On CCFL in collider test facility: CCFL results at PWR hot-leg pipe geometry with 190 mm diameter

CCFL (Countercurrent Flow Limitation) at a PWR hot-leg pipe geometry with a 190 mm channel diameter was experimentally and numerically investigated in recent three years at COLLIDER test facility of the technical university Munich (TUM).This article will summarize most important CCFL features concluded at COLLIDER test facility and try to explain reasons for obtaining different descriptions/results/conclusions at different CCFL experimental investigations. Factors that can affect CCFL experimental results are explained in detail including scale effects. Necessary preconditions to compare two sets of data from different experimental investigations are discussed in details. The difference among different CCFL-related limits is clarified taking data at COLLIDER as an example. This includes: The limit of the transition from a supercritical into a subcritical flow (SSTL), the onset of CCFL limit (iCCFL) inside the hot-leg pipe, the onset of CCFL limit (eCCFL) at the entrance of steam generator, deflooding limit (CCFLd), the CCFL characteristics curve (CCFLch, which predicts the water delivery rate after the onset of iCCFL), and the onset of hysteresis limit (OHL). It will be shown that among these limits only the SSTL, CCFLch, and eCCFL are original limits while the rest are derivatives of them. In particular, it will be shown that the iCCFL limit is a combination of SSTL and CCFLch limits. The role of scale upon the existing eCCFL’s physical mechanism (whether a water accumulation or droplets entrainment at the entrance to the steam generator) is clarified via a comparison to a similar 50 mm CCFL facility at Kobe university. The article tests the scalability of interface distribution at quasi-stationary conditions (points along the CCFLch curve) via a comparison of time-averaged interface distributions obtained at similar inlet conditions (^{∗0.5 ∗0.5}) at COLLIDER 190mm and Kobe 50 mm channels. The comparison will show that the interface distributions (which is directly linked to the pressure drop and interfacial momentum transfer) cannot be scaled at the bend/riser/entrance region because of the influence of the channel diameter upon occurring CCFL mechanism. Meanwhile the water level gradient can be similar at the horizontal part, but not the absolute value of water depth.