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Abstract: This paper presents different CFD-simulations on flows which are relevant for nuclear reactor safety using a new modeling approach for the interfacial drag at free surfaces. The developed drag coefficient model was implemented together with the Algebraic Interfacial Area Density (AIAD) model () into the three-dimensional (3-D) computational fluid dynamics (CFD) code ANSYS-CFX. The applications considered include the prediction of counter-current flow limitations (CCFL) in a PWR hot leg, the development of hydraulic jump during the air–water co-current flow in a horizontal channel, and pressurized thermal shock (PTS) phenomena in a PWR cold leg and downcomer. For the modeling of these tasks, an Euler–Euler approach was used. This approach allows the use of different models depending on the local morphology. In the frame of an Euler–Euler simulation, the local morphology of the phases has to be considered in the drag model. To demonstrate the feasibility of the present approach, the computed main parameters of each case were compared with experimental data. It is shown that the CFD calculations agree well with the experimental data. This indicates that the AIAD model combined with new drag force modeling is a promising way to simulate the phenomena in frame of the Euler–Euler approach. Moreover the further validation of the model by including mass transfer effects should be carried out. [Copyright &y& Elsevier]

The study on the interfacial phenomena of flooding during the counter-current flow of gas-liquid in a complex pipe was carried-out experimentally. It consists of a combination of the horizontal pipe, elbow and inclined pipes to simulate the hot leg pipe in a PWR. The inner diameter was 25.4 mm as scale down of real hot leg pipe of German Konvoi type reactor. The visual observations were performed using high-speed video cameras. The measured parameters were the liquid film thickness and the pressure difference between reactor pressure vessel (RPV) simulator and steam generator (SG) simulator. The film thickness inside the test pipe was determined by using the processed image of visual data. As a result, three different mechanisms at the onset of flooding in counter-current were clarified, and are strongly affected by liquid superficial velocity. At low liquid superficial velocity (J L ), the flooding mechanism is triggered by the presence of wavy interface along the horizontal pipes in the form of a liquid blockage on the elbow. At the medium J L , the slug was detected along the horizontal pipe. However, the slug collapses immediately since the water is insufficient to maintain the blockage. At high J L , the slug occurs and travels along the horizontal pipe until the elbow. Moreover, the effect of the length of horizontal pipe on the interfacial behavior of air water counter current flow also can be explained. As the length of the horizontal pipe decreases, the onset of flooding occurs at a higher gas liquid mass flow rate. [ABSTRACT FROM AUTHOR]

• The experiment was conducted during the liquid–gas counter-current two-phase flow in a 1/30 scaled-down of PWR hot leg. • The liquid thickening on the upper edge of the hydraulic jump triggers the liquid blockage to initiate the CCFL. • The statistical characterizations were investigated on the basis of both the locus and the flow parameters. • The time-domain and frequency-domain analyses implies a good agreement with the visual observations, while the wavelet analysis is able to differ the obtained flow regimes. The present work investigates the statistical characteristics of the fluctuations of the liquid film thickness obtained by resistive-conductance probe signals of a complex geometry representing a 1/30 down-scaled of a PWR hot leg. Here, four locations at the horizontal part of the test section were assessed, while the flow development was investigated through a close observation by using a high-speed video camera. Next, the normalized voltages, indicating the liquid film thickness fluctuations, are shown in the time-series. From the fluctuations, the amplitudes representing the film thickness were recorded. Hence, such statistical diagnostic tools were applied on the basis of time-domain, frequency-domain and also time–frequency domain analyses. From the probability density function, the distributions of the fluctuations were assessed, whereas the power spectral density function describes the dominant frequencies. In addition, the Kolmogorov entropy denotes the chaotic level of the signal, meanwhile the wavelet energy corresponds to several details on the wavy interfaces on the basis of its frequency scales. Next, those parameters were statistically employed to elaborate the interfacial behaviors due to the signal characteristics. The results indicate that the low dominant frequency corresponds to the occurrence of either liquid blockage or slugs during the acquisition, meanwhile the largest scale fluctuations reveal the wave movements. [ABSTRACT FROM AUTHOR]