Heat transfer in microchannels

In the last years a large amount of experimental analyses have been addressed to the study of the fluid-dynamical and heat transfer characteristics of single-phase flows in micro channels. These reviews show clearly how the results are far from univocal, sometimes agreeing with the classical correlations, at other times contradicting them. In this paper the main aspects which can be responsible for these deviations will be analysed and discussed. From a chronological analysis of these experimental works it is possible to note that the discrepancy between the experimental obtained for micro channels and the predictions of the conventional theory is decreasing. This fact can be partially explained by considering the dramatic improvements in micro fabrication techniques with the consequent more appropriate control of the roughness and geometry of the micro channel and with the increase in the accuracy of the experimental tests made. For this reason the results of the older studies are sometimes fraught with errors and can be misleading. In this work the role played by the main scaling effects and micro-effects on the pressure drop and on the convective heat transfer for single-phase flows in micro channels will be analysed with the aim to explain that many experimental results published in the open literature can be justified by using the conventional theory. The tremendous progress over the past five years in sophisticated measurement techniques and analytical methodologies in the area of micro scale heat transfer have greatly added to the understanding of the phenomena unique to micro channel flow and has helped to clarify those situations where conventional theoretical models can be used to accurately describe and predict the flow and heat transfer behavior in micro channel flows. Presented here is a review of the published literature along with discussions of several ongoing research projects involving micro channel flow and heat transfer. Theoretical and experimental studies, computational analyses, a description of some of the more innovative instrumentation techniques, and important findings from all of these are presented, compared and discussed. Key Words: Microchannels, Minichannels, Phase Change Heat Transfer, Pressure Drop, Single-Phase, Two-Phase, Convection, Scaling Effects, Micro Effects.