Condensation in minichannels is widely used in air-cooled condensers for the automotive and air-conditioning industry, in heat pipes and other applications for cooling electronics. The knowledge of pressure drops in such small channels is important in order to optimize heat transfer surfaces. Most of the available experimental work refers to measurements obtained within multiport smooth extruded tubes and deal with the average values over the number of parallel channels. In this context, the present authors have set up a new test apparatus for heat transfer and fluid flow studies in single minichannels. This paper presents new experimental frictional pressure gradient data, relative to single-phase flow and adiabatic two-phase flow of R134a inside a single horizontal mini tube, with 0.96 mm inner diameter and with not-negligible surface roughness. The new all-liquid and all-vapour data are successfully compared against predictions of single-phase flow models. Also the two-phase flow data are compared against Cavallini et al. [1, 2] model, valid for adiabatic flow or for flow during condensation of halogenated refrigerants inside smooth minichannels. Surface roughness effects on the liquid-vapour flow are discussed. In this respect, the friction factor in the proposed model is modified, in order to take into consideration also effects due to wall roughness.

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