Direct contact condensation of steam on slowly moving water

A study of direct contact condensation of stagnant saturated steam on slowly moving subcooled water has been performed with reference to a horizontal flat geometry. Inlet water mass flowrate and temperature together with inlet steam temperature have been investigated, as experimental variables, in the following ranges:

1. (a) pressure up to 6 bar,

2. (b) inlet steam temperature up to 160°C

3. (c) inlet water mass flowrate up to 120 kg/h,

4. (d) inlet water temperature up to 70°C,

5. (e) available steam mass flowrate up to 20 kg/h.

Condensation heat transfer coefficients have been determined as functions of inlet water mass flowrate, inlet water and steam temperature. Heat transfer coefficient does not show, practically, dependence either on inlet water temperature or inlet steam temperature but only on inlet water mass flowrate. Correlations are given for the Nusselt number, as a function of Reynolds and Prandtl numbers.An evaluation of thermal non-equilibrium degree between the phases is also presented, together with a correlation for its prediction.     

Experimental and analytical study of direct contact condensation of steam in water

The object of this work is to improve our understanding and analysis capability for direct contact condensation of steam in water. Transition criteria between regimes of direct contact condensation have been proposed. A transition criterion for the onset of chugging has been developed from the transient conduction model as well as a criterion for the existence of a condensing jet from the two-layer turbulent eddy transfer model. In order to analyze the effect of non-condensable gas on the chugging boundary, a transient conduction-diffusion model for steam and steam-gas mixtures can be calculated. In particular, the amount of non-condensable gas required to suppress chugging can be quantified by use of this model. The critical gas content is found to be of the order of a few percent. In addition, a methodology is suggested for calculating the product of the interfacial area and the heat transfer coefficient for oscillatory jets. All the models and transition criteria developed are applicable for upward steam injection only with the exception of the methodology used for calculating the heat transfer of high velocity steam jets.     

Cavitation model of choked flow of underheated water

A necessary condition for cavitation to appear in fluid flow is a local drop of the pressure below the saturation pressure at the flow temperature. Such a pressure drop can be due to centrifugal acceleration or more accurately the centrifugal force engendered by this acceleration and directed along the radius of curvature of a convex surface bounding the flow, across the flow direction. A mathematical model of cavitation is constructed on this basis in the form of a dependence of the critical flow rate on the pressure, density, and underheating of water at the entrance into a hydraulic apparatus and concretized for nozzles and Venturi tubes with a smooth entry as well as for a stop valve in a RBMK process channel. This model is used to evaluate the state of the flow in the valve in different RBMK-1000 operating regimes.     

Experimental and theoretical study of steam condensation induced water hammer phenomena

We investigate the steam condensation induced water hammer (CIWH) phenomena and present experimental and theoretical results. The experiments were performed in the PMK-2 facility, which is a full-pressure thermo-hydraulic model of the primary loop of the VVER-440/312 type nuclear power plant and located in the Atomic Energy Research Institute Budapest, Hungary.The present experimental setup is capable to measure CIWH phenomena in a wide range of steam pressure, cold water temperature and mass flow rate at a high level of accuracy. On the theoretical side CIWH is studied and analyzed with the WAHA3 model based on two-phase flow six first-order partial differential equations that present one-dimensional, surface averaged mass, momentum and energy balances. A second order accurate high-resolution shock-capturing numerical scheme was applied with different kind of limiters in the numerical calculations. Our study clearly shows that Relap5 and Cathare which are used in the nuclear industry to simulate nuclear power plant accidents cannot resolve the narrow pressure peaks created during a CIWH event. Only WAHA3 can model CIWH properly. Experimentally measured and theoretically calculated pressure peaks are in good agreement, however simulations always show additional pressure peaks. As a new feature in this study we present calculations without additional unphysical reflections caused by boundary conditions.     

Evaluation of heat-transfer coefficient at direct-contact condensation of cold water and steam

The estimation of the heat transfer coefficient at the direct-contact condensation of cold water and steam is a very hard task since the phenoma are essentially undsteady and the interface motion is so complicated that an exact estimation of its area is almost impossible. The present study shows the heat transfer coefficient evaluated experimentally by assuming simple interface shapes for complicated surfaces and estimated those through comparison of the numerical analyses to the data of experiments related to the loss of coolant accidents of light water reactors.At chugging, the heat transfer coefficient reached up to 2 × 10⁶W/(m^{2 K}). At condensation oscillation, it ranged between 10⁵–10⁶ W/(m² K). At a jet region of cold water injected into the steam flow in a pipe or the stationary steam in a vessel, the value was around 2 × 10⁵W/(m²K), and at the surface of stratified flow, it was between 3 × 10³–3 × 10⁴W/(m²K).     

Aerosol growth by steam condensation in a PWR containment

The fraction of steam which condenses as an aerosol in the cooling through surfaces of a well-mixed steam-air mixture confined in a cavity is calculated. A previous theory which predicts this fraction is extended so as to allow supersaturations, S, to occur in boundary layers, and the fraction is shown to be rapidly varying towards its maximum value which occurs in the saturated limit, S = 1. Only when the cooling surfaces are cold is the fractional aerosol condensation predicted to be large, and the aerosol is expected to evaporate slowly for small bulk-wall temperature differences. Results applicable to the cooling of PWR containments by water sprays are obtained. They indicate that it is unlikely that any net aerosol growth is produced.     

LOCA steam condensation loads in BWR Mark II pressure suppression containment system

Hydrodynamic loads induced in the BWR Mark II pressure suppression containment system during a loss-of-coolant accident (LOCA) were investigated using a large scale test facility. The maximum-bounding loading conditions on the pressure suppression pool-boundary structures were defined by conducting experiments for a wide range of parameters. The maximum-bounding loads occurred when steam, with air concentration less than 1% in weight, was injected at moderate rates ( 30 kg/m²·s) into a low-temperature (below 310 K) pool. Such conditions are most likely to be encountered during LOCAs with intermediate break sizes.     

Analytical thermal hydraulic model for oscillatory condensation of steam in presence of air

An analytical thermal hydraulic model has been developed from fundamental conservation laws, for the process of oscillatory condensation of steam in a pool of water, in presence of non-condensable gas (air). The oscillatory condensation phenomena addressed here is steam chugging, with an emphasis laid on studying the effect of small amount of air present in steam, on the phenomena. The objective of developing the model is to present an approximation of the real phenomena and to obtain an analytical solution. At the outset, a parametric study was conducted by using the developed model to capture and identify the salient features of steam chugging and compare the wave shapes obtained with those available in open literature. Subsequently, the effect of presence of air in steam was studied in detail using the non-condensable gas model. An attempt has been made to show numerically that the presence of a small amount of air in steam would effectively stabilize condensation and prevent inception of chugging. Typical results are presented in this paper to bring out the difference in oscillatory behavior due to presence and absence of non-condensable gas.     

Dynamic stresses in spherical containments with pressure suppression system during steam condensation

Different physical phenomena which control the structural integrity of the containment shell in case of a postulated failure of the primary coolant system are discussed. Detailed analyses are carried through for the chugging phase, where steam will be blown into the water pool of the pressure suppression system. As reference geometry a German boiling water reactor type 69 is used. The fluid dynamics of the water pool is described by a boundary integral equation method. For the structural dynamics of the thin spherical containment shell analytical solutions of Flügge's shell equations are obtained. The feedback of structural deformations on the fluid-dynamic loadings, i.e., the effect of fluid-structure interaction is considered. It increases the loadings significantly in comparison to calculations without this effect. The steam condensation in the water pool is treated as a parameter.As results upper limit steam condensation scenarios are presented where the resulting stresses reach the admissible values. If these upper limit scenarios cover all the steam condensation events which may occur, the dimensions of the containment shell are adequate to steam condensation loadings. (According to many experiments the upper limit scenarios cover indeed the expected condensation events.)The computer programs used in this work are carefully checked by comparisons with other analysis methods and relevant experimental findings.     

Studies on steam condensation and particle diffusiophoresis in a heat exhanger tube

In this paper, studies on steam condensation and aerosol behaviour in a heat exchanger are presented. The heat exchanger is a model of one single tube of a Passive Containment Condenser (PCC) that is used in the European Simplified Boiling Water Reactor (ESBWR). A hot carrier gas containing nitrogen and steam with Ag and CsOH particles flows through the heat exchanger. The walls of the heat exchanger are cooled with a water jacket, thus causing steam condensation and diffusiophoretic particle deposition. The amount of condensed steam is measured, as well as the temperatures, particle mass concentrations and size distributions before and after the heat exchanger. The experiments are done with different proportions of steam and nitrogen in the carrier gas. Heat and mass transfer in the system are modelled with well known engineering correlations, producing results that agree nicely with the experimental results. The diffusiophoretic particle deposition velocity is shown to be proportional to the steam condensation rate, as expected.     

CFD modelling of wall steam condensation by a two-phase flow approach

Condensation heat transfer in the presence of non-condensable gases is a relevant phenomenon in many industrial applications. The present work is focused on the condensation heat transfer that plays a dominant role in many accident scenarios postulated to occur in the containment of nuclear reactors. The aim of the study is to contribute to the understanding of the heat and mass transfer mechanisms involved in the problem. The modelling proposed in the paper assumes that liquid droplets form along the wall at nucleation sites. Vapor condensation on droplets makes them to grow. Once the droplet diameter reaches a critical value, gravitational forces compensate surface tension force and then droplets slide over the wall. Droplets can also join the surrounding droplets and form a film layer.As a consequence of the modelling adopted in the paper, the starting point is the balance of heat and mass transfer between droplets and the gas mixture surrounding the droplet. So, the flow in the simulation domain is modelled as a two-phase flow. This approach allows taking into account simultaneously heat and mass transfer on droplets in the core of the flow and condensation or evaporation phenomena at the wall.Two tests were performed to validate the condensation model against experimental data: the COPAIN experiment (CEA Grenoble) and the TOSQAN ISP47 experiment (IRSN Saclay). Calculated profiles compare favourably with experimental results particularly for the helium and steam volume fraction. Nevertheless the cross-comparison of the gas velocities profiles should be improved in plume-jet configuration. Hence more investigations are needed in turbulence modelling for accurate predictions of heat transfer in the whole containment.     

Reflux condensation behavior in a U-tube steam generator with or without noncondensables

A series of reflux condensation experiments on the mid-loop inventory were conducted in the Institute of Nuclear Energy Research integral system test facility with various steam flow rates and noncondensables contents. The focus of this paper is on reflux condensation behavior in a U-tube steam generator with or without noncondensables. In particular, this paper explains the effect of noncondensables on integral system responses, U-tube flow modes, control mechanism for liquid holdup/breakdown, and loop seal clearance/reformation phenomena.     

Effect of steam condensation on countercurrent flow limiting in nearly horizontal two-phase flow

An analytical model that includes the steam condensation effect has been derived and a parametric study has been performed. In addition, a series of experiments were performed and a total of 34 experimental data for the onset of countercurrent flow limiting (CCFL) in nearly horizontal countercurrent two-phase flow have been obtained for various flow rates of water. Comparisons of the present CCFL data with slug formation models show that the agreement between the present as well as the existing model and the data is about the same. However, the deviation between Taitel and Dukler’s model predictions and the data is the largest when j_f<0.04 m s⁻¹. A parametric study of the effect of condensation using the present model shows that, when all local conditions are similar, the model predicted local gas velocities that cause the onset of flooding are slightly lower when condensation occurred. Based on the visual observation and the evaluation of the present work, it has been concluded that the criterion derived for the onset of slug flow can be directly used to predict the onset of inner flooding in nearly horizontal two-phase flow within the experimental ranges of the present work.     

Modeling of high pressure steam condensation in inclined horizontal tubes of PAFS in APR+

A Single-tube COndensation exPeriment (SCOP) was performed at Korea Advanced Institute of Science and Technology (KAIST) to simulate the condensation phenomena of Passive Auxiliary Feedwater System (PAFS), which is introduced in Advanced Power Reactor Plus. SCOP tests were conducted for two pipes of different diameters [22.6 and 44.8 mm inside diameter] with 8.2 m length and 2.5–4 degrees of the inclined angles of the upper and lower section of each tube. The experiment were performed at 1.5–6.5 MPa of steam pressure and 0.12–0.45 kg/s of steam flow. Based on the Kim and No correlation (KAIST, 2000), we developed a new turbulent–condensation correlation for the turbulent region of the liquid film in the inclined tube. We introduced the factors to account for the different inclined angle effect, different diameter effect. The Nusselt correlation is adopted for condensation near the tube inlet where the laminar region of the liquid film is dominant due to the small amount of condensed water. The present correlation was compared with the Shah and RELAP5 condensation model using the SCOP data. The root mean square errors of the present correlation, Shah model, and RELAP5 model are 17.41%, 57.88%, and 43.07%, respectively: the present correlation can predict well the condensation characteristics in comparison with other correlations in PAFS.     

Admitting cold water into steam filled pipes without water hammer due to steam bubble collapse

Water hammer due to steam bubble collapse when cold water is admitted to vertical upward flowing, vertical downward flowing, and nominally horizontal pipes has been studied both experimentally and analytically. The work in horizontal pipes included a study of the effect of a slight downward inclination, a slight upward inclination, and the length of the pipe on the initiation of water hammer. Stability maps showing the combinations of filling velocities and liquid subcooling that cause water hammer and those which do not for each flow geometry were obtained from experiments. Analytical models were developed to predict those stability boundaries in the stability maps. All these models were tested with experimental data. Based on the verified models, a step-by-step approach for each flow geometry is presented for plant engineers and designers to follow in avoiding water hammer induced by steam bubble collapse.     

A diffusion layer model for steam condensation within the AP600 containment

Steam condensation plays a key role in removing heat from the atmosphere of the Westinghouse AP600 containment in case of a postulated accident. A model of steam condensation on containment surfaces under anticipated accident conditions is presented and validated against an extensive and sound database. Based on the diffusion layer theory and on the use of the heat/mass transfer analogy, one can deal with large temperature gradients across the gaseous boundary layer under high mass flux circumstances. The thermal resistance of the condensate film, as well as its wavy structure, have also been considered in this model. As compared to Anderson et al. (1998) (Experimental analysis of heat transfer within the AP600 containment under postulated accident conditions. Nucl. Eng. Des. (submitted)) experimental database, an average error lower than 15%, within the experimental confidence range, has demonstrated its remarkable accuracy. In particular, the model has shown a good response to the influence of primary variables in steam condensation (i.e. subcooling, noncondensable concentration and pressure), providing a mechanistic explanation for effects such as the presence of light noncondensable gas (i.e. helium as a simulant for hydrogen) in the gaseous mixture. In addition, the model has been contrasted against correlations used in safety analysis (i.e. Uchida, Tagami, Kataoka, etc.) and occasionally to Dehbi’s database. This cross-comparison has pointed out several shortcomings in the use of these correlations and has extended the model validation to other databases.     

The vent-to-vent desynchronization effects on LOCA steam condensation loads in BWR pressure suppression pool

During a loss-of-coolant accident of a boiling water reactor (BWR), direct-contact steam condensation in the pressure suppression pool will induce undesirable dynamic pressure loads onto pool boundary structures. The magnitudes of the pool boundary loads will be influenced by finite desynchronization among the condensation processes, which take place at the outlets of multiple (about one hundred) vent pipes. This paper investigates the effects of the condensation desynchronization on the pool boundary loads, with regard to test data obtained from large-scale tests which represented seven full-sized vent pipes in a BWR Mark II pressure suppression containment system. The desynchronization effects are studied by analyzing the test data in the time and frequency domains. Attempts are made to extrapolate the experimental results to the actual plant geometry. Mechanisms responsible for the desynchronization are also discussed.     

An assessment of PWR steam generator condensation at the Oregon State University APEX facility

This report describes the testing to assess steam generator U-tube steam condensation conducted at the Oregon State University Advanced Plant Experiment Test Facility from 2005 to 2007. Six separate SG condensation (without non-condensable gas) tests were conducted as part of this test program. These tests were designed to evaluate steam condensation rates in a scaled Pressurized Water Reactor steam generator at various primary and secondary side pressures and inlet steam mass flow rates. The experimental data will provide a basis to assess TRACE steam generator modeling techniques and to assist in development of improved models for condensation and steam generator thermal-hydraulics.     

Experimental study on the direct contact condensation of steam jet in the passive safety injection tank

The direct contact condensation and subsequent thermal mixing by the injected steam jet onto a quiescent coolant inside a tank were examined experimentally to simulate the phenomena in passive safety injection systems. Specifically, the influence of the steam injection velocity was studied. Even though the total flow rate of injected steam was unchanged, the pressure inside the tank increased quickly at the larger nozzle diameter. Additionally, at a larger nozzle diameter, the thickness of the thermal mixing zone decreased because the amount of direct contact condensation decreased. For the in-depth study on the role of the nozzle size for the thermal mixing, the particle image velocimetry method was used to understand the flow field of water inside the tank. The visualization results demonstrated the formation of a flow field in the coolant due to the expansion and contraction of the steam–air mixture boundary. Furthermore, the thermal mixing zone was found to be closely related to the penetration depth. Finally, a variety of penetration models were examined and compared against the experimental observation. The correlations based on the steam condensation approach under-predicted the penetration depth, whereas the approach that considers the momentum of non-condensable gas gave the reasonable prediction capability.