The objective of the present paper is to provide a general overview of the research carried out so far in single-phase heat transfer and flow in capillary (micro) pipes. Laminar flow and laminar-to-turbulent flow transition are analyzed in detail in order to clarify the discrepancies among the results obtained by different researchers. Experiments performed in the ENEA laboratory indicate that in laminar flow regime the friction factor is in good agreement with the Hagen-Poiseuille theory for Reynolds number below 600–800. For higher values of Reynolds number, experimental data depart from the Hagen-Poiseuille law to the side of higher f values. The transition from laminar-to-turbulent flow occurs for Reynolds number in the range 1800–2500. Heat transfer experiments show that heat transfer correlations in laminar and turbulent regimes, developed for conventional (macro) tubes, are not properly adequate for heat transfer rate prediction in microtubes.

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