Reflux condensation heat transfer of steam-air mixture under turbulent flow conditions in a vertical tube

Under certain hypothetical accident circumstances in a pressurized water reactor, reflux condensation in the steam generator U-tubes may be an effective heat removal mechanism. Non-condensable gas (air) may be present in the reactor coolant system and is well known to inhibit steam from condensing. In a previous study, local condensation heat transfer coefficients for reflux condensation with air were measured, and an empirical correlation intended for a laminar flow of steam-air mixtures was developed, which was a function of the partial pressure ratio, (P-steam/P-air) In this study, heat transfer coefficients measured under turbulent flows were used to modify the empirical correlation for improved prediction accuracy. The modified empirical correlation considers convection and is proportional to (P-steam/ P-air)Re-0.75(g)0.8 where Re-g is the steam-air mixture Reynolds number. The correlation was verified against the data of Moon et al. and then incorporated into the transient analysis code RELAP5/SCDAPSIM/MOD3.2. The temperature distributions of the steam-air mixture calculated by the code for six reflux condensation tests are shown to agree well with the measured results.