Experimental analysis and modelling of R1233zd(E) condensation heat transfer and pressure drop in small diameter channels

Hydrofluoroolefins such as R1234yf, R1234ze(E) and hydrochlorofluoroolefin R1233zd(E) were suggested as replacements for hydrofluorocarbons in refrigeration and air-conditioning applications. In particular, R1233zd(E) has been indicated for the substitution of R123 and R245fa, since it displays a much lower value of global warming potential but a similar saturation curve. R1233zd(E) is also adopted as a replacement for R245fa in organic Rankine cycles. In the present study the condensation process of R1233zd(E) has been investigated in small channels with internal diameter lower than 4 mm. Condensation experiments have been conducted at 40 °C saturation temperature and mass flux ranging from 30 kg m^-2 s^-1 to 400 kg m^-2 s^-1. Therefore, this study holds particular significance for the design of condensers both in direct Rankine cycles and inverse vapour compression cycles. The adoption of channels with small diameter is intended for reducing the refrigerant charge. The present study is particularly focused onto condensation occurring at reduced mass flux, at 100 kg m^-2 s^-1 and below. Experimental investigations at these mass fluxes are relatively scarce, despite their significant relevance considering that predicting tools are less accurate at these working conditions. The present experimental activity, besides the measurement of the heat transfer coefficients and two-phase frictional pressure drop, includes the visualization of the flow patterns, in a 3.38 mm inner diameter tube. The detected flow regimes have been used to assess a modified flow pattern map and discuss the predictions of three semi-empirical models. Since R1233zd(E) is a low-pressure refrigerant displaying high pressure drop, the condensation heat transfer performance is also affected by the saturation temperature drop due to friction. Therefore, the comparison with other fluids is carried out first at the same mass flux and then at operating conditions that properly account for the pressure drop.