Heat transfer and pressure drop of natural refrigerants in minichannels (low charge equipment)

The paper presents a comprehensive overview of the most recent research works on heat transfer with natural refrigerants in minichannels, aimed at proper design of heat transfer equipment. About boiling heat transfer, so far no predicting procedure finds wide acceptance either for HT coefficients or for dryout conditions; experimental HT results are mainly fitted by empirical correlations referring to the common mechanisms used in more conventional geometries (nucleate and convective boiling); evaporation heat transfer through thin liquid film around vapour plugs is at times considered. About shear dominated condensation heat transfer, suggested design tools again mostly refer to the extension of the semi-empirical correlations earlier established for conventional geometries. As two-phase heat transfer cannot be regarded disjointed from the associated frictional pressure drop, recent experimental results on this subject are considered as well. For CO2, heat transfer at supercritical conditions, such as in a gas cooler, is also treated. Finally the concept of the Penalty Factor is applied to shear condensation in minichannels to establish the heat transfer performance of the different working fluids, the superior effectiveness of minitubes over macrotubes, and the optimization of minichannel condensers.