Droplets entrainment ratio in a PWR hot-leg pipe geometry

Abstract

Droplets entrainment was found to contribute to the occurrence of countercurrent flow limitation (CCFL) at COLLIDER test facility which has a PWR hot-leg pipe geometry with a large diameter value of 190 mm. Previous investigations showed that droplets entrainment starts to become observable at medium water inlet velocities (In Wallis parameters Jw,in∗0.5>0.2). In current article, the occurrence of droplets entrainment at COLLIDER test facility is discussed along with a detailed descriptions of visual observations that corresponds to entrainment mechanisms at the junction between the hot-leg and the steam generator. These mechanisms are compared to typical mechanisms found in literature for annular flows. The significance of COLLIDER large-diameter and pipe geometry against rectangular and narrow channel geometry is explained. Experimental measurements of the droplet entrainment ratio at COLLIDER test facility (using air/water at atmospheric pressure) are introduced and discussed. Measurements cover a range of air superficial velocities between 10.6 and 15.2 m/s, and water superficial velocities between 0.05 and 0.12 m/s. A correlation of the critical air velocity at which the droplets entrainment start to occur is given. An empirical correlation of the entrainment ratio was developed for obtained data. The correlation shows a stronger dependency upon the water inlet velocity in comparison to correlations found in literature for annular flows. A comparison between the prediction of available entrainment ratio correlations in literature and COLLIDER experimental data is made. The comparison shows the need of a new correlation for the droplets entrainment that takes place at the entrance of a hot-leg pipe geometry. The obtained correlations are useful to predict the onset of entrainment and to quantify its possible contribution to the reduction of water delivery into the reactor core during the occurrence of an SBLOCA accident. The last result becomes of a special importance knowing that CCFL caused by droplets entrainment occurs at lower air velocities than those at which a typical CCFL occurs (caused by large roll-waves at the bend), and that droplets entrainment does not require a transition from a supercritical into a subcritical flow condition in order to take place.