Countercurrent Flow Limitation Phenomenon Research Articles

Experimental investigation of countercurrent flow limitation (CCFL) in a 1/19 scale of VVER-1000 geometry

Countercurrent flow limitation (CCFL) has been experimentally considered in a VVER-1000 hot-leg. The proposed test facility consists of a hot-leg pipe, a reactor vessel simulator (RVs), and a steam generator simulator (SGs). It was designed and constructed based on power-to-volume scaling approach by preserving the Froud number in the hot leg. The scaling ratio was considered 1/19 of the Bushehr NPP (VVER-1000) geometry. The hot leg of the proposed facility provides good optical observation of the gas/liquid interface. The SG simulator has also a side view for optical observation. Experimental results were captured at atmospheric pressure with air/water as fluid. CCFL was obtained by analyzing the pressure differences between RV and SG simulators.This CCFL data was compared with empirical data and correlations available in the literature. With increasing liquid superficial velocity, the gas superficial velocity curve is continuously decreased, which is consistent with Navarro's results. In this study, an experimental correlation is proposed and compared to other correlations. It was very close to the Navarro and Richter correlations.The results of experimental investigations clarified the onset of the CCFL phenomenon in a Bushehr NPP (VVER-1000) hot-leg geometry. Moreover, the achieved patterns and behaviors of the air/water mixture would be used in computational fluid dynamics (CFD) code validations.

Analysis of RELAP5 prediction of countercurrent flow limitation in downcomer at upper plenum test facility

The relevance of the countercurrent flow limitation phenomenon is known in nuclear (emergency core cooling and accident situations) industries, such as a Loss of Coolant Accident (LOCA). The occurrence of countercurrent flow limitation (CCFL), which can exist in a host of places around the nuclear reactor system, will prevent the emergency cooling water, which is necessary to cool the core successfully flowing into the core from the downcomer of Pressurized Water Reactor (PWR) in the LOCA. Accurate prediction of the downcomer CCFL phenomenon is significant for the design and analysis of reactor safety. Among the experiments at Upper Plenum Test Facility (UPTF), various tests, test 6 and test 7 are studied in this research. Those tests are conservatively evaluated to predict the CCFL phenomenon at the downcomer. Above all, the methodology of node partition used in the RELAP5 model for the channels with cross-connections to represent the circumferential downcomer annulus was also studied. In particular, the nodes along the circumferential direction had a vital effect on the prediction of the steam velocity profile of downcomers. The influence of the CCFL model, including the most widely used Wallis-type and Kutateladze-type correlations, and the cooling-limited liquid-downflow behavior were analyzed. In addition, the pressure on the downcomer and steam flow rate of the break were monitored, and the predicted results were also analyzed.

Experimental Investigations and Numerical Studies of Two-Phase Countercurrent Flow Limitation in a Pressurized Water Reactor: A Review

Gas–liquid two-phase countercurrent flow limitation (CCFL) phenomena widely exist in nuclear power plants. In particular, the gas–liquid countercurrent flow limitation phenomena in a pressurized water reactor (PWR) during a loss-of-coolant accident (LOCA) or a small-break loss-of-coolant accident (SBLOCA) play an important role in nuclear reactor safety research. Over several decades, a series of experimental investigations and numerical studies have been carried out to study the CCFL phenomena in a PWR. For the experimental investigations, numerous experiments have been conducted, and different CCFL mechanisms and CCFL characteristics have been obtained in various test facilities simulating different scenarios in a PWR. The CCFL phenomena are affected by many factors, such as geometrical characteristics, liquid flow rates, and fluid properties. For the numerical studies, more and more numerical models were presented and applied to the calculations of two-phase countercurrent flow over the past several decades. It is considered that the computational fluid dynamics (CFD) tools can simulate most of the two-phase flow configurations encountered in nuclear power plants. In this paper, the experimental investigations and the numerical studies on two-phase countercurrent flow limitation in a PWR are comprehensively reviewed. This review provides a further understanding of CCFL in a PWR and gives directions regarding future studies. It is found that relatively fewer investigations using steam–water under high system pressures are performed due to the limitation of the test facilities and test conditions. There are a number of numerical studies on countercurrent two-phase flow in a PWR hot leg geometry, but the simulations in other flow channels were relatively rare. In addition, almost all of the numerical simulations do not include heat and mass transfer. Thus, it is necessary to investigate the effects of heat and mass transfer experimentally and numerically. Furthermore, it is of significance to perform numerical simulations for countercurrent two-phase flow with a fine computational grid and suitable models to predict the formation of small waves and the details in two-phase flow.

Numerical Simulation Study on the Air/Water Counter- current Flow Limitation in Nuclear Reactors

After a loss-of-coolant accident (LOCA) in a Pressurized Water Reactor (PWR), the temperature of the fuel elements cladding increases dramatically due to the heat produced by the fission products decay, which is not adequately removed by the vapor contained in the core. In order to avoid this sharp rise in temperature and consequent melting of the core, the Emergency Core Cooling System is activated. This system initially injects borated water from accumulator tanks of the reactor through the inlet pipe (cold leg) and the outlet pipe (hot leg), or through the cold leg only, depending on the plant manufacturer. Some manufacturers add to this, direct injection into the upper plenum of the reactor. The penetration of water into the reactor core is a complex thermofluidodynamic process because it involves the mixing of water with the vapor contained in the reactor, added to that generated in the contact of the water with the still hot surfaces in various geometries. In some critical locations, the vapor flowing in the opposite direction of the water can control the penetration of this into the core. This phenomenon is known as Countercurrent Flow Limitation (CCFL) or Flooding, and it are characterized by the control that a gas exerts in the liquid flow in the opposite direction. This work presents a proposal to use a CFD to simulate the CCFL phenomenon. Numerical computing can provide important information and data that is difficult or expensive to measure or test experimentally. Given the importance of computational science today, it can be considered a third and independent branch of science on an equal footing with the theoretical and experimental sciences.

Experimental Investigation of Air-Water CCFL in the Pressurizer Surge Line of AP1000

Countercurrent flow limitation (CCFL) may occur under certain flow conditions in the surge line, restricting the draining of water from the pressurizer and thus affecting the coolant inventory and water level in the reactor pressure vessel (RPV). The complexity of the AP1000 pressurizer surge line structure makes predicting CCFL fairly difficult, and there are still not enough CCFL studies on this complex structure. Based on an extensive literature survey, the authors of this paper are particularly aware of the need for improved CCFL models for the pressurizer surge line of AP1000. To investigate the CCFL phenomenon in the surge line assembly fixture of AP1000, a whole-visual test model of the surge line is designed with a scaling ratio of 1:4, and a test loop is established to carry out visualization experiments with air-water countercurrent flow (CCF). The whole-visual test section made of acrylic material is composed of a pressurizer simulator, a surge line tube, a hot leg T-type tube, and an RPV simulator. The air-water CCF experiments are conducted at atmospheric pressure and room temperature with the pressurizer simulator water level varying from 150 to 900 mm. The visual CCF experimental processes and CCFL phenomena are filmed by a high-speed camera and analyzed in detail. The pressure drops at different CCFL locations are measured and evaluated to explore the relationships between the CCFL characteristics and flow patterns in the surge line. The development process of the CCFL is defined as the CCFL region, which can be divided into different regions according of the changes in water mass flow and CCF flow behavior. The CCFL data are analyzed and compared using the air and water superficial velocities to study the effects of hysteresis and water level. Small discrepancies are found between the data of different water levels, reflecting the small but not-negligible influence of the upper tank water level. Empirical models for the CCFL in the surge line assembly fixture are explored preliminarily using Kutateladze-type correlation and Froude-Ohnesorge correlation. Deficiencies still exist in the present semiempirical models, inspiring a more in-depth study on the empirical models for CCFL in the surge line assembly fixture that considers the complex two-phase flow behaviors in the upper tank and near the joint between the upper tank and surge line tube. The present CCFL data are compared broadly and in detail with groups of CCFL data of similar former experiments to demonstrate the applicability of the present air-water CCFL data to the development of a CCFL prediction model for the prototype large-diameter surge line assembly fixture of the AP600/AP1000. We will perform much more experimental and theoretical work to study the detailed mechanism of these special phenomena and to develop a more applicable CCFL model for the geometry and conditions of the prototype large-diameter surge line assembly fixture.

Experimental study of gas–liquid two-phase flow through packed bed under natural circulation conditions

Dry-out phenomena in packed beds or porous media may cause a significant digression of cooling/reaction performance in heat transfer/chemical reactor systems. One of the phenomena responsible for the dry-out in packed beds is known as the counter-current flow limitation (CCFL). In order to investigate the CCFL phenomena induced by gas–liquid two-phase flow in packed beds inside a pool, a natural circulation packed bed test facility was designed and constructed. A total of 27 experimental conditions covering various packing media sizes (sphere diameters: 3.0, 6.4 and 9.5 mm), packed bed heights (15, 35 and 50 cm) and water level heights (1.0, 1.5 and 2.0 m) were tested to examine the CCFL criteria with adiabatic air–water two-phase flow under natural circulation conditions. Both CCFL and flow reversal phenomena were observed, and the experimental data including instantaneous and time-averaged void fraction, differential pressure and superficial gas–liquid velocities were collected. The CCFL criteria were determined when periodical oscillations of void fraction and differential pressure appear. In addition, the Wallis correlation for CCFL was utilized for data analysis, and the Wallis coefficient, C, was determined experimentally from the packed bed CCFL tests. Compared to the existing data-sets in literature, the higher C values obtained in the present experiment suggest a possibly higher dry-out heat flux for natural circulation debris systems, which may be due to the water supply from both top and bottom surfaces of the packed beds. Considering the effects of bed height and hydraulic diameter of the packing media, a newly developed model for the Wallis coefficient, C, under natural circulation CCFL is presented. The present model can predict the experimental data with an averaged absolute error of ±7.9%.

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 (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.

Experimental study of counter-current gas–droplet flow limitation in a 30 cm pipe

In the present study, gas–liquid two-phase flow at the counter-current flow limitation (CCFL) condition in a relatively large pipe is investigated analytically and experimentally. Droplet behavior and stable counter-current flow criteria are analyzed by studying the interaction between downward moving liquid droplets and upward gas flow. An analytical model based on the Drift Flux Model was developed to predict the onset of CCFL phenomena. The model takes into account various factors that may be encountered in the industry including gas velocity profile, droplet fraction profile, and velocity fluctuation factor. Analyses show that maintaining a uniform gas velocity profile and a stable gas flow rate is the key to prevent droplet carry-over and hence delaying the CCFL condition. For droplet–gas interaction, droplet size in the distorted particle regime is the major contribution. An experimental facility consisting of a 30cm test section pipe was designed and constructed to perform a fundamental study of counter-current droplet–gas flow. A dataset for the onset of CCFL at varying operating conditions was established. The proposed model showed the ability to predict the onset of CCFL in the counter-current two-phase flow.

Evaluations of the CCFL and critical flow models in TRACE for PWR LBLOCA analysis

Abstract This study aims to develop the Maanshan Pressurized Water Reactor (PWR) analysis model by using the TRACE (TRAC/RELAP Advanced Computational Engine) code. By analyzing the Large Break Loss of Coolant Accident (LBLOCA) sequence, the results are compared with the Maanshan Final Safety Analysis Report (FSAR) data. The critical flow and Counter Current Flow Limitation (CCFL) play an important role in the overall performance of TRACE LBLOCA prediction. Therefore, the sensitivity study on the discharge coefficients of critical flow model and CCFL modeling among different regions are also discussed. The current conclusions show that modeling CCFL in downcomer has more significant impact on the peak cladding temperature than modeling CCFL in hot-legs does. No CCFL phenomena occurred in the pressurizer surge line. The best value for the multipliers of critical flow model would be 0.5 and the TRACE could consistently predict the break flow rate in the LBLOCA analysis as shown in FSAR.

Numerical Simulations of Counter Current Flow Experiments Using a Morphology Detection Algorithm

In order to improve the understanding of counter-current two-phase flows and to validate new physical models, CFD simulations of 1/3rd scale model of the hot leg of a German Konvoi PWR with rectangular cross section was performed. Selected counter-current flow limitation (CCFL) experiments at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) were calculated with ANSYS CFX 12.1 using the multi-fluid Euler-Euler modeling approach. The transient calculations were carried out using a gas/liquid inhomogeneous multiphase flow model coupled with a SST turbulence model for each phase. In the simulation, the surface drag was approached by a new correlation inside the Algebraic Interfacial Area Density (AIAD) model. The AIAD model allows the detection of the morphological form of the two phase flow and the corresponding switching via a blending function of each correlation from one object pair to another. As a result this model can distinguish between bubbles, droplets and the free surface using the local liquid phase volume fraction value. A comparison with the high-speed video observations shows a good qualitative agreement. The results indicated that quantitative agreement of the CCFL characteristics between calculation and experimental data was obtained. To validate the model and to study scaling effects CFD simulations of the CCFL phenomenon in a full scale PWR hot leg of the UPTF test facility were performed. Also these results indicated a good agreement between the calculation and experimental data. The final goal is to provide an easy usable AIAD framework for all ANSYS CFX users, with the possibility of the implementation of their own correlations.

Validation of Heat Transfer Models in Narrow Gap for RELAP/SCDAPSIM/MOD3.2

In a severe accident of a light water reactor (LWR), heat transfer models in a narrow gap between superheated core debris and reactor pressure vessel (RPV) are important for evaluating the integrity of the RPV and emergency procedures. Newly developed heat transfer models are discussed that take into account both the local heat flux on a heated surface, which is characterized by the boiling regime, and the average critical heat flux (CHF) on a heated surface, which is restricted by countercurrent flow limitation (CCFL), including the effect of an inclination angle of the gap. The models were incorporated into the mechanistic detailed code RELAP/SCDAPSIM/MOD3.2. The local heat flux was applied to the outer surface of the debris and the inner surface of the RPV wall. The average CHF was evaluated through the CCFL phenomenon at each junction in the gap. For the assessment, an analysis of Japan Atomic Energy Research Institute’s ALPHA test was performed. The calculated peak temperature response of the vessel showed good agreement with the experimental data. It was validated that the new models effectively simulate the coolability in a narrow gap, which could be an effective means of cooling the vessel wall and thereby preventing RPV failure, as was demonstrated in the TMI-2 accident.

Thermal-hydraulic phenomena during reflux condensation cooling in steam generator tubes

The transitions of cooling mechanism in steam generator tubes during reflux condensation are studied. It is found that the transitions are closely related to the occurrence of flooding or counter-current flow limitation phenomena in steam generator tubes. As shown in the previous studies of other researchers, the transition from filmwise reflux condensation into total reflux condensation occurs when the flooding criterion suggested by Wallis is met. In this study, it is suggested that the transition from total reflux condensation to complete carry-over occurs depending on the tube height and cooling conditions. It is also shown that the flooding at SG tubes occurs before the flooding at hot leg when a reflux condensation mode is existing in steam generator. Though the thermal-hydraulic conditions during reflux condensation after a small-break loss-of-coolant accident have enough margin to the transition into carry-over, considerations for the prevention of primary coolant relocation should be provided.

Countercurrent flow limitation in inclined channels with bends

The countercurrent flow limitation phenomenon in an inclined round channel connected to bends at both ends is experimentally studied. The channel inner diameter is 1.9 cm, the bend radius divided by channel diameter is 10, and the channel length divided by channel diameter is varied in the 105–315 range. Countercurrent flow rates are measured with liquid and gas superficial velocities in the 0.015–0.21 m s −1 and 0.1–3.1 m s −1 ranges respectively. Air and water at room temperature and atmospheric pressure are used in the experiments. The liquid injection system is a constant-head plenum, and the channel angle of inclination with respect to the vertical line is varied in the 0°–60° range. The measured liquid and gas flow rates for all the angles of inclination are correlated using Wallis' flooding correlation, with a unique value for each of the two constants in the correlation.