A Study On Boiling And Condensation Co-existing Phase Change Heat Transfer In Closed Small Spaces

In this study, theorectical analysis, numerical simulations and experimental testswere used to investigate the heat sink as well as boiling and condensationco-existing phase change heat transfer in closed small spaces. The main contents areas follows.A one-dimensional steady state heat transfer model of heat sinks isestablished. The analytical solutions of the maximum heat source temperaturethat can be used for design the size of heat sinks are presented. A gravity assistedheat pipe with high heat transfer capacity used for electronic cooling is developed.The two most important dimensionless parameters that describe the geometry and heattransfer characteristics of the heat sinks are defined and their influences on themaximum temperature of heat sinks are analyzed. The results show that the maximumheat source temperature decreases with the increase of heat sink size. It is also shownthat if the heat sink size is large enough then the effectiveness in reducing themaximum heat source temperature by increasing the heat sink size is significantlydecreased. A gravity assisted heat pipe for electronic cooling is developed based onthe analysis of the innerside structure and the heat transfer processes of the heat pipe.The heat transfer performance of the heat pipe can be significantly improved byenhancing the heat transfer processes of evaporation and condensation as well asreducing the flow resistance of the condensate liquid.An experimental system that used to investigate boiling and condensationco-existing phase change heat transfer phenomenon in closed small spaces isdesigned and set up. The reliability of experimental system and the method oftests and data deduction are confirmed by both experiments and numericalsimulations. The test section is a closed and confined phase change chamber thatconsists of a boiling surface, a condensation surface and a sidewall. An extended thinfilm is manufactured directly from the top end of the copper rod that used to avoid oreliminate the edge effects. A transparent quartz glass tube is the sidewall of theconfined chamber that used for high quality of observation. The bottom end of thecooling copper block, whose top is manufactured with the water-cooling fin array, isused as the condensation surface of the confined chamber. The assumption ofone-dimensional temperature distribution of the heated copper rod as well as thereliability of the method to calculate the heat flux and the temperature of boilingsurface are all verified by numerical simulation and experiments.The characteristics of boiling and condensation co-existing phase changeheat transfer phenomenon in closed small spaces are studied. The influences of liquid levels and heat fluxes on heat transfer performances are analyzed andtypical images are presented. Experimental results show that there exist an optimumliquid level at which both boiling and condensation heat transfer coefficients acquiredtheir maximum values for the fixed height of confined space of33mm. Experimentalobservation shows that boiling and condensation processes have significant influencesover each other that include the interaction between vapor bubbles and the condensateliquid film, the interaction between vapor bubbles and condensate liquid drops, thedisturbance of the liquid surface caused by the dripping of condensate drops and so on.Besides, some particular phenomenon are found, such as most of the isolated bubblesstay on the water surface after departed from the heating surface and exist for quite awhile rather than breakdown immediately, the processes of the bubble coalescentoccurring on the water surface rather than below the water surface, the liquid bridgesbetween condensation surface and water surface and so on. Based on the theories ofheat transfer and vapor dynamics, the typical images of the particular phenomenonand boiling-condensation interaction are discussed and analyzed.The effects of different gap sizes on boiling and condensation co-existingphase change heat transfer characteristics are experimentally investigated. Theinfluences of gap sizes, filling rate and heat fluxes have on boiling and condensationco-existing phase change heat transfer performances are investigated. Experimentalresults show that for different height of confined chamber, there always exists anoptimum filling rate which discloses that the optimum filling rate is the generality forboiling and condensation co-existing phase change heat transfer in small and closedspaces. Visualization study and analysis show that the influence of boiling andcondensation processes have significant impact on heat transfer performances of theconfined spaces.Boiling and condensation co-existing phase change heat transfer in confinedspaces with an enhanced boiling surface are studied. Factors that affect the heattransfer performances are analyzed and discussed. The enhanced boiling surfaceconsists of the fin arrays (1.4×1.4×1.4, mm) directly on the top end of the heatedcopper rod and two different heights of confined spaces under the three differentfilling rates are studied. Results show that the heat transfer resistance of the confinedspace decreases with the increase of liquid filling rate. Analysis of the experimentresults and typical images indicate the condensation heat transfer process is the keylink for the heat transfer ability of the confined phase change chamber.A heat transfer mechanism model for boiling and condensation co-existingphase change heat transfer in small confined spaces is proposed. The factors thataffect the heat transfer of the confined space are analyzed and discussed. Themodel is established based on the characteristics and influences between boiling andcondensation processes. The mechanism model includes the heat flux caused by the vapor bubbles, natural convection, condensate liquid drops and microlayerevaporation. The effects of liquid levels, gap sizes, and the departure diameter anddeparture frequency of bubbles are analyzed and discussed. The mechanism of theinteraction between boiling and condensation processes is explained by thequantitative relations of each parameter.