Proposed Design for Simultaneous Measurement of Wall and Near-wall Temperatures in Gas Microflows

Abstract

Gas behavior in systems at microscale has been receiving significant attention from researchers in the last two decades [1-4]. Today, there is an enhanced emphasis on developing new experimental techniques to capture the local temperature profiles in gases at rarefied conditions. The main underlying reason behind this focus is the interesting physics exhibited by gases at these rarefied conditions, especially in the transition regime. There is the onset of local thermodynamic disequilibrium, which manifests as velocity slip and temperature jump [1-4] at the wall. However, there is limited experimental evidence on understanding these aforementioned phenomena. With the advances in experimental facilities, it is today possible, at least in principle, to map the local temperature profiles in gases at rarefied conditions. Molecular tagging approach is one such technique which has shown the potential to map the temperature profile in low pressure conditions [5]. In molecular tagging approach, a very small percentage of tracer molecules are introduced into the gas of interest, referred as carrier gas. In gas flow studies, the typical tracers employed are acetone and biacetyl. These tracer molecules, assumed to be in equilibrium with the carrier gas, are excited with a source of energy at a specific wavelength, typically a laser. The excited molecules are unstable and tend to de-excite in a radiative and non-radiative manner, which is manifested as fluorescence and phosphorescence. Following the deformation with time of a tagged line permits to obtain the flow velocity. In addition, the dependence of the phosphorescence and fluorescence intensity to the gas temperature could also allow to use this technique for local temperature measurements. The objective of this study is to develop an experimental setup capable of simultaneously mapping the wall and fluid near-wall temperatures with the final goal to measure temperature jump at the wall when rarefied conditions are reached. The originality of this setup shown in Figure 1 is to couple surface temperature measurements using an infrared camera with Molecular Tagging Thermometry (MTT) for gas temperature measurements. The bottom wall of the channel will be made of Sapphire substrate of 650 $\mu$m thickness coated with a thin film of Indium Tin Oxide (ITO). The average roughness of this ITO layer is about 3 nm. The top wall of the channel will be made of SU8 and bonded with the bottom wall with a layer of PDMS. The channel will be filled in with acetone vapor,