Numerical Simulations of Boiling Jet Impingement Cooling in Power Electronics

This paper explores turbulent boiling jet impingement for cooling power electronic components in hybrid electric vehicles

A CFD-DEM solver to model bubbly flow. Part I: Model development and assessment in upward vertical pipes

[EN] In the computational modeling of two-phase flow, many uncertainties are usually faced in simulations and validations with experiments. This has traditionally made it difficult to provide a general method to predict the two-phase flow characteristics for any geometry and condition, even for bubbly flow regimes. Thus, we focus our research on studying in depth the bubbly flow modeling and validation from a critical point of view. The conditions are intentionally limited to scenarios where coalescence and breakup can be neglected, to concentrate on the study of bubble dynamics and its interaction with the main fluid. This study required the development of a solver for bubbly flow with higher resolution level than TFM and a new methodology to obtain the data from the simulation. Part I shows the development of a solver based on the CFD-DEM formulation. The motion of each bubble is computed individually with this solver and aspects as inhomogeneity, nonlinearity of the interfacial forces, bubble-wall interactions and turbulence effects in interfacial forces are taken into account. To develop the solver, several features that are not usually required for traditional CFD-DEM simulations but are relevant for bubbly flow in pipes, have been included. Models for the assignment of void fraction into the grid, seeding of bubbles at the inlet, pressure change influence on the bubble size and turbulence effects on both phases have been assessed and compared with experiments for an upward vertical pipe scenario. Finally, the bubble path for bubbles of different size have been investigated and the interfacial forces analyzed. (C) 2017 Elsevier Ltd. All rights reserved.The authors sincerely thank the ''Plan Nacional de I + D+ i" for funding the project MODEXFLAT ENE2013-48565-C2-1-P and ENE2013-48565-C2-2-P.Peña-Monferrer, C.; Monrós Andreu, G.; Chiva Vicent, S.; Martinez-Cuenca, R.; Muñoz-Cobo, JL. (2018). A CFD-DEM solver to model bubbly flow. Part I: Model development and assessment in upward vertical pipes. Chemical Engineering Science. 176:524-545. https://doi.org/10.1016/j.ces.2017.11.005S52454517

RANS simulation of bubble coalescence and break-up in bubbly two-phase flows

In bubbly flows, the bubble size distribution dictates the interfacial area available for the interphase transfer processes and, therefore, understanding the behaviour and the average features of the bubble population is crucial for the prediction of these kinds of flows. In this work, by means of the STAR-CCM+ code, the Sγ population balance model is coupled with an Eulerian–Eulerian two-fluid approach and tested against data on upward bubbly pipe flows. The Sγ model, based on the moments of the bubble size distribution, tracks the evolution of the bubble sizes due to bubble break-up and bubble coalescence. Good accuracy for the average bubble diameter, the velocity and the void fraction radial profiles is achieved with a modified coalescence source. Numerical results show that better predictions are obtained when these flows are considered to be coalescence dominated, but, nevertheless, additional knowledge is required to progress in the development of coalescence and break-up models that include all the possible responsible mechanisms. In this regard, there is a requirement for experimental data that will allow validation of both the predicted bubble diameter distribution and the intensity of the turbulence in the continuous phase which has a significant impact on coalescence and break-up models. An advanced version of the model, that includes a Reynolds stress turbulence formulation and two groups of bubbles to account for the opposite behaviour of spherical bubbles, which accumulate close to the pipe wall, and cap bubbles, that migrate towards the pipe centre, is proposed. The Reynolds stress model is found to better handle the interactions between the turbulence and the interphase forces, and the use of only two bubble groups seems sufficient to describe the whole bubble spectrum and the bubbly flow regime up to the transition to slug flow

Accuracy of Eulerian–Eulerian, two-fluid CFD boiling models of subcooled boiling flows

Boiling flows are frequently found in industry and engineering due to the large amount of heat that can be transferred within such flows with minimum temperature differences. In the nuclear industry, boiling affects in different ways the operation of almost all water-cooled nuclear reactors. Recently, the use of computational fluid dynamic (CFD) approaches to predict boiling flows is increasing and, in the nuclear area, CFD is being developed to solve thermal hydraulic safety issues such as establishing the critical heat flux, which is perhaps the major threat to the integrity of nuclear fuel rods. In this paper, the accuracy of an Eulerian–Eulerian, two-fluid CFD model is evaluated over a large database of subcooled boiling flows, avoiding the rather popular case-by-case tuning of descriptive models to a limited number of experiments. The model includes a Reynolds stress turbulence model, the method of moments-based S-gamma population balance approach and a boiling model derived using the heat flux partitioning approach. The database covers a large range of conditions in subcooled boiling flows of water and refrigerants in vertical pipes and annular channels. Overall, a satisfactory predictive accuracy is achieved for some quantities of interest, such as the void fraction and the turbulence and liquid temperature fields, but results are less satisfactory in other areas, more specifically for the average bubble diameter and the mean velocity profiles close to the wall in annular channels. Agreement may be improved with advances in the treatment of large bubbles and bubble break-up and coalescence, as well as in improved modelling of the boiling region close to the wall, and more specifically the bubble departure diameter, the wall treatment and the contribution of bubbles to turbulence

Multiphase turbulence in bubbly flows: RANS simulations

The ability of a two-fluid Eulerian–Eulerian computational multiphase fluid dynamic model to predict bubbly air–water flows is studied. Upward and downward pipe flows are considered and a database of 19 experiments from 6 different literature sources is used to assess the accuracy of the model, with the aim of evaluating our ability to predict these kinds of flows and to contribute to ongoing efforts to develop more advanced simulation tools. The particular focus in the work described is on the prediction of multiphase turbulence due to its relevance in the modelling of bubbly flows in general, including bubble coalescence and break-up, and boiling at a wall. Overall, a satisfactory accuracy is obtained in the prediction of liquid velocities and void fraction distributions in all conditions, including upward and downward flows, and wall-peaked and core-peaked void profiles, when values of the bubble diameter are specified from experimental data. Due to its importance for the correct prediction of the turbulence level in these flows, a bubble-induced turbulence model is introduced, starting from an existing formulation. Source terms due to drag are included in the turbulence kinetic energy and the turbulence energy dissipation rate equations of the k-ε turbulence model, and optimisation of the turbulence source gives velocity fluctuation predictions in agreement with data. After comparisons with data, improvement in the predictions of other turbulence models is also demonstrated, with a Reynolds stress formulation based on the SSG (Speziale et al., 1991) pressure–strain model and the same bubble-induced turbulence model accurately predicting the two-phase flows and the anisotropy of the turbulence field. The same database is also exploited to evaluate different drag models and the advantages of including the effect of the bubble aspect ratio. Following experimental evidence, the model of Tomiyama et al. (2002) is used which assumes that the bubble shape is closer to spherical near a wall and employs a correlation to calculate the aspect ratio. An increase in the drag coefficient due to the higher aspect ratio increases the accuracy of calculated velocity profiles in the near-wall region, even if additional validation is still required due to the possible loss of accuracy in the pipe centre

Influence of multiphase turbulence modelling on interfacial momentum transfer in two-fluid Eulerian-Eulerian CFD models of bubbly flows

Eulerian-Eulerian two-fluid computational fluid dynamic (CFD) models are increasingly used to predict bubbly flows at an industrial scale. In these approaches, interface transfer is modelled with closure models and correlations. Normally, the lateral void fraction distribution is considered to mainly result from a balance between the lift and wall lubrication forces. However, and despite the numerous models available that achieve, at least in pipe flows, a reasonable predictive accuracy, agreement on a broadly applicable and accurate modelling approach has not yet been reached. Additionally, the impact of turbulence modelling on the lateral void fraction distribution has not, in general, been examined in detail. In this work, an elliptic blending Reynolds stress model (EB-RSM), capable of resolving the turbulence field in the near-wall region and improved to account for the contribution of bubble-induced turbulence, is evaluated against best-practice k-ε and high-Reynolds second-moment turbulence closures. Lift and wall lubrication forces are initially deliberately neglected in the EB-RSM. Comparisons for flows in pipes and a square duct show that the EB-RSM reproduces the lateral void fraction distribution, including the peak in the void fraction in the near-wall region, and reaches an accuracy comparable to the other two models noted above. In rod bundles, even if none of the models considered performs with sufficient accuracy, the EB-RSM detects features of the flow that are not predicted by the other two approaches. Overall, the results demonstrate a much more prominent role of the turbulence structure and the induced cross-sectional pressure field on the lateral void fraction distribution than is normally considered. These effects need to be accounted for if more physically-consistent modelling of bubbly flows is to be achieved. The lift force is added to the EB-RSM in the final part of the paper, to provide a two-fluid formulation that can be used as the basis for additional developments aimed at improving the accuracy and general applicability of two-fluid CFD models

Simulation of the loss of the residual heat removal of an integral test facility using computer code Cathare7

Due to the character of the original source materials and the nature of batch digitization, quality control issues may be present in this document. Please report any quality issues you encounter to [email protected], referencing the URI of the item.Includes bibliographical references: 51-52.Issued also on microfiche from Lange Micrographics.Assessment of the ability of thermal hydraulic codes to correctly predict complex phenomena during reactor transients can be performed only by means of comparison against experimental data. The thermal hydraulic CATHARE V1.3U computer program has been used to simulate International Standard Problem (ISP38) experiment conducted at BETHSY Integral Test Facility located in Grenoble, France. BETHSY is an integral test facility modeling the three-loop, 2775 thermal NM Framatome Pressurized Water Reactor (PWR). Its main objectives are to contribute to the assessment of CATHARE code and of the physical basis of PWR Emergency Operating Procedures. This investigation dealt with experimental simulation of the loss of the Residual Heat Removal System (RHRS) during midloop operation. It involved opening of the pressurizer manway and the steam generator outlet plenum manway simultaneously with switching on the power of heating rods, thus simulating loss of the RHRS. The total power was kept unchanged with the level 138 kW throughout the test. Mass discharge through both manways led to core boiling and uncovery. A gravity feed injection of cold water was actuated in the cold leg when cladding temperature at the top of heating rods reached 250 C. The test was stopped when the primary cooling system was filled back to rnidloop level. The transient was executed on the HP6400 workstation at Texas A&M University. The code predictions underestimated the time of the core uncovery and the actuation of the gravity feed injection due to the ovepredicted mass discharge through steain generator manway during the initial stage of the transient. This was probably caused by CATHARE's miscalculation of the phase separation effect at the hot leg/surge line tee junction and significant water entrainment into the surge line in the beginning of the test. It was found that the model of the upward tee junction needs to be refined for the low pressure range. Overall, the code's predictions were in a qualitative agreement with the experimental data

ANALYSIS OF THE IMPACT OF ELECTRIC VEHICLES ON THE URBAN STRUCTURE OF RUSSIA IN THE CONTEXT OF INDUSTRY 4.0

This study will present a study on the impact of electric vehicles on Russia’s urban infrastructure. In the course of studying this issue, the current state of the electric vehicle market, the factors contributing to its development, and the specifics of the impact of this market on Russia’s urban infrastructure will be examined