Experimental investigation of countercurrent flow limitation (CCFL) in a large-diameter hot-leg geometry: A detailed description of CCFL mechanisms, flow patterns and high-quality HSC imaging of the interfacial structure in a 1/3.9 scale of PWR geometry

• Countercurrent flow limitation is experimentally investigated in a large-diameter hot-leg pipe geometry (Din = 190 mm, ∼1/3.9 scale of a PWR hot-leg).
• High-speed and high-quality imaging was implemented to study the interfacial structure and flow patterns during flooding and deflooding.
• Two main mechanisms were identified and described in details: bend-CCFL and ADE-CCFL. Four regions were recognized for jwater, in*0.5=0.085−0.305..
• Experimental curves of the onset of CCFL, and deflooding were discussed. High-quality images provide valuable validation data for CFD.
• Form a safety point of view: onset of ADE-CCFL precedes the onset of bend-CCFL and its influence cannot be neglected for jwater, in*0.5>0.27..

Countercurrent flow limitation (CCFL) is experimentally investigated in large-diameter hot-leg geometry. COLLIDER test facility consists of reactor vessel simulator (RVs), hot-leg pipe, and a steam generator simulator (SGs) and was constructed to be a 1/3.9 scale model of a real PWR geometry. The facility is completely transparent providing excellent conditions for optical observation of the gas/liquid interface. Experiments were performed at atmospheric pressure using air/distilled water as flow mediums. High-quality high speed recording (HSC) was implemented to acquire the air/water interface which provided a unique and detailed look into the interface structure within the entire hot-leg geometry. This allowed a precise and a high-qualitative identification and description of the flow patterns and mechanisms by which the onset of CCFL occurs. A systematic experimental investigation of the onset of CCFL and deflooding was carried out for water inlet velocities in the range Jw,in*0.5=0.085−0.305,   ΔJw,in*0.5=0.01. Air velocity was increased and decreased stepwise to identify the onset of CCFL and deflooding limits precisely. Two onset of CCFL mechanisms were identified and labeled as: bend-CCFL and ADE-CCFL. The results were arranged into four regions according to the inlet water velocity Jw,in*0.5. The four regions differentiate by sub-mechanisms that appear during bend-CCFL and ADE-CCFL. A detailed description of the successive events and mechanisms by which the onset of CCFL during the increase of air velocity, and deflooding occur is provided. The description is supported by high-quality images of the interface structure. The resulting experimental onset of CCFL curve and deflooding curve are provided and discussed. The experimental investigation clarifies the onset of CCFL phenomenon in a large-diameter hot-leg pipe geometry and provides valuable images of the air/water interface for future CFD validation. The current study shows that from a safety point of view, the onset of ADE-CCFL precedes the onset of bend-CCFL as Jw,in*0.5>0.185; its intensity and the amount of backflow water cannot be ignored for Jw,in*0.5>0.27.