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• A thin liquid film is observed beneath the vapor slug. • This liquid film is evaporated completely when CHF occurs. • The increase in coolant subcooling is conductive to the improvement of CHF. • There is decrease trend of CHF near the position closing to 90°. It is essential to conduct theoretical and experimental research on the heat transfer limit for External Reactor Vessel Cooling (ERVC), which aims to implement In-Vessel Retention (IVR) severe accident mitigation strategy successfully. In this paper, a theoretical model for IVR-ERVC condition is proposed to predict the critical heat flux (CHF) based on the experimental observation. According to the experimental observation on the one-dimensional full height IVR-ERVC experimental facility, it is found that there is a thin liquid film between the heating surface and the vapor slug. And this film is evaporated completely when CHF occurs. In addition, a large number of bubble condensation collapses were observed under subcooling condition. According to these experimental observations, a CHF model based on the mechanism of liquid film dryout is developed. The CHF trigger mechanism of this model is that the supply of fresh liquid into the liquid film is not sufficient to prevent depletion of the liquid film due to the boiling. The influence of heating surface geometry and the heat flux shape are considered in this model. The predicted values of the model are compared with the CHF experimental result from different test facilities with different conditions, and the comparison result shows that the relative errors are basically within 20 %. The analysis results of the model indicate the increasing of coolant subcooled degree is conductive to the improvement of CHF. When the coolant subcooled degree increases by 15 ℃, more than 30 % maximum increase in CHF is obtained. In addition, this model can reflect the "exit phenomena" to a certain extent. [ABSTRACT FROM AUTHOR]

• Flow and rupture study of liquid film on corrugated plate wall are reviewed. • Chaotic analysis of film thickness series will be a prospect research direction. • Film rupture mechanism cannot be explained by surface wave disturbance model. • More study will be focused on rupture phenomenon at the corrugated plate corner. Steam generated by steam generator in a nuclear power plant must firstly be subjected to steam-water separation treatment before entering the turbine. The research on flow and rupture characteristics of the liquid film on the wall surface of corrugated plate has important and practical significance for improving the separation efficiency. Since the 1940s, researchers from the United States, France, Japan and other countries have carried out related researches. With the rapid development of steam-water separation technology, many scholars have carried out a large number of experiments, theoretical study and numerical calculations on this phenomenon. Based on the published papers, the research progress and significant achievements of the flow and rupture characteristics of the liquid film on corrugated plate wall are reviewed. Shortcomings of past research and prospects for future research are summarized and proposed, respectively, which provides a reference for improvement of separation efficiency of the corrugated plate dryer. [ABSTRACT FROM AUTHOR]

• Droplet impacting on super-hydrophilic cylindrical surface. • Effect of incident direction and position on dynamic and thermal behaviors. • The distribution of liquid film thickness varies with impact types. • Abnormal impact leads to reduced contact area and heat transfer capacity. • High heat flux region is concentrated at lower circumferential position. Abnormal impact of droplet with different incident directions and positions can be found in evaporator, desalinator and nuclear engineering etc. The researches devoted to droplet abnormal impingement on a super-hydrophilic cylindrical surface are limited. In this paper, a numerical study of individual droplet impacting on super-hydrophilic cylindrical heated surface was conducted by applying VOF method. The incident direction and position of droplet were characterized by the impact angle φ and horizontal offset δx , the effect of which on dynamic and heat transfer behaviors was analyzed. Results show that there is an anisotropic spread during droplet impingement. Compared to no-offset vertical impact, the peaks of liquid film thickness for impact with an impact angle or horizontal offset are not the same at two edges of liquid film along circumferential direction. Impact angle or horizontal offset leads to a reduced contact area, a lower average temperature of liquid film and smaller total droplet input heat flow. For φ from 0° to 75° and δx from 0 mm to 5 mm, the maximum total droplet input heat flow decreases by 42.1% and 27.6% respectively. The local heat flux shows apparent fluctuations along the circumferential direction, and the high heat flux region is concentrated at a lower position in the circumferential direction of the cylinder for a larger φ or δx due to the auxiliary effect of gravity. [ABSTRACT FROM AUTHOR]

• - A wall boiling model able to predict flow on a downward-facing heated wall, is suggested. • - The paper presents a detailed description of this new hybrid model. • - The validation of this model against existing experimental data is presented. • - The proposed wall boiling model returns predictions superior to existing wall boiling models. • - This enhanced model is able to mimic the heat transfer rate better than the existing correlations. This paper presents a mechanistic wall boiling model for the prediction of flow boiling on a downward-facing oriented heating surface containing sliding slug bubbles on the heating surface at the pre-critical heat flux condition. Essentially, this boiling model is an extension of the well-known RPI boiling model which was initially developed based on dispersed small spherical bubbles on the heated wall. The fundamental assumption of the RPI model sharply contrasts with the real situation during flow boiling on a heated wall facing downward because of the existence of large-scale interface bubbles (vapor slugs) that are virtually attached (separated from the wall by a thin liquid film) to the wall due to buoyancy effect. Therefore, the existing wall boiling model might not be adequate for a situation like this because the heat transfer governing mechanisms of vapor slug (deformable shape) is dissimilar to that of dispersed spherical bubbles. In this article, a liquid film conduction model that represents the wall heat transfer of slug bubble is coupled with the existing wall boiling model and the new extended wall heat flux partitioning model is administered within the solution framework of hybrid multiphase flow model in OpenFoam. The hybrid model combines Eulerian multi-fluid model (for the dispersed regime) and volume of fluid model (for the large-scale interface vapor slug regime). With the hybrid multiphase flow model, different multiphase morphologies coexisting in the flow are duly accounted for. The extended wall boiling model shows an improved prediction of the experimental data during flow boiling on a downward-facing heated surface when compared with the prediction obtained with the existing wall boiling models. [ABSTRACT FROM AUTHOR]

The estimation of dryout power holds a lot of importance in safety of boiling water reactors (BWRs). The basic mechanism of dryout is the depletion of the liquid film adhering to the walls under the combined action of deposition, entrainment and vaporization. In literature, the rate processes of entrainment and deposition have been correlated empirically and woven into a model for dryout prediction. The greatest advantage of this model is that the formulation for tubular geometry can be extrapolated to rod-bundles, with certain modifications and modeling assumptions. An accurate model thus, has the potential to make redundant the numerous dryout correlations which exist for various rod-bundle geometries. This is the reason why this model has gained a lot of popularity. Initial entrainment fraction (IEF), i.e., the fraction of liquid entrained as droplets at the onset of annular flow is an important boundary condition required for the model. However there is very little theoretical or experimental work towards prediction of IEF. In this paper, a new methodology is presented whereby the IEF can be determined. It is seen that predictions are improved upon using the new method. [ABSTRACT FROM AUTHOR]

• A novel probe is developed for local measurement in liquid dispersed flows. • Locl measurements in churn-turbulent to annular transition region are performed. • The probe's capability in measuring gas-dispersed flows is demonstrated. In churn to annular transition, the liquid could exist as a dispersed phase (droplets). But the conventional probe is not able to distinguish the dispersed liquid phase from the gas phase due to electrode design. To extend the measurement capability of the conductivity probe, the Two-Sensor Droplet-Capable Conductivity Probe (DCCP-2) is developed, which incorporates one more sensor to distinguish the dispersed liquid phase from the gas phase and the continuous liquid phase. This paper presents the design of DCCP-2, such as the circuit design, signal processing, and probe manufacturing process. Its capability in measuring liquid-dispersed flow is demonstrated by experiments in churn to annular transition conditions. Also, it still preserves the capability of measuring bubbly flow, which is demonstrated through benchmark experiments with the conventional probe. Therefore, the DCCP-2 serves as a reliable measurement instrument covering a wide range of two-phase flow regimes from bubbly flow to annular flow. [ABSTRACT FROM AUTHOR]

• Method to delay rupture of the liquid film on corrugated plate wall is given. • Increasing radius of curvature can suppress the occurrence of liquid film rupture. • PLIF method is used to measure liquid film thickness on the wall. • Effect of corrugated plate spacing on liquid film rupture is not obvious. Corrugated plate dryer is an important steam-water separation equipment in secondary loop of nuclear power plants. Rupture of liquid film on corrugated plate wall will inevitably affect steam-water separation efficiency. In this paper, based on the liquid film rupture model that we derived earlier, influencing factors of liquid film rupture are given. Effects of liquid film thickness, linear distance between two corrugated plate walls, radius of curvature at the position of inflection angle of corrugated plate on critical airflow speed were experimentally analyzed. Results show that the larger the radius of curvature, the higher the critical airflow speed. When inflection angle increases to a certain value, the increase in the radius of curvature has little effect on film rupture. As the linear distance between corrugated plate walls increases, critical airflow speed will increase. However, effect of linear distance between corrugated plate walls on liquid film rupture is not obvious. [ABSTRACT FROM AUTHOR]

• The CLSVOF method is used. • The velocity field and pressure field in liquid film are analyzed. • The successive droplets impacting on liquid film are analyzed. In the corrugated plate separator, the droplets are separated by impacting on the wall of the corrugated plate, so it is necessary to study the droplet impacting on liquid film. In this paper, CLSVOF method is used to analyze the liquid droplets impacting on different thicknesses liquid film. The morphology of crown, the velocity field and pressure field distribution inside the liquid film have been studied, and the morphology, velocity field and pressure field of successive droplets impacting on liquid film are analyzed. The conclusions are following: With the increase of liquid film thickness, the radius of crown increases but the height of crown decreases. Liquid film thickness and distance from the free surface of the liquid film are influence on velocity and pressure in liquid film; when successive droplets impact on liquid film, with the increase of time interval of two droplets, the second splashing occurs more easily. [ABSTRACT FROM AUTHOR]

• Film lateral movement distance of the film reflux is measured by PLIF method. • The theoretical model of the distance of liquid film reflux is derived. • Theoretical and experimental results agree well for small and medium Reynolds number. The corrugated plate is a vital steam-water separation device in nuclear power plants. Research on steam-water separation mechanism and separation efficiency of corrugated plates has been a hot research direction. In this paper, the film lateral movement distance of the film reflux on the corrugated plate wall is measured by PLIF method. Based on experimental results, calculation equation of film lateral movement distance when the film reflux with good applicability is fitted. The degree of liquid film reflux is characterized by the lateral distance between the apex of the liquid film reflux profile and film mainstream. This parameter is defined as distance of liquid film reflux. Based on the law of conservation of mass, theoretical equation of distance of liquid film reflux is derived. Results show that theoretical results of distance of liquid film reflux agree well with experimental results in small and medium Reynolds number regions (Re < 2625). [ABSTRACT FROM AUTHOR]

• A three-field mechanistic model is developed for dryout prediction in annular flow. • The model is verified by experimental data of the circular and rectangular channel with relatively large parameter range. • Some key constitutive relations in the model are discussed for the better prediction. • The model is effective on prediction of dryout in the rod bundles by coupling with subchannel code. Critical heat flux (CHF) is one of the most important thermal criteria for nuclear power plants. It has traditionally been evaluated using look-up tables or empirical correlations. In this paper an annular film dryout (AFD) mechanistic model has been developed based on the interaction of three fields in annular flow region, i.e. the liquid film, entrained droplets and vapor core. The model describes the mass and momentum conservation equations of three fields together with a series of constitutive relations. The effect of some constitutive correlations (the entrainment and deposition of droplets and the onset of annular flow) on the prediction accuracy of the model is studied. The results are compared with CHF experimental data in flow boiling. Fairly good agreements are observed for the CHF in circular tubes with uniform and axially non-uniform heating, as well as rectangular channels with uniform heating. Through coupling with the subchannel analysis method, the model is used to predict the dryout-type CHF in the rod bundles with a good precision. [ABSTRACT FROM AUTHOR]