The rules of single-phase forced convection in microchannels

This paper deals with the experimental analysis of forced micro-convection features of liquid and gas flows through microchannels. An overview of the main effects that tends to play an important role on the determination of the Nusselt numbers in microchannels is presented. Some experimental data obtained at the Microfluidics Lab of the University of Bologna together with the main results appeared recently in the open literature are used in order to highlight the characteristics of the convective heat transfer through microchannels with inner diameter from 0.75 mm down to and 0.15 mm. It is shown that the behavior of gas and liquid flows through microchannels in terms of convective heat transfer coefficients can be strongly affected by scaling and micro-effects as by practical issues linked to the geometry of the test rig, fittings, position of the sensors and so on. It is demonstrated that the comparison with the conventional correlations for the prediction of the convective heat transfer coefficients gives good results only if one has verified beforehand that the main scaling and micro-effects are negligible.