HEAT TRANSFER AND PRESSURE DROP OF NATURAL REFRIGERANTS IN MINICHANNELS (LOW CHARGE EQUIPMENT)

摘要

The paper presents a comprehensive overview of the most recent research works on heat transfer withrnnatural refrigerants in minichannels, aimed at proper design of heat transfer equipment.rnAbout boiling heat transfer, so far no predicting procedure finds wide acceptance either for HT coefficientsrnor for dryout conditions; experimental HT results are mainly fitted by empirical correlations referring to therncommon mechanisms used in more conventional geometries (nucleate and convective boiling); evaporationrnheat transfer through thin liquid film around vapour plugs is at times considered. About shear dominatedrncondensation heat transfer, suggested design tools again mostly refer to the extension of the semi-empiricalrncorrelations earlier established for conventional geometries. As two-phase heat transfer cannot be regardedrndisjointed from the associated frictional pressure drop, recent experimental results on this subject arernconsidered as well. For CO_2, heat transfer at supercritical conditions, such as in a gas cooler, is also treated.rnFinally the concept of the Penalty Factor is applied to shear condensation in minichannels to establish thernheat transfer performance of the different working fluids, the superior effectiveness of minitubes overrnmacrotubes, and the optimization of minichannel condensers.