Bekhmard H. Heat transfer characteristics of miniature two-phase thermosyphons for cooling elements of electronic equipment

Behmard Gholamreza. Heat transfer characteristics of miniature two-phase thermosyphons for cooling elements of electronic equipment. - Qualifying scientific work on the rights of the manuscript. The dissertation for the degree of the candidate of technical sciences (PhD) on a specialty 05.14.06 “Technical thermal physics and industrial heat-power engineering”. – National Technical University of Ukraine “Igor Sikorsky Kyiv Polytechnic Institute” MES of Ukraine, Kyiv, 2021. The dissertation is devoted to the decision of an actual scientific and technical problem of intensification of heat exchange in miniature evaporating and condensing cooling systems for the purpose of increase of their heat transfer properties and creation of effective devices of heat transfer on the basis of miniature thermosyphons. The dissertation work is devoted to the study of regularities of heat transfer and heat transfer characteristics of miniature thermosyphons with different heat carriers including nanofluids. The rapid development of radio electronics, and especially micro radio electronics, has posed an urgent task for instrument and equipment developers to create reliable cooling systems capable of providing efficient heat dissipation by increasing the maximum heat fluxes diverted from miniature semiconductor devices. The most effective in this case are cooling using evaporation-condensation systems as elements of the tract, which have an effective thermal conductivity, which is several orders of magnitude higher than the most thermally conductive existing materials (copper, silver, etc.). The use of thermosyphons in technology can significantly improve the technical and economic parameters of heat-loaded equipment, to solve at a higher level the problems arising as a result of scientific and technological progress to cool the elements of electronic equipment. The main heat transfer characteristics of closed two-phase evaporation-condensation systems (heat pipes and thermosyphons) are the maximum heat flow and the minimum thermal resistance. The difference between miniature thermosyphons and "conditionally large" ones is determined by the Bond number (Bo <4), which takes into account the influence of small sizes on the processes of vaporization during boiling. According to the analysis of international and national experience scientific publications on the general heat transfer characteristics of closed two-phase thermo-siphons, it is shown that the data for the development of optimal designs of miniature thermo-siphons are insufficient. The influence of various factors on their heat transfer characteristics, temperature regimes, different diameters, lengths and heat carriers is little presented in the literature. Taking into account this, the main directions of studying the flow of heat transfer processes in closed evaporation-condensation systems of small size (miniature thermosyphons) are determined. The design of the experimental stand and the method of conducting research and processing experimental data have been developed. The temperature regimes of miniature thermosyphons are presented, and it is shown that as a result of the formation of the thermal boundary layer, conditions arise in the heating zone when the liquid is in a metastable state when the inner diameter decreases. This is due to the fact that the thickness of the thermal boundary layer becomes commensurate with the size of the internal space in the area of the heating zone. The resulting steam bubbles after separation from the heat exchange surface increase in size and reach the inner diameter of the thermosyphon. It is shown that as the heat flux increases, the temperature pulsations in the heat exchange zones decrease and then disappears. The influence of the filling coefficient and the influence of thermophysical properties of heat carriers on the temperature regime of thermosyphons are revealed. The physical picture of the movement of the condensate film and its thickness, which affects the reduction of the inner diameter of the vapor space of the miniature thermos-background, is presented. It is shown that the film thickness depends on the thermophysical properties of heat carriers. Based on the dimensional analysis, new dimensionless complexes are obtained that affect the values of thermal resistance and heat transfer intensity in the heat exchange zones of a miniature thermosyphon. The dependences for the calculation of thermal resistance and heat transfer coefficients in the heat exchange zones of miniature thermosyphons are obtained. It is shown that the minimum thermal resistance increases with decreasing inner diameter of the thermosyphon. Comparison of data on thermal resistance with existing dependences showed a large discrepancy between the results.