Main issues on the experimental analysis of forced convection in microchannels and micro heat exchangers

A large amount of papers dealing with the measurement of the friction factor and convective heat transfer coefficients for single phase flows in microchannels appeared in the scientific literature during the last twenty years present incorrect and/or inaccurate results in complete disagreement with the conventional correlations. In many cases, once demonstrated that the data were not in agreement with the classical predictions of the conventional theory, new “specific” correlations have been proposed for friction factors and/or Nusselt numbers by avoiding a theoretical justification of the new correlations proposed. Unfortunately, more than 90% of the correlations proposed for microchannels were not able to explain the further data obtained by other researchers and they were in clear disagreement with each to other. The diffusion of many correlations for the prediction of friction factor and of Nusselt number in microchannels and micro heat exchangers has lead to confusion in this field and this trend has delayed, more or less, of 10 years the complete understanding of these phenomena. In this lecture a critical analysis of the main features of forced micro-convection of liquid and gas flows through microchannels and micro heat exchangers is given. A critical overview of the main effects (scaling and micro effects) which tend to play an important role in the determination of Nusselt number in microdevices is presented. In addition, a systematic review of the specific measurement techniques proposed for the analysis of micro-convection is given with the aim to put in evidence the main advantages and limitations of each one. Experimental data appeared in the open literature both for liquids and gases are used in order to highlight the peculiar characteristics of the convective heat transfer through microchannels and micro heat exchangers and to suggest the guidelines for a physically-based interpretation to the experimental results which takes into account the intrinsic limitations of the measurement techniques available for this kind of analysis.