Khmil D. Heat transfer and spatial structure of distribution of thermal physical properties of supercritical water in tubes under mixed convection conditions

In the introduction, the relevance of the dissertation topic is substantiated, the goal and objectives of the research are formulated, the scientific novelty and practical value of the obtained results are presented, the personal contribution of the candidate is defined, and information regarding the approval, structure, and volume of the dissertation is provided. The first chapter of the dissertation is devoted to the analysis of professional sources concerning the study of heat exchange patterns in channels under supercritical pressures. The peculiarities of CFD modeling of the studied physical situation are considered. The results of the analysis of the state of research on the flow and heat exchange of supercritical water under conditions of mixed convection are presented. Particular attention is paid to the study of distributions of thermophysical properties in flows of supercritical heat carriers.Based on the analysis of the state of the considered problem, the purpose and main tasks to be solved in the dissertation are formulated. The second chapter highlights the peculiarities of the methodology of mathematical and computer modeling according to the tasks of the work. A mathematical model of the studied process is presented, which corresponds to an axisymmetric nonlinear problem of flow and heat transfer of supercritical water in vertical smooth heated tubes. Data on the verification of the turbulent transfer model for the studied physical situation are presented. The results of the analysis regarding the application of the selected turbulence model for different heat exchange regimes under conditions of ascending flow of supercritical water in tubes are also presented. The third chapter is dedicated to the study of mixed convection of supercritical water during ascending flow in vertical heated tubes under conditions of average water mass velocities and different magnitudes of heat fluxes supplied to the wall. Results of CFD modeling of the flow patterns of supercritical water for the studied physical situation are presented. In particular, it is shown that the regime of mixed convection is observed only on a certain central length of the tube. Within the localization zone of mixed convection, its manifestation along the length of the channel increases, reaches a maximum, and then decreases. Data on studies concerning the peculiarities of the flow of supercritical water when varying the magnitude of the heat flux q on the wall of the tube from 239 to 310 kW/m² are presented. It is shown that the length of the mixed convection zone in the central part of the channel significantly increases with the increase of the heat flux q. The larger the value of q, the closer to the inlet cross-section of the tube this zone begins and the further from it it ends. A more pronounced local maximum on M-shaped profiles of the speed of supercritical water corresponds to larger values of the heat flux q supplied to the wall of the channel. The fourth chapter presents the results of the conducted studies on determining the spatial distributions of properties of supercritical water in channels under different values of the heat flux supplied to the wall under conditions of mixed convection. Based on the solution of the axisymmetric nonlinear problem of heat transfer during the flow of supercritical water in channels, the peculiarities of its temperature fields are established, on the basis of which these distributions are constructed. An analysis of the patterns of movement of pseudo-phase transition fronts, which largely determine the character of the distribution of thermophysical properties of supercritical water in channels, is carried out. The scientific novelty of the obtained results: • Based on the results of a comparative analysis of modeling convective heat exchange with and without consideration of Archimedean forces for ascending flows of supercritical water in vertical smooth tubes, the effect of a local lengthwise manifestation of the noticeable influence of Archimedean forces on the flow pattern is revealed, and its dependence on determining factors is established. • The presence of a correlation of radial profiles of the axial speed of supercritical water with the change along the length of the channel of the local Richardson number is established. • The patterns of the influence of the magnitude of the heat flux density on the wall of the tube on the length along the channel of the mixed convection zone, the speed of the pseudo-phase transition front movement, etc., are revealed. • The peculiarities of the structure of spatial distributions of physical properties of supercritical water (thermal conductivity coefficient, specific heat capacity, viscosity, and density) during its ascending flow in vertical channels are revealed, and the patterns of the influence on these distributions of Archimedean forces and the density of the heat flux supplied to the walls of the channel are established