Turbulence in Microchannels (original) (raw)
Related papers
Characterization of transition to turbulence in microchannels
International Journal of Heat and Mass Transfer, 2006
This paper reports on an experimental study characterizing the laminar-turbulent transition for water flow in circular microtubes. Microtubes with diameters in the range 16.6-32.2 lm of varying length were employed over the Reynolds number range 300-3400. The volume flowrate was measured for an imposed pressure differential using a timed displacement technique. Additionally, the viscous heating-induced mean fluid temperature rise was measured. Two independent approaches were used to identify transition from laminar to turbulent flow. Both methods showed transition to occur in the Reynolds number range 2100-2500, consistent with macroscale tube flow behavior.
Piv analysis of turbulent flow in a micro-channel
2007
Turbulent flow of water in a short 0.4 mm high micro-channel of an emulsifier is investigated experimentally using a micro-PIV technique and compared with numerical predictions. The micro-flow measurements are based on epi-fluorescence illumination and high-speed imaging. Velocity fields obtained from the measurements and direct numerical simulations indicate that flow turbulization is delayed and develops only at the outlet region of the micro-channel.
Turbulent Flow in a Micro-Channel
ASME 4th International Conference on Nanochannels, Microchannels, and Minichannels, Parts A and B, 2006
Turbulent flow of water in a narrow gap of an emulsifier is investigated experimentally using micro-PIV (micro Particle Image Velocimetry) technique and compared with numerical predictions performed using the commercial code Fluent. The purpose of the investigations is to develop a procedure for well-controlled generation of mono-disperse suspension of micro droplets. These droplets will form a matrix for collection of nano-particles into well-structured configuration [1]. The micro-flow measurements are based on epi-fluorescence illumination and high-speed imaging. The experimental data are compared with the numerical results obtained using both turbulent and laminar flow models. It was found that, due to small channel dimensions and very small flow development length, the turbulent energy dissipation takes place mainly in the gap and shortly behind it. Very low amount of oil-phase fraction in investigated emulsions justifies us to use mean energy dissipation estimated for pure wat...
Journal of Micromechanics and Microengineering, 2003
Experimental observations of liquid microchannel flows are reviewed and results of computer experiments concerning channel entrance, wall slip, non-Newtonian fluid, surface roughness, viscous dissipation and turbulence effects on the friction factor are discussed. The experimental findings are classified into three groups. Group I emphasizes 'flow instabilities' and group II points out 'viscosity changes' as the causes of deviations from the conventional flow theory for macrochannels. Group III caters to studies that did not detect any measurable differences between micro- and macroscale fluid flow behaviors. Based on numerical friction factor analyses, the entrance effect should be taken into account for any microfluidic system. It is a function of channel length, aspect ratio and the Reynolds number. Non-Newtonian fluid flow effects are expected to be important for polymeric liquids and particle suspension flows. The wall slip effect is negligible for liquid flows in microconduits. Significant surface roughness effects are a function of the Darcy number, the Reynolds number and cross-sectional configurations. For relatively low Reynolds numbers, Re < 2000, onset to turbulence has to be considered important because of possible geometric non-uniformities, e.g., a contraction and/or bend at the inlet to the microchannel. Channel-size effect on viscous dissipation turns out to be important for conduits with Dh < 100 µm.
Examination of large-scale structures in turbulent microchannel flow
Experiments in Fluids, 2006
Microscopic particle image velocimetry was performed on turbulent flow in microchannels of various diameters and aspect ratios to evaluate the characteristics of large-scale turbulent structures. Spatial correlations of velocity fluctuations were measured along the channel centerlines and at four other locations, and characteristic turbulent length scales were defined. For square microchannels, excellent agreement was observed between the measured length scales and results for macro-scale duct flow. Along the centerline of the square microchannels the normalized longitudinal length scale, 2Lx uu /W, ranged from 0.30 to 0.37, the lateral length scale, 2Ly uu /W, ranged from 0.16 to 0.18, and the ratio between the two length scales, Lx uu /Ly uu ranged from 1.88 to 2.00, results which agree well with macroscale results. Results for non-square microchannels indicate that as aspect ratio increases, the ratio Lx uu /Ly uu also increases, ranging from 2.29 for an aspect ratio of 2.09 up to 3.75 for an aspect ratio of 5.68. Measurements were repeated at various distances from the side walls of the microchannels. For the square microchannels the turbulent structures are smaller near the side walls than near the center of the microchannel with 2Lx uu /W ranging from 0.30 to 0.38 along the centerline, but dropping to 0.04-0.06 at y/(W/2)=0.94. Similar results were observed for the rectangular microchannels. For the rectangular microchannels 2Lx uu /W ranged from 0.32 to 0.42, compared to 0.30-0.38 for the square microchannels.
Effects of compressibility and transition to turbulence on flow through microchannels
A detailed experimental study of flow through long microchannels of hydraulic diameter ranging from 60.5 to 211 lm has been carried out. The internal pressure distribution along the length of the channel has been measured to analyze the local flow behaviour. The effects of compressibility and transition to turbulence occurring in the microchannel flow were investigated in detail. In addition, the resulting flow has been analyzed numerically using a commercially available CFD code, FLUENT. It has been shown that there are no special micro-scale effects, including early transition to turbulence at least in the present range of hydraulic diameters after the significant effects of compressibility are accounted appropriately.
The effect of surface character on flows in microchannels
BioMEMS and Nanotechnology, 2004
A technique for quantifying velocity profiles of fluids flowing in circular microchannels is presented. The primary purpose of this technique is to provide a robust method for quantifying the effect of surface character on the bulk fluid behaviour. A laser-scanning confocal microscope has been used to obtain fluorescent particle images from a 1 micron thick plane along the centreline of hydrophobic and hydrophilic glass capillaries. The velocities of fluorescent particles being carried in pressure-driven laminar flow of a Newtonian fluid have been evaluated at the centreplane of 57.5 micron capillaries using a variation of particle tracking velocimetry (PTV). This work aims to clarify inconsistencies in previously reported [1-12] slip velocities observed in water over hydrophobically modified surfaces at micron and submicron lengthscales. A change in the velocity profile is observed for water flowing in hydrophobic capillaries, although the behaviour appears to be a result of an optical distortion at the fluid-wall interface. This may point to previous suggestions of a thin layer of air adsorbing to the surface. Notwithstanding, the results do not confidently suggest evidence of slip of water on hydrophobic surfaces in microchannels.
Developing of laminar fluid flow in rectangular microchannels
Two dimensional elliptic differential equations are solved numerically to investigate laminar fluid flow in rectangular microchannels. The model employs the Navier-Stokes equations with velocity slip at the wall boundary condition to simulate the flow behavior in microchannels. The numerical solution is obtained by discretizing the governing equations using the finite-volume technique. A numerical code was developed. The developed code is used to evaluate the effects of velocity slip, the flow rate and size of microchannel on the fluid flow in rectangular microchannels. The numerical simulations are done on a wide range of the Reynolds number (Re), the Knudsen number (Kn) for three different values of the width of microchannel. The results shows good agreement with previous published experimental data. The effects of rarefaction and flow rate on the flow behavior and the hydrodynamic developing fluid field are presented and discussed. The developing profile of velocity and pressure due to different Re and Kn, are shown. It is found that at a given Re, increasing the Knudsen number causes to decrease the dimensionless pressure along the microchannel. An increasing in the Knudsen number reduces the maximum velocity in the microchannels while the entrance length increases at any Re.