct phosphorescence is applied to argon and helium flows at low pressures in a 1-mm deep channel. MTV has provided accurate measurements of the molecular displacement of the gas at average pressures of the order of 1 kPa. To the best of our knowledge, this work reports the very first flow visualizations of a gas in a confined domain and in the slip flow regime, with Knudsen numbers up to 0.014. MTV is cross-validated with mass flowrate measurements by the constant volume technique. The two diagnostic methods are applied simultaneously, and the measurements in terms of average velocity at the test section are in good agreement. Moreover, preliminary results of the slip velocity at the wall are computed from the MTV data by means of a reconstruction method.

Review of Optical Thermometry Techniques for Flows at the Microscale towards Their Applicability to Gas Microflows

“…Therefore, the lower acquisition time limit can be defined by the characteristic lifetime of the fluorescence (a few ns) or phosphorescence emission (a few ms). Furthermore, previous molecular tagging velocimetry works have demonstrated that enough spatial resolution can be achieved when measuring gradients of luminescent signals at the microscale in gases [162]. Another important point to be added is that the main technological barriers with respect to manufacturing techniques for obtaining leak tight fluidic chips for gas applications are being bypassed on a day-to-day basis, thus making possible the fabrication of new interesting devices where thermography measurements can be performed.…”

Editorial for the Special Issue “Selected Papers from the ISTEGIM’19—Thermal Effects in Gas Flow in Microscale”

“…Advantages and disadvantages of the current state-of-the-art approaches are analyzed in that review and guidelines are proposed for the selection of a suitable measuring system according the application. In Fratantonio et al [ 4 ] molecular tagging velocimetry (MTV) by direct phosphorescence, another non-invasive optical measurement technique, is applied to argon and helium rarefied flows in a millimetric channel, providing the first flow visualizations ever reported of a confined gas flow in the slip regime (for Knudsen numbers down to 0.014). Despite the new challenges encountered in carrying out local velocity measurements in gas flows characterized by high molecular diffusion, this work demonstrated that MTV is currently the most promising technique for providing direct measurements of the slip velocity at the wall in rarefied gas flows.…”