Horizontal convective boiling of pure and mixed refrigerants within a micro-fin tube

This paper presents local convective boiling measurements in a micro-fin tube** for four pure refrigerants: R22, R32, R125, and R134a; and four refrigerant mixtures: R410B (R32/125, 45/55% mass), R32/RI34a (27/73% and 30/70% mass), and R407C (R32/125/134a, 25/23/52% mass). All testing was conducted using a counterflow water-heated horizontal 9.5 mm (Do) U-tube with helical micro-fins. Saturation temperatures ranged from 274.5 K to 293.6 K. Flow boiling heat transfer coefficients for the mixtures' pure components and R22 were measured to establish a baseline for the heat transfer degradation calculations. The measured convective boiling Nusselt numbers for all of the test refrigerants were correlated to a single expression consisting of a product of dimensionless properties valid for mass ve2 2 locities ranging from 70 kg/m. s to 370 kg/m s and for vapor qualities between 0 and 0.7. These measurements were within 20% of the new correlation predictions for 94.6% and 87.3% of the pure refrigerant and mixed refrigerant measurements, respectively. The correlation was shown to predict some existing data from the literature to within 20%. The degradation in heat transfer performance of the mixtures was found to range from 1% to 50% for all refrigerants tested.