Local measurements were made in a droplet-laden vapor core in upward R-134a annular flow in a high aspect ratio vertical duct. These detailed measurements are unique in that they were performed at high pressures and low liquid-to-vapor density ratios. Using a gamma densitometer, hot-film anemometer and laser Doppler velocimeter, profiles of void fraction, liquid droplet frequency, and droplet velocity were acquired across the narrow test section dimension. At relatively high flows, the measured void fraction was highest near the wall, due to the thinning of the liquid film. The dip in the void fraction in the vapor core at these flows suggests significant droplet entrainment. The entrainment fractions for these refrigerant flows fall in the range measured for pressurized steam-water systems. The average drop size, calculated from direct measurements of void fraction, droplet velocity, and frequency, compares favorably with previous experimental results from the literature. These data are useful for developing an improved understanding of practical two-phase flows, and for assessment of advanced two-fluid computer codes.

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