Turbulence in Microchannels

Rands C, Webb BW, Maynes D (2006) Characterization of transition to turbulence in microchannels. Int J Heat Mass Transfer 49 2924-2930 Ren L, Qu W, Li D (2001) Interfacial electrokinetic effects on liquid flow in micro-channels. Int J Heat Mass Transfer 44 3125-3134... 

For Reynolds numbers larger than the critical Reynolds number, the flow becomes turbulent and the fluid velocity at a point can be defined as the mean value of the velocity at this point with respect to time by taking a time interval to larger than the typical time of one turbulent fluctuation and shorter than the typical time of pressure variations. Since mass and momentum must be conserved in turbulent as well as in laminar flow, the conservation equations (Eq. 1) hold equally for turbulent flow, provided the velocities and pressures in these equations are interpreted as instantaneous velocities and pressures of the turbulent field. From a practical standpoint such equations are of little value because there is no interest in knowing the complete and complex history of instantaneous velocities and pressures but only their time-mean values that can be measured and observed (see also > Turbulence in Microchannels). In microchannels with hydraulic diameters less than 500 pm the maximum value that the Reynolds number can assume drops to between 10,000 and 30,000. In fact, in order to reach these values of the Reynolds number, a very large pressure difference... 

Turbulence in Microchannels, Fig. 1 (a) Rms velocity fluctuations and (b) Reynolds shear stress as measured by microPIV for turbulent flow in a 536 )im diameter glass microtube... 

Large-Scale Structures and Velocity Correlations The previously mentioned microPIV studies suggest that the turbulence in microchannels is statistically similar to flow in macroscale pipes and charmels, but it is also important to consider if the microscale flows are structurally similar to... 

Turbulence in Microchannels, Fig. 2 An instanta- A convective velocity of 91 % of the center line velocity... 

Turbulence in Microchannels, Table 1 A comparison of turbulent length scales for turbulent microchannel and macroscale channel flows ... 

While at the present time, these two studies are the only measurements of spatial correlations and turbulent length scales for microscale flows, the agreement observed in these measurements compared to large-scale pipe and channel flows suggest that the turbulence in microchannels and microtubes is structurally similar to the turbulence observed in similar macroscale flows. 

Rands C, Webb BW, Maynes D (2006) Characterization of transition to turbulence in microchannels. Int J Heat Mass Transf 49 2924-2930... 

As noted earlier, the absence of turbulence in microchannel flows means that the traditional means of enhancing heat transfer via the use of baffles or periodic roughness must be modified or avoided due to the very different construction methods in microscale systems. The flow channels must be designed in such a way that good mixing is caused by the geometry of the channels or some other means that disrupts the thermal boundary layer. 

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