Flow boiling heat transfer capabilities of hydrocarbons inside a 4 mm id horizontal smooth tube

Several HydroCarbons (HCs), such as propane (R290), isobutane (R600a), and propylene (R1270), have been applied as refrigerants since the beginning of refrigeration, due to their good thermodynamic and thermophysical properties and their favorable heat transfer performance. However, their high flammability limited their use to applications that require a reduced refrigerant charge, such as domestic refrigerators, or for larger refrigerating machines, that work with a secondary loop fluid configuration. In the last decades, HC refrigerants attracted a new interest, first as substitutes for CFC and HCFC refrigerants subjected to phase-out for their large Ozone Depleting Potential (ODP) and, more recently, as alternatives for HFC refrigerants with large Global Warming Potential (GWP). Nevertheless, the open literature collects just a few experimental data on HCs two-phase heat transfer and pressure drop inside small diameter tubes (ID<6 mm), most of them are quite recent. This paper presents some new experimental data collected during R600a flow-boiling inside a 4 mm ID horizontal copper smooth tube. Tests were carried out at 5 °C of saturation temperature, with heat flux ranging from 15 to 30 kW m−2 and refrigerant mass flux in the range 100–300 kg m −2 s −1 at increasing vapor quality up to incipient dryout. The effect of heat flux, refrigerant mass flux, and mean vapor quality, have been evaluated separately. On the basis of the heat transfer data, the influence of forced convection and nucleate boiling contributions was investigated. The experimental data were then used to assess some classical literature correlations. Finally, isobutane heat transfer performance was compared with other hydrocarbons (propane, and propylene). These latest data were obtained by the same authors under similar working conditions [1].