*2.2. Test Conditions*

The condensation and evaporation heat transfer characteristics of annulus flow, using R134A, were investigated. During the condensation experiments, mass velocities varied in the range of 50–120 kg/(m2·s), with a vapor quality in the range of 0.8–0.1. Heat flux increased with increasing mass velocities in order to keep a constant vapor quality variation for the test section. Mass velocities ranged from 40–80 kg/(m2·s) for the evaporation study, with the vapor quality ranging from 0.2 to 0.8. Two different annulus gaps were considered, 2.975 mm and 3.475 mm. Experimental results were sampled for a continuous period of 5 min (after the fluctuation of pressure was less than 3 kPa and the temperature variation was less than 0.1 K). A summary of the experimental conditions is given in Table 2.

**Table 2.** Experimental conditions.

