Unsteady flows in milli- and microsystems: analysis of wall shear rate fluctuations

Abstract

To cite this version:Abstract The particular benefits of microfluidic systems, in terms of heat and mass transfer enhancement, require conducting local flow diagnostics, especially when unsteady properties of the microflow can play a critical role at the reaction interface, as currently observed in the fields of bioengineering and chemical engineering. The present paper focuses on unsteady confined flows within microsystems characterized by various geometries of crossing channels and exhibiting high surface-to-volume ratios. An experimental analysis of the signal measured at microsensors embedded to the wall of microsystems is discussed. In the objective of performing flow diagnostics, including regime identification and wall flow structure recognition, two methods for electrochemical signal processing are investigated and compared within an experimental network of crossing minichannels. One method is based on the use of a transfer function, while the other, the so-called Sobolik solution (Sobolik et al. in Coll Czech Chem Commun 52:913-928, 1987), consists of finding a direct solution to the mass balance equation. Sobolik's method has been selected given its ability to provide a description, over a wide range of Reynolds numbers (317 \ Re \ 3,535), for all wall shear rate fluctuations, as well as for the associated mixing scales in the power spectra density (PSD). This technique is then applied to flow within micromixers composed of two crossing microchannels in order to study highly unsteady and inhomogeneous microflows. The hydraulic diameters of the studied channels are 500 and 833 lm, respectively. Two flow patterns are investigated herein: the crossing-flow type and the impinging flow (or so called co-flow) for a Reynolds number range between 173 and 3,356. The PSD of wall shear rate fluctuations reveals various flow characteristics depending on the microchannel aspect ratio.

A revisit of the electro-diffusional theory for the wall shear stress measurement

“…It is based on the assumption of similarity of the concentration fields over the probe surface under steady and unsteady flow conditions. This method was verified numerically for small amplitudes of the fluctuation rate [22] and experimentally at higher amplitudes of the fluctuation rate [23] showing its relevancy in turbulent flows compared to the transfer function.…”

The frequency response of rectangular ducts

“…Recently, rectangular ducts are studied again in relation to microchannels in microfluidic devices and the cooling of micro-electronic devices. 15,16 Basic equations The velocity distribution Figure 1 illustrates the rectangular ducts that will be considered in this article. Figure 2 shows cross sections of circular, square, and rectangular ducts whose flow rates are equal when the fluid is the same, and the steady-state pressure gradients along the ducts are equal.…”

“…Let us find an explicit formula giving Q. Referring to equations (13) to (16), the partial flow rate due to the velocity component W n is…”

“…Recently, rectangular ducts are studied again in relation to microchannels in micro-fluidic devices and the cooling of micro-electronic devices. 15,16…”

The electrodiffusional theory for the wall shear stress measurement by two-strip probe

“…In general, the importance of this interaction on the overall behavior of the system grows as the specific surface area increases. For this reason, it is necessary to deal seriously with contact between wall and fluid, especially in microfluidic devices [2], porous structures [3], or reactors containing packed beds [4] or structured layers [5].…”