Single-phase Convective Heat transfer in Laminar and Transitional Regime in Microchannels

In this paper the role played by the main scaling effects and micro-effects on the convective heat transfer for single-phase flows in microchannels, the micro-convection, are analyzed in a systematic way in order to evidence, starting from the governing equations, the main features of the forced convection in microchannels having hydraulic diameters less than 1 mm. No new governing equations are required to study micro-convection but the relative weight of the different terms can be very different going from macro to micro-scale: for example, in microtubes no free and mixed convection are evidenced in laminar regime also for large temperature differences between the fluid and the heated walls. The correlations proposed by Gnielinski for pure forced convection are presented and compared with a series of experimental data obtained by using water and FC-72 as working fluids. All the experimental Nusselt numbers have shown an agreement with the classical correlations for pure forced convection within the experimental uncertainty: this fact puts in evidence that it is possible to use the conventional correlations to study the microconvection if no scaling effects occur under the test operative conditions. It has been observed that in the transitional region the average Nusselt number exhibits an abrupt increase in correspondence of the critical Reynolds number. This behaviour is more pronounced for microtubes than for the conventional ones where the onset of the mixed convection reduces the step variation of the average Nusselt number when the flow regime changes.