Raman Thermometry of Confined Gas Milli-Flows

The temperature of rarefied gas flows inside small channels is hard to measure without disturbing the flow. We present here an optical approach with submillimeter spatial resolution, based on Raman spectroscopy, and several examples of its application to gas flows within millimeter-sized channels. For this work, we have designed and built several channels with different geometries, and forced thermal gradients between some of their surfaces while flowing N₂, CO₂ and H₂ through the channels. The pressures ranged from atmospheric down to 11 mbar, and the flow velocities ranged from 8 to over 450 cm/s. The most rarefied flows lie in the slip regime (Kn $\ge 0.001$). Raman spectra of the flowing gases were recorded inside the channels, and the intensities of the rotational lines were analyzed in terms of Boltzmann plots to retrieve the local gas temperature. It is demonstrated that Raman spectroscopy is able to measure the temperature profiles across dimensions as short as 1 mm inside the channels. The achieved precisions (1-$\sigma$ uncertainties) at 100 mbar are 0.8 K for CO₂, 2.0 K for N₂, and 3.7 K for H₂.