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Open-source environments such as the Computational Fluid Dynamics software OpenFOAM are very appealing for research groups since they allow for an efficient prototyping of new models or concepts. However, for downstream developments to be sustainable, i.e. reproduci-ble and reusable in the long term, a significant amount of maintenance work must be account-ed for. To allow for growth and extensibility, the maintenance work should be underpinned by a high degree of automation for repetitive tasks such as build tests, code deployment and validation runs, in order to keep the focus on scientific work. Here, an information technology environment referred to as OpenFOAM_RCS is presented that aids the centralized mainte-nance of simulation code and setup files for OpenFOAM developments concerned with reac-tor coolant system safety research. It fosters collaborative developments and review processes. State-of-the-art tools for managing software developments are adapted to meet the require-ments of OpenFOAM. A flexible approach for upgrading the underlying installation is pro-posed, based on snapshots of the OpenFOAM development line rather than yearly version releases, to make new functionality available when needed by associated research projects. The process of upgrading within so-called sprint cycles is accompanied by several checks to ensure compatibility of downstream code and simulation setups. Furthermore, the foundation for building a validation data base from contributed simulation setups is laid, creating a basis for continuous quality assurance. The content of this article was initially presented at the 33rd German CFD Network of Com-petence Meeting, held on March 22-23 2022 at GRS in Garching, Germany.

The Euler-Euler model is widely used in bubbly flow simulations up to industrial dimensions. The standard Euler-Euler model is based on the phase-averaging method. After averaging, the bubble forces in the field equations are functions of the local gas volume fraction. In simulations, when the bubble diameter is larger than the computational cell spacing, the forces can transport the gas belonging to the same bubble in different directions. By contrast, a closure model for the bubble force is typically developed based on the assumption that the force is a resultant force that acts on the bubble's center-of-mass. This inconsistency can lead to a nonphysical gas concentration in the center of a channel or near the channel wall if the bubble diameter is larger than the cell spacing. The purpose of the present contribution is to developed an Euler-Euler model where the bubble force consistency is recovered for two-phase flow simulations where the diameter of the disperse phase can be larger than the cell spacing. Such an Euler-Euler model is developed by combining an existing particle-center-averaged Euler-Euler framework with a Gaussian convolution method. To validate this Euler-Euler approach, a comparison is made with experimental data for the bubbly flows in two different vertical pipes. The results show that the proposed Euler-Euler model recovers the bubble force consistency and alleviates the over-prediction of the gas volume fraction peak near the wall, while its simulation results in the axial gas and liquid velocity and the liquid turbulence kinetic energy are similar to the results of the standard Euler-Euler model.

Bubble size and deformation are important factors for the closure models required in Euler-Euler simulations of bubbly flows. To properly simulate polydisperse bubbly flows where the bubble size spectrum may cover a range of several millimeters, several velocity groups with different sizes have to be considered. To this end, the theory for the particlecenter-averaged Euler-Euler model is generalized for the simulation with multiple bubble velocity groups. Furthermore, bubble deformation effects have been included in appropriate bubble force models. The particle-center-averaged Euler-Euler model provides additional freedom to consider the bubble shape during the conversion between the bubble number density and the gas volume fraction. Therefore, the theory is also generalized to consider an oblate ellipsoidal bubble shape in simulations. A bubbly pipe flow is used to validate the theory and to demonstrate the improvements of the proposed generalizations.

Effects of surfactants on lift coefficients, CL, of single ellipsoidal bubbles rising through linear shear flows were investigated. Two types of surface-active agents, Triton X-100 and 1-octanol, were used. The liquid properties except for the surface tension were identical to those in a clean system of logM = -5.5, where M is the Morton number. The range of the bubble Reynolds number was 0.1 < Re < 70. Bubble shapes were either spherical or ellipsoidal. Comparing with clean bubbles, less deformation of contaminated bubbles was confirmed. A shape correlation without taking the dimensionless shear rate into account gave good evaluations of the bubble aspect ratio, which means that the shear rate is not a dominant factor causing the change of shape deformation. However, drag coefficients were affected by the shear rate. A new correlation of drag coefficients was deduced, which agreed well with the experimental data. Both clean and contaminated CL data showed a similar tendency: after a drastic decrease in CL, CL slightly increases and then decreases to a negative value while increasing the bubble Reynolds number, Re. A difference in concentration of Trion X-100 resulted in only a slight change in CL at high Re regime. Different types of surfactant resulted in noticeably different values of CL especially at low Re. The CL of small bubbles in contaminated systems could be reproduced by a correlation for solid particles, supporting that fully contaminated spherical bubbles behave like solid spheres. For deformed bubbles, CL can be expressed by relating the negative lift force due to shape deformation with the drag force.

Pressure relief by blowdown is one of the most important measures to prevent excessive pressures in the primary circuit or containment in severe nuclear accidents. Pool scrubbing can significantly reduce the release of radioactive materials, e.g., aerosols, to the environment during the pressure relief. The decontamination factor indicating the particle retention efficiency depends, among other factors, on the hydrodynamic conditions of the gas-liquid two-phase flow inside the pool. In the present work, the hydrodynamics in two typical pool scrubbing experiments is investigated with the two-fluid model, and the influence of some key factors including bubble diameter, nozzle submergence as well as interaction models are analysed. One case is a rectangular pool and the other is a cylindrical column, and their injection Weber number is around 2×103 and 4×105, respectively. The numerical results show that as the distance from the nozzle exit increases, the void fraction and velocity field expand from the central region, where the nozzle is located, to the whole cross section. The profile and its development depends largely on the bubble size and the interaction force model. It reveals that in the monodisperse simulation, the tuning of bubble diameter is necessary for achieving good agreement, although it is difficult for high velocity gas injection. More information is required to properly describe the bubble size distribution as well as its evolution in pool scrubbing conditions. Furthermore, the experimental data show clear drag reduction in the bubble swarm generated by the gas jet, and the mechanism and model improvement possibilities need to be investigated.

Polydispersity is a challenging feature of many industrial and environmental multiphase flows, influencing all related transfer and transport processes. Besides their size, the fluid or solid particles may be distributed with respect to other properties such as their velocity or shape. Here, a population balance model based on the method of classes is combined with a multifluid solver within the open source computational fluid dynamics library OpenFOAM. The model allows for tracking the evolution of one or more size-conditioned secondary properties. It is applied to two different problems, the first being bubbly flow of air and water in a vertical pipe, where considering the velocity as a secondary property allows to resolve the size-dependent radial segregation. The second application is the gas phase synthesis of titania powder, where non-spherical particle aggregates appear whose shape is modeled through a collision diameter, leading to an improved prediction of the size distribution.

The properties of bubble-laden turbulent flows at different scales are investigated experimentally, focusing on the flow kinetic energy, energy transfer and extreme events. The experiments employed particle shadow velocimetry measurements to measure the flow in a column generated by a homogeneous bubble swarm rising in water, for two different bubble diameters ( $2.7$ mm and $3.9$ mm) and moderate gas volume fractions ( $0.26\,\%\sim 1.31\,\%$ ). The two velocity components were measured at high resolution, and used to construct structure functions up to twelfth order for separations spanning the small to large scales in the flow. Concerning the flow anisotropy, the velocity structure functions are found to differ for separations in the vertical and horizontal directions of the flow, and the cases with smaller bubbles are the most anisotropic, with a dependence on void fraction. The degree of anisotropy is shown to increase as the order of the structure functions is increased, showing that extreme events in the flow are the most anisotropic. Our results show that the average energy transfer with the horizontal velocity component is downscale, just as for the three-dimensional single-phase turbulence. However, the energy transfer associated with the vertical component of the fluid velocity is upscale. The probability density functions of the velocity increments reveal that extreme values become more probable with decreasing Reynolds number, the opposite of the behaviour in single-phase turbulence. We visualize those extreme events and find that regions of intense small-scale velocity increments occur near the turbulent/non-turbulent interface at the boundary of the bubble wake.

Heat transfer on a vapor bubble rising in superheated liquid is investigated by direct numerical simulation. The vapor–liquid system is described by the one-fluid formulation with the level set method capturing the interface. The proportional-integral-derivative controller is employed to keep the bubble's location fixed and evaluate interfacial forces. The heat transfer performance featured by the Nusselt number is evaluated based on the energy balance. Simulations are carried out for the bubble Reynolds number ranging from 20 to 500 and Morton number from 1.10 × 10−10 to 3.80 × 10−4. The aim of this paper is to shed some light on the effect of bubble deformation and oscillation on interfacial heat transfer. The results show that the front part of the bubble contributes to the majority of the interfacial heat transfer, while the rear part mainly affects the oscillation amplitude of the total heat transfer. The interface stretch during bubble oscillation is considered as a key mechanism in enhancing the instantaneous Nusselt number. The potential flow solution of the averaged Nusselt number is corrected by considering the influence of the aspect ratio. This research provides additional insights into the mechanism of interfacial heat transfer, and the results apply equally to interfacial mass transfer.