Flow Boiling and Pressure Drop Measurements for R-134a/Oil Mixtures Part 1: Evaporation in a Microfin Tube

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Abstract
Intube flow boiling experiments for refrigerant R-134a mixed with a lubricating oil are reported. The tests were run at a nominal inlet pressure of 340 kPa over a wide range of vapor qualities at mass velocities of 100, 200 and 300 kg/(s·m2) (73,500, 147,000, and 220,500 lb/h·ft2) for inlet oil concentrations from 0, 0.5, 1.0, 3.0 and 5.0 mass% oil. At high vapor qualities (x > 0.75 to 0.85), the local boiling heat transfer coefficients dropped off rapidly with increasing oil concentration. At low to intermediate vapor qualities (0.2 < x < 0.60), oil tended to increase the local boiling coefficient at 300 kg/(s·m2) (220,500 lb/h·ft2) while significant deterioration in boiling performance occurred at the two lower mass velocities. There was strong evidence that oil holdup occurred inside the microfin tube test section and was responsible for the sharp falloff in performance at the mass velocity of 100 kg/(s·m2) (73,500 lb/h·ft2), which means there may be a lower mass velocity limit for effective use of microfin tubes.