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A time-averaged Eulerian-Eulerian (E-E) multiphase approach for the simulation of boiling flows is presented. A previously developed bubble dynamics model is implemented in the E-E framework as a sub-model to improve the accuracy of the simulations and reduce the case dependency. This mechanistic model, which is based on the balance of forces applied on a single bubble, considers the evaporation of the microlayer underneath the bubble, thermal diffusion and condensation around the bubble cap as well as geometry changes and dynamic inclination and contact angles. The advantage of this model is that it does not require a recalibration of parameters to predict the bubble departure size. However, its implementation in the E-E framework needs an extension of the current nucleation site activation and heat partitioning model. Further on, we use a population balance model in our approach. With the new modelling approach, we are able to analyse the impact of bubble sliding on the heat transfer, which has been rarely considered in other modelling approaches. Validation was made with experimental data from flow in a vertical pipe and an annulus. The experimental test cases cover a wide range of operating parameters such as wall heat flux, fluid velocity, subcooling temperature and pressure.

The condensation of saturated steam bubbles in sub-cooled water inside a vertical pipe was studied by poly-disperse CFD simulations. Six test cases with varied pressure, liquid sub-cooling and diameter of the gas injection orifices were simulated. Baseline closures presented for non-drag forces in previous work were found to be reliable also in non-isothermal cases. The effect of bubble coalescence and breakup is over-weighting in the region close to steam injection in case of small orifice diameter. With the increase of orifice diameter, breakup becomes dominant in determining bubble size change. The effect of interphase heat transfer coefficient correlations was investigated. The widespread Ranz–Marshall correlation was found to under-estimate the condensation rate, especially at high pressure levels. In contrast, satisfying agreement with the experimental data was obtained by the Tomiyama correlation.

This paper describes a CFD based strategy for the modeling of stratified two-phase flows with heat and mass transfer across a moving steam-water interface due to direct contact condensation. Such flows have been of major importance for example in connection with the analysis of nuclear reactor safety systems, in particular during two-phase Pressurized Thermal Shock (PTS) scenarios. The approach is based on the two-fluid phase-average model. The interfacial friction was modeled by using an Algebraic Interfacial Area Density (AIAD) framework where the drag coefficient is a function of the local flow characteristics. To show the impact of the modeling of interfacial friction the simulation with the AIAD model was compared with a simulation where a constant drag coefficient of 0.44 was used in the whole domain. For the modeling of interfacial heat and mass transfer two correlations for the water heat transfer coefficient based on the penetration theory were utilized. The CFD simulations were validated against a steady-state TOPFLOW-PTS steam/water experiment. In the experiment, very detailed temperature measurements were conducted using special thermocouple lances and infrared thermography. Total condensation rate was determined indirectly by using three different methods. The simulations have shown that the results obtained with the AIAD model are considerably closer to the experimental observations than the results obtained with the constant drag coefficient. The condensation models used in the current study predict quite different total condensation rates. That caused significant differences in the temperature field. The simulations of the TOPFLOW-PTS steam/water experiment with condensation have shown that the proposed CFD modeling approach can be successfully applied for the prediction of temperature field and condensation rate during two-phase Pressurized Thermal Shock scenarios. However, the modeling of turbulent interfacial heat transfer should be improved.

The ultrafast electron beam X-ray computed tomography was developed during the last years to obtain detailed data on two-phase flows. In a recent study we investigated different gas–liquid flow regimes in a vertical pipe at the Transient Two-phase Flow test facility (TOPFLOW). The study includes experiments on gas–liquid flows with varied superficial velocities for both phases and different flow directions. The obtained data is required for understanding fundamental physics of two-phase flow phenomena and for the development and validation of CFD codes. To extract quantitative data from the reconstructed three-dimensional data array, a new segmentation algorithm was developed, due the results of existing segmentation algorithms aren't satisfyingly. The originality of this new algorithm bases on a stepwise creation of new bubbles using pixel agglomeration in shrinking steps without defining markers or starting points. The results were compared with threshold and gradient methods using two different bubble phantoms and real two-phase flow measurements. The new algorithm shows the best qualitative and quantitative results.

A comprehensive database was obtained for stationary upward air–water flows in a vertical pipe with an inner diameter of 195.3 mm using the wire-mesh sensor technology. During the experiments the sensor was always mounted on the top of the test section while the distance between gas injection and measuring plane was varied to up to 18 different L/D by using gas injection chambers at different vertical positions. The gas was injected via holes in the pipe wall. The pressure was kept at 0.25 MPa (absolute) at the location of the active gas injection while the temperature was constant at 30 °C ± 1 K. This procedure exactly represents the evolution of the flow along the pipe, as it would be observed for an injection at a constant height position and a shifting of the measurement plane. The experiments were done for 48 combinations of air and water superficial velocities varying from 0.04 m/s to 1.6 m/s for water and 0.0025 m/s to 3.2 m/s for air. From the raw data time-averaged data as: radial gas volume fraction profiles, bubble size distributions, radial volume fraction profiles decomposed according to the bubble size and the radial profiles of the gas velocity were calculated. Due to the combination of the new experimental procedure with the high spatial and temporal resolution of the wire-mesh sensor technology the data have new quality especially regarding their consistency in the evolution with increasing L/D. This closes a gap for data suitable for CFD code development and validation for two-phase flows, especially for models on bubble coalescence and break-up.

During two-phase flow blowdown from pressure vessels considerable pulsations of the discharged mass flow rate were found. Regions of mass flow rate instability were predicted by a linear stability analysis. The pulsations are caused by the following closed feedback circuit: boil up – level swell – void fraction of the discharged mixture – critical discharge rate – velocity of pressure decrease – boil up. They were also observed in transient simulations of the depressurization. Finally, the regions of instability were confirmed by experiments. The possibility of the occurrence of pulsations increases with the volume of the ventline and the volume void fraction of the discharged mixture. They may influence the rate of pressure relief from pressure vessels as well as from chemical reactors.

The wire-mesh sensor developed by the Forschungszentrum Rossendorf produces sequences of instantaneous gas fraction distributions in a cross section with a time resolution of 1200 frames per second and a spatial resolution of about 2–3 mm. At moderate flow velocities (up to 1–2 m·s−1), bubble size distributions can be obtained, since each individual bubble is mapped in several successive distributions. The method was used to study the evolution of the bubble size distribution in a vertical two-phase flow. For this purpose, the sensor was placed downstream of an air injector, the distance between air injection and sensor was varied. The bubble identification algorithm allows to select bubbles of a given range of the effective diameter and to calculate partial gas fraction profiles for this diameter range. In this way, the different behaviour of small and large bubbles in respect to the action of the lift force was observed in a mixture of small and large bubbles.

A method for the prediction of the radial gas profile for a given bubble size distribution is presented. It is based on the assumption of the equilibrium of the forces acting on a bubble perpendicularly to the flow direction. These forces strongly depend on the bubble size [14, 18]. For the simulation of transient flow regime effects, the modelling of several bubble classes in a 1D model and consideration of their radial profiles seems to be more promising than a detailed 3D modelling. The radial profile of the liquid velocity is calculated by the model of Sato [21, 22]. On the basis of this velocity profile, radial distributions are calculated separately for all bubble classes according to the given bubble size distribution. The sum of these distributions is the radial profile of the gas fraction. It is used in an iteration process to calculate a new velocity profile. There is a strong interaction between the profiles of liquid velocity and gas volume fraction. The model is the basis of a fast running one-dimensional steady state computer code. The results are compared with experimental data obtained for a number of gas and liquid volume flow rates. There is a good agreement between experimental and calculated data. In particular, the change from wall peaking to centre peaking gas fraction distribution is well predicted.

A new one-dimensional particle-in-cell transport model for multiphase flow in a vessel is presented. The model aims at the consistent simulation of discontinuities as the top level of a multi-phase mixture. Thus it is possible to include models for the transient behaviour of a foam layer on top of a mixture, for example. The transport model, which is the basic component of a new computer code will be described. Flexible interfaces allow the implementation of models, constitutive laws or correlations for extra effects like phase transfer, generation and coalescence of bubbles or drops, foam behaviour, heat transfer, discharge from the vessel, etc. Due to these interfaces and a transparent code structure the code is a suitable basis for the development, testing and validation of models. It allows the completion or the replacement of such models according to the specific application.