title: Computational study of thermotechnical two-phase flow configurations with scale-resolving turbulence models
Creators: name: Joksimović, Ivan
Publisher: name: UNSPECIFIED
Year: 2023

description: The present work deals with the further development of a novel scale-resolving model of turbulence, in terms of its computational validation and in-depth analysis of its predictive performance under the conditions of complex interacting flow and thermal fields, as encountered in the thermotechnical two-phase flow systems. Computational capturing of the phenomena being presently of interest that commonly characterize industrial flow configurations and require coupled modeling of multiple flow fields include: high turbulence intensity and large-scale instabilities,heat transfer and interphase interaction accounting also for free surface-induced effects.Their correct capturing is either beyond the capabilities of classical RANS (Reynolds-Averaged Navier-Stokes) models with respect to their inherent time-averaged theoretical foundation or is too costly for correspondingly well-resolved LES and DNS approaches. By resolving the spectrum to a reasonable extent in a grid-free spacing free manner, it is expected that the most important flow features will be captured directly, while the non-resolved residual turbulence will be modeled with highest possible accuracy. Particularly suitable for the latter are the models relying on the second-moment closure concept, as presented in Jakirlić and Maduta (2015). The key question behind the outlined research is whether the accuracy of the model and competitive resources for its performance (reflected in the relatively modest grid size, as compared with LES methods) can be extended far beyond the parameter space used for its calibration and development. This dissertation addresses this question by examining some specifically configured thermotechnical flow configurations using a higher-order, scale-resolving turbulence model, termed as Improved Instability-Sensitive Reynolds-Stress Model (IIS-RSM), and associated numerical algorithms in the context of the Sensitized RANS framework.In total, six complex flow configurations, involving a variety of all the above-mentioned phenomena, are covered by the modeling paradigm adopted. Systematical testing of the scale-resolving capability of the model scheme is initially performed over a range of canonical, but relevant pipe configurations. Afterwards, the IIS-RSM is systematically validated in simulating thermal mixing in three differently configured T-junction configurations that exhibit a complex flow topology resulting from structural variations in inflow properties and strong temperature gradients causing high-level turbulence instabilities. Moreover, the flow cases were chosen that cover the widest possible range of Reynolds numbers consistent with practically relevant operational conditions. The eddy-resolving Reynolds-stress model was further coupled with the Euler-Lagrangian methodological framework to evaluate its suitability for predicting two-phase flow systems. Accordingly, three gas-liquid two-phase flow configurations were selected, representing differently arranged bubbly columns and a bubble stream generated by an emerged water jet exiting into a pool. The latter flow configuration occurring at a high Reynolds number is characterized by a jet-induced secondary motion. Additionally, model formulations describing the Bubble-Induced-Turbulence (BIT), including the model recently proposed by Ma et al. (2020), are tested for the first time as part of the complete computational model operating in a scale-resolving mode.In addition to the common results interpretation showing the evolution of global properties and individual profiles of various variables characterizing the underlying flow and thermal fields as well as those of turbulence quantities, Proper-Orthogonal-Decomposition (POD) is used throughout the work to analyze and extract coherent flow features as a means of identifying prevalent flow mechanisms.All simulation results show a high degree of accuracy with respect to the avail-able experimental or otherwise numerically determined reference data. Both the statistical properties of the flow and its dynamic behavior are correctly captured qualitatively and quantitatively by the model, with remarkable reductions in the necessary spatial and temporal resolution. In the calculations of the two-phase bubbly columns, the quality of the previous studies is maintained, while the robustness and stability of the calculations have been significantly improved. Certain weak points of the IIS-RSM have been identified, and indications for future improvements are proposed and presented in the conclusion. In this way, a reliable computational tool is obtained that is capable of accurately predicting a variety of computationally challenging flow phenomena.