LGWP, Refrigerants, Evaporation, Heat Transfer, Pressure Drop
To meet the terms of the Montreal Protocol, CFCs and HCFCs have been gradually phased out and they have been replaced by refrigerants that have zero ozone depletion potential. However, some of these fluids, such as R-410A, have global warming potential (GWP) that might still be of concern from an environmental perspective in case of leakage or improper charge management. Few studies of refrigerants that have zero ozone depletion potential and GWP less than 500 are available in the literature. Preliminary findings from these studies suggested that new development refrigerants were viable options. System COP and capacity were promising but there is not much information on the heat transfer and pressure drop characteristics of these low GWP refrigerants. This paper contributes to address this gap and provides new data for the two phase flow boiling heat transfer coefficient (HTC) and pressure drop of two new low GWP developmental refrigerant alternatives for R-410A. Heat transfer measurements were conducted for a copper tube commonly used in direct expansion evaporators of air conditioning systems with a 9.5 mm (0.375 in.) outside diameter and internally enhanced micro-finned surface. Data of local two phase flow HTC and pressure drop are presented for refrigerants R-410A, R-32, R-1234yf and the two new developmental refrigerants referred to as DR-5 and DR-5A. The experimental findings from this work indicated that the refrigerant R-32 had similar and slightly higher heat transfer coefficient than that of R-410A at same refrigerant mass flux and similar heat flux conditions on the outer surface of the tube. Refrigerant R-1234yf had about 15 to 20% lower heat transfer coefficient than R410-A at 4?C saturation temperature. For this saturation temperature the developmental refrigerants DR-5 and DR-5A had heat transfer coefficients between R-32 and R-1234yf when the vapor quality ranged from 0.2 to 0.7. An increase of the saturation temperature from 4°C to 9°C decreased the heat transfer coefficients for all of the refrigerants tested. The two phase flow boiling pressure drops increased monotonically if the vapor quality of the refrigerant increased. The pressure drops of refrigerant R-410A were the lowest while the pressure drop for refrigerant R-1234yf were the highest measured among the fluids investigated. The developmental refrigerants DR-5 and DR-5A showed identical characteristics in terms of pressure drop at both saturation temperatures of 4°C and 9°C.