04. Boiling and Multi-Phase Flow

HFO-1233zd(E) is a potential replacement for HFC-245fa in the organic Rankine cycle due to its low global warming potential value. However, there are limited studies of the condensation of HFO-1233zd(E) outside integral-fin tubes available in the literatures. This study experimentally investigates the condensation of HFC-245fa and HFO-1233zd(E) integral-fin tubes with rectangular section fins having a thickness of 0.3 mm, a height of 1.0 mm, and 2.0, 1.5, 1.0, and 0.5 mm fin spacings. The results demonstrate that HFC-245fa exhibits higher condensing coefficients than HFO-1233zd(E) across all tested tubes. Notably, the disparity in heat transfer coefficients between HFC-245fa and HFO-1233zd(E) becomes more pronounced with decreasing fin spacing. The experimental data also reveals a decrease in heat transfer coefficients with increasing wall subcooling temperature for larger fin spacings, contrasting with behavior observed at 0.5 mm fin spacing, where heat transfer coefficients increase with rising subcooling temperature. The experimental results deviate from existing models, particularly for 0.5 mm finned tubes, due to shifts in condensate flow modes. A small fin spacing caused the space between fins to be filled with retained condensate, and the effect of the surface tension force significantly holds up the liquid condensate. The condensate drainage shifted from droplet to column mode at a higher subcooling temperature. Therefore, the surface rejects the condensate faster, and the thickness of the condensate decreases. A 0.5 mm finned tube continuously shed condensate liquid, and a higher condensation volume flow rate was obtained. The heat transfer coefficients increased at high heat flux.