Hysteresis Phenomena at Boiling in Liquid Film Flowing down the Tubes with Microarc Oxidation Coating

A study of heat transfer at evaporation and boiling in liquid film flowing down the horizontal tube bundle in tube-and-shell evaporator is reported. This bundle includes duralumin tubes with micro-arc oxidizing (MAO) treatment, which heat transfer characteristics are the subject of conducted tests. Refrigerant R21 at the pressure of  \(\sim\) 3 bar of absolute pressure is used as working liquid. Hot water pumped through bundle tubes used as a heat carrier. The boiling curves obtained for the tested tubes with MAO treatment showed considerable heat transfer enhancement in comparison with plain tube; however, hysteresis phenomena due to a porous structure of micro-arc oxidizing coatings are revealed. These phenomena should be taken into account at operating heat transfer equipment.