Characterization of the liquid phase distribution in a minichannel during vertical downflow condensation: from experiments to modelling

Two-phase heat transfer in small diameter channels can be exploited in various applications, taking advantage of reduced volumes and lower refrigerant charge in refrigeration systems. For design purposes in practical applications, it is fundamental to experimentally determine the liquid phase distribution inside the channels and the implications in heat transfer. During downward annular flow, at small mass flux the heat transfer is mainly controlled by the thermal conduction through the film of condensate, i.e. by the thickness of the liquid film itself. With increasing mass flux, disturbance waves can appear at the liquid-vapor interface, enhancing turbulence and heat transfer and promoting the liquid film thickness reduction. Therefore, the knowledge of the liquid film thickness value is essential to better understand the heat transfer phenomena. The first purpose of this work is to assess available models for the prediction of film thickness, based on experimental data previously acquired with R245fa, R134a and HFE-7000 in downward annular flow condensation inside a 3.38 mm inner diameter tube, at mass flux between 30 and 150 kg m-2 s-1. In order to predict liquid film thickness with higher accuracy, two new correlations are proposed: one based on the combination of the physical models by Nusselt (1916) and Kosky and Staub (1971), and another obtained combining dimensionless groups. The first correlation has a mean absolute error below 13% for all three fluids analyzed in Padova, while it is below 16% for the second one. In the last part of the work, liquid film thickness of condensing R1233zd(E) inside a 3.38 mm inner diameter horizontal channel measured by means of confocal sensor and interferometer in microgravity conditions at G = 30-40 kg m-2 s-1, during the 84th ESA Parabolic Flight Campaign, has been analyzed and compared with data taken using HFE-7000 as working fluid, from a previous experimental investigation. It was found that R1233zd(E) displays higher average thickness than HFE-7000 in microgravity, particularly at G = 30 kg m-2 s-1. Experimental microgravity data were compared with the two models developed for vertical downflow, showing excellent agreement. This result confirms that the effect of gravity is negligible in the case of downflow condensation, under the operating conditions considered in the present Thesis.