Christine Barrot

Analysis of Gaseous Flows in Minichannels by Molecular Tagging Velocimetry

The Molecular Tagging Velocimetry (MTV) technique has been widely used for analyzing velocity fields in liquid mini- and microflows. Concerning gaseous flows, only few works describe the implementation of MTV at millimetric scale, and these studies are limited to the analysis of external flows, such as jet flows. The goal of the present work is to develop this technique for the analysis of internal gas flows in minichannels. It is a first step toward the visualization of velocity profiles in rarefied conditions, and direct measurement of velocity slip at the walls.A specific experimental setup has been designed. Its features are detailed. Velocity profiles are obtained in a pressure driven steady flow of argon through a long rectangular minichannel of 1.2 × 5 mm2 cross-section and 15 cm length using acetone molecules as tracer. Experiments are carried out at atmospheric pressure, in a laminar continuum flow regime. The accuracy of the method is discussed by comparison between experimental and theoretical velocity profiles.The potential of the MTV technique for analyzing mini or micro gaseous internal flows is commented on. Perspectives of the work for discussing the validity of boundary conditions in the slip flow regime are presented.Copyright

Quantitative measurement of gas pressure drop along T-shaped micro channels by interferometry

The study of gas flows in microchannels has received considerably more attention in the literature from a simulation perspective than an experimental. The majority of the experimental work has emphasis on the global measurements at the inlet or exit of the microchannel instead locally along it. In this paper some efforts were made to measure the pressure drop along T-shaped micro channel by using interferometry. The two side channels were served as gas entrances and they were both open to air and the channel outlet was being vacuumed during experiments. A Mach-Zehnder interference microscopy was built for the measurement of gas pressure drop along the mixing channel. Some points along the mixing channel were selected for interferometric measurements. Simulations were first developed in unsteady condition by using Ansys Fluent to verify the nonexistence of transient phenomena of gas flow in the defined condition and then run again in steady condition to get the theoretical pressure drop that was would be used for comparison with experimental results.

Design of Tree-Shaped Microchannel Networks Submitted to Simultaneous Pressure Driven and Electro-Osmotic Flows

Microchannel networks can be efficiently used for several applications. For example, they can be the main elements of micro chemical reactors or micro heat exchangers for cooling electronic chips. In such networks, the flow of liquid can be generated either by a pressure difference, by electro-osmosis or by both of them. The design of the network can be optimized in order to deliver a maximum flowrate. In this paper, an analytical study of a pressure driven and electro-osmotic flow in tree-shaped microchannel network is developed. The network is built with a series of rectangular microchannels with high aspect ratio. Each bifurcation connects a parent microchannel to a couple of twin child microchannels. The objective of this work is to determine the geometrical configuration which offers the highest flowrate. The efficiency of the tree-shaped network is compared to the efficiency of a series of parallel microchannels, for the same inlet and outlet values of electric potential and pressure and for the same network volume. Focusing on one bifurcation, the influence of the thickness of the electrical double layer is discussed. The optimal geometric dimensions, such as the ratio of the child over parent microchannel widths and the ratio of the parent over total microchannel lengths, are calculated. The influence of the number of bifurcations is also analyzed and optimal design rules are proposed.Copyright

Gas Mass Flow Rate Measurement in T-Shaped Microchannels in Slip Flow Regime

A new setup was developed for gas mixing analysis in T-shaped microchannels. The principle of the flow rate measurement was based on the Constant Volume (CV) method [1]. The mass flow rate measurements of two gases N2 / CO2 mixing in a T mixer were carried out in the slip flow regime and followed by a simulation work for comparison. The mass flow rate has a magnitude of 10−8 or 10−7 kg/s and has good agreement with simulation for the lowest inlet over outlet pressures ratios and moderate agreement for the highest inlet over outlet pressures ratios.Copyright

Electroosmotic Flow in Tree-Shaped Microchannel Networks

An analytical model of electroosmotic flow in tree-shaped microchannel networks is developed. The aim of the study is to determine the best network architecture to maximize the electroosmotic flowrate for a given electric field and a given total microchannel volume. The network consists of rectangular microchannels with high aspect ratio. The paper shows under what conditions the tree structure offers a higher flowrate than a series of parallel microchannels. The influence of the electric double layer (EDL) thickness is pointed out. As long as the EDL thickness is negligible compared with the microchannel width, the tree-shaped architecture does not present any particular interest. But as soon as the EDL thickness is significant, it is shown that the flowrate can be largely enhanced by increasing the number of bifurcations in the tree-shaped network. Two configurations of bifurcations (V-shaped and U-shaped) are considered and compared. Each bifurcation is composed of a parent microchannel connected to two identical daughter microchannels. The optimal value of the daughter over parent microchannels widths is calculated. The influence of the daughter over parent microchannels lengths and of the number of bifurcations is pointed out. Guidelines for the design of tree-shaped microchannel networks are finally proposed.Copyright