Wall superheat at the onset of nucleate boiling of pure refrigerants and zeotropic refrigerant mixture.

For further improvement of COP and reduction of refrigerant charge, the use of compact heat exchangers using aluminum multiport tubes are spreading. Multiport tubes have non-circular mini-channels with a hydraulic diameter of less than 1.0 mm, which results in a thicker liquid film at the corners due to surface tension when used as an evaporator. In this study, pool boiling experiments had been conducted to evaluate the wall superheat at the onset of nucleate boiling (ONB). A pure refrigerant of R1234yf and zeotropic mixture of R1234yf and R32 were used. The heat transfer surface was a smooth circular surface with a diameter of 10 mm. The surface was polished with #400 sandpaper to simulate a surface of general industrial products. As a result, the wall superheat at the ONB became lower for the zeotropic mixture than R1234yf.