Preliminary Tests about heat trasfer during flow boiling of a carbon dioxide

Refrigerating plants working with carbon dioxide (R744) in hot climates require the use of cascade systems or transcritical cycles, when the ambient temperature is lower than the critical temperature. The use of particular blends of R744 and hydrocarbons (HCs) increases the critical temperature over the ambient temperature, allowing to work with a subcritical cycle. At the same time safety problems rise related to the HCs flammability; moreover large changes in thermophysical properties are encountered requiring more attention in system and components design. About R744 and HCs blends, the most promising ones seem to be those with propane (R290). In this work preliminary tests to investigate the heat transfer during evaporation of a blend (75% of R744 and 25% of R290 in weight) are presented. The test section is a smooth, horizontal, stainless steel tube (6.00 mm ID) uniformly heated by Joule effect with a constant heat flux equal to 10 kW/m2. The tests are related to evaporating temperatures that range between 7 and 12°C varying the refrigerant mass flux from 200 to 350 kg/m2.s.