Gas flow and heat transfer CFD modeling in microchannels

During the last decade, the development of microfabrication technologies for the fabrication of Micro Electro-Mechanical Systems (MEMS) has led to the development of an increasing number of microfluidic technologies. However, the emergence of MEMS, has highlighted that the behavior of fluids in micro-channels is not necessarily the same as the one experienced at the macroscopic scale. This has led to numerous questions regarding the applicability of the predictions based on conventional assumptions for gas-phase microflows. Previous experiments on a thermal micromachine based on a Stirling cycle were carried out in our institute, and have highlighted thermal issues, high pressure drops and difficulties to understand oscillating flows at these microscales. In the literature, the effects of various parameters such as working fluid, cross-sectional geometry of duct, channel wall roughness and aspect ratio have already been studied for unidirectional permanent flows. However, fluid flow and heat transfer in oscillatory flows are still not yet mastered and require further research work.

Thus, the objective of the present study is to investigate numerically permanent and oscillating gas flows in microchannels with and without temperature gradient. The aim is to obtain reference data that will be compared to the correlations obtained in the literature, before being compared later on with experimental results. A three-dimensional model of microchannel is established and simulated using the commercial code ANSYS Fluent. The influences of working fluids, fluid temperature, geometrical parameters (such as hydraulic diameter, length of the channel and cross-sectional geometry of duct) are investigated, as well as the associated minor and major losses. The pressure drop data are used to characterize the friction factor over a range of aspect ratio from 0.024 to 1 and Reynolds number from 0.1 to 100 with Air, Helium, Hydrogen and Nitrogen as working fluids.

This numerical analysis will be completed by an experimental study with micro-channels that are currently being microfabricated in the MIMENTO cleanroom facility of the FEMTO-ST Institute. They will be instrumented with temperature and pressure sensors, manufactured and calibrated in our institute, that will provide local measurements inside the micro-channel. The experimental setup, that will allow both the studies for permanent and alternate gas flows, is currently under construction.