Microengineering and Microtechnology

If the characteristic linear dimension of the flow field is small enough, then the measured hydrodynamic data differ from those predicted by the Navier-Stokes equations [79]. With respect to the value in macrocharmels, in microchannels (around 50 microns of section) (i) the friction factor is about 20-30% lower, (ii) the critical Reynolds number below which the flow remains laminar is lower (e.g., the change to turbulent flow occurs at lower linear velocities) and (iii) the Nusselt number, for example, heat transfer characteristics, is quite different [80]. The Nusselt number for the microchannel is lower than the conventional value when the flow rate is small. As the flow rate through the microchannel is increased, the Nusselt number significantly increases and exceeds the value for the fully developed flow in the conventional channel. These effects have been investigated extensively in relation to the development of more efficient cooling devices for electronic applications, but have clear implications also for chemical applications. 

New reaction pathways considered too difficult for application in conventional equipments could be pursued because, if the microreactor fails, the small amount of chemicals released accidently could be easily contained. The high wall to volume ratio 

In the case of multiphase reactions, such as those involving gas-liquid, gas-liquid-solid and gas-liquid-liquid systems, microreaction technology is still in an early stage of development with respect to single-phase applications. However, this is also a rapidly developing area, but the fluidodynamics in microchannels have to be better understood. 

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