Melnyk R. Capillary structure parameters influence on heat transfer intensity for sub-atmospheric boiling and capillary feeding conditions

In the introduction, the relevance of the work is justified, the purpose is defined, and the object and subjects of research are identified. The scientific novelty of the obtained results is indicated, and information about the researcher's personal contribution is provided. Information about the approval of the research results is also given. The structure and scope of the dissertation work are described. In the first chapter, a literature review is presented on the main types of heat transfer devices based on the vapor-condensation cycle, which use porous structures. The most common types of porous structures and the materials used in their manufacture are analyzed. In general, they are made from various materials such as copper, titanium, nickel, and others. The influence of the type of structure and its parameters on the heat transfer intensity during boiling of different heat carriers is also analyzed. The second chapter is dedicated to determining the coefficients of liquid permeability of metal fiber porous structures along the plane of felting. A scheme of the experimental setup and research methodology is presented. Although the determination of permeability coefficients of metal fiber capillary structures (MFCS) over a wide range of parameters has been conducted previously, the conditions under which the research was carried out did not fully correspond to the conditions in heat pipes and steam chambers, where the heat transfer fluid mainly moves along the plane of felting. Previous studies were conducted under conditions where the heat transfer fluid moves perpendicular to the plane of felting. As a result, it was determined that the direction of liquid filtration through a porous sample affects the coefficient of liquid permeability. Explanations for this deviation in the results were proposed, and dependencies for calculating the coefficient of liquid permeability for filtration conditions along the plane of felting were also proposed. The third chapter presents research on capillary pressure for metal fiber porous structures over a wide range of structural characteristics, including fiber diameters and porosities. Capillary pressure studies were conducted for porous structures with a porosity range from 60% to 90% and fiber diameters ranging from 10 to 50 micrometers. The influence of the filtration direction on the permeability coefficient was determined by comparing the results with published results of similar studies. The fourth chapter presents the results of determining the heat transfer coefficients depending on operational, structural, and geometric factors. The main type of capillary structures (CS) chosen for the study was the metal fiber structure, which has advantages over other types of porous structures and has not been studied under conditions close to those in heat pipes or steam chambers. In addition, this type of porous structure has been well studied over a wide range of parameters under boiling conditions in a large volume at atmospheric pressure, which allows for result comparison and verification of the research methodology. For the purpose of comparing heat transfer efficiency, several samples were manufactured from sintered powder. As a result of the comparison, it was found that the heat transfer efficiency of powder capillary structures (PCS) is higher than that of metal fiber capillary structures (MFCS) under boiling conditions in a large volume and under capillary transport conditions, but the maximum heat flux densities for MFCS were 12-96% higher. With a decrease in saturation temperature, the heat transfer efficiency for samples made from sintered powder was lower than that for MFCS samples. The maximum heat flux densities of samples made from metal felt also exceeded the values obtained for powder CS samples. The fifth chapter is dedicated to the development of a physical model of vapor formation on porous structures and the generalization of the obtained data. As a result of visual observations of samples during the main experiments and their comparison with temperature values, it was proposed for the first time to classify the vapor formation processes on CS under capillary transport conditions into stages, with a description of these stages and possible causes of their occurrence. Visual observations were conducted for both powder structures and metal fiber structures. Qualitatively, the results of observations on them did not differ, which allows us to assume that other types of porous structures with aperiodic structure behave in a similar way. Dimensionless dependencies of the form Nu=f(Re) for different saturation temperatures were obtained, describing the experimental points with a maximum deviation of ±30%.