Capillary Pumping Performance Of Porous Structure And Its Preparation And Optimization For Loop Heat Pipe

Loop heat pipe (LHP) is a new type of heat pipe which is a high efficient heat transfer device using the phase transition to transfer heat. LHP has the advantages of high capability of transferring heat efficiency in long distance, good ability to work against gravity, and suitable layout and so on. It is now trending widely used in thermal management of satellites and spacecrafts as well as cooling of high heat flux electrical and electronic devices.This study firstly developed a new heat transfer analysis model of LHP. It concluded that the heat transfer capability of LHP could be evaluated by the power of the latent heat of vaporization of working fluid and the mass flow rate of working fluid through the capillary wick. It showed theoretically that both working fluid and capillary wick play an important role in the thermal performance of LHP. The behavior of porous wick pumping working fluid by capillary force is an important fundamental problem. The capillary pumping model of porous structure was developed. The math expression of capillary pumping amount was derived and verified by experiments. The result shows that the changing of capillary pumping amount accords well with an exponential increase equation. The offset and the amplitude of the exponential increase equation are opposite numbers and theirs value is equal to the power of density of the working fluid, porosity, height and cross sectional area of the porous wick. Besides, the reciprocal value of the time constant in the exponential increase equation is equal to the power of density of the working fluid, the ratio of relative permeability to porosity of the porous wick and the acceleration of gravity. That is to say, the final capillary pumping amount is determined by the total volume of the pores in the porous structure; the capillary pumping rate increases with the increasing density of the working fluid and the increasing ratio of relative permeability to porosity of the porous wick. Both experiments of qualitative analysis and quantitative calculation were performed for verifying the capillary pumping model. All the experimental results accorded well with the results analyzed by the capillary pumping model developed before.The applications of the heat transfer analysis model of LHP and the capillary pumping model of porous structure were investigated. The result shows that the rapidness or tardiness of the capillary pumping behavior represents the comprehensive performance of the capillary force and the permeability of the porous wick, and such capability is named capillary pumping performance in this study. The more amount of working fluid can be pumped into the porous wick at the same time, that is to say, the quicker it can pump, the better capillary pumping performance it has. It is used to considering that both capillary force and permeability of the wick are the bigger the better; however, they are antinomy. The capillary pumping performance can be considered as the best balance of them. It was proposed that the capillary pumping performance experiment can be used to measure the permeability of porous structure. Compared with other traditional methods, it needn't be sealed during the measuring process and the flow was drove by capillary force as it was in a LHP and the measurement was convenient, so it was supposed to be better than the traditional method in evaluating the flow property of porous wicks for loop heat pipe applications.Based on the former study of the capillary pumping performance of porous structure, the effects of thermophysical properties such as surface tension coefficient, viscosity and density of working fluids on the capillary pumping performances of porous structure under room temperature were further studied. The result shows that when porous structure pumps different working fluids, the final capillary pumping amount is determined by the total pore volume of the porous structure when its height is shorter than its capillary rise limit, and the capillary pumping rate increases with the increasing surface tension coefficient and the increasing density of the working fluid, and decreases with the increasing viscosity of the working fluid. This result accords well with the result that analyzing by merit number without considering the effect of latent heat of vaporization of the working fluid. The merit number only evaluates the performance of working fluid. The relationship among heat transfer capability of LHP, merit number and physical properties of working fluid and capillary pumping performance of capillary wick was found in this thesis. The capillary pumping performance method was proposed to evaluate the thermal performance of LHP. It was useful for the design and optimization of LHP.Besides the above applications, the heat transfer analysis model of LHP, the capillary pumping model of porous structure and their experimental methods can also be used in guiding the optimization of capillary wick parameters. For example, the research results show that there is an optimization value of the thickness of capillary wick for LHP. This result accords well with the results reported in many literatures. When the pore diameter and the pore size distribution are the same, and the capillary force also meets the requirements, then, the bigger porosity, the better capillary pumping performance the capillary wick has, which is supposed to be helpful to improve the thermal performance of LHP.Based on the above results, porous wicks with high porosities were prepared by powder metallurgy method and space-holder technology with microcrystalline cellulose (MCC) as the space-holder. The effect of fabricating parameters on properties of porous wicks was investigated. The result shows that porosity increases about 6.32% when forming pressure decreases 10MPa and increases about 6.64% when microcrystalline cellulose addition increases 10wt%. The pore size distribution ranges become wider and the mean pore diameters increase with the increasing space-holder addition.The thermal conductivity, the thermal diffusivity and the volume specific heat of porous wicks was studied by using the transient plane source method. The relationships between thermal properties and porosity, forming pressure and space-holder addition were investigated. The result shows that thermal conductivity decreases with the increasing porosity, but there are no obvious similar relationship between thermal diffusivity, volume specific heat and porosity. Both thermal conductivity and volume specific heat of water saturated state porous wicks are bigger than those of dry state ones, but there is no obvious similar rule for thermal diffusivity. Thermal conductivity increases with the increasing forming pressure, while there are no obvious changing trends for thermal diffusivity and volume specific heat. Thermal conductivity and volume specific heat decrease while thermal diffusivity increases with the increasing space-holder addition. Porous wicks with high porosity, good strength and low thermal conductivity can be prepared by using the space-holder technology.Based on the above studies of the effects of fabricating parameters on the pore properties and the thermal properties of porous wicks, a new method for controlling the parameters of porous wicks during the preparation was proposed and verified by experiment. The result showed that the error of porosity was less than 6% in the case study that used the porosity control method to fabricate porous wicks with the expected porosity of 75%. It was found further that the capillary pumping performances and the strength were different while both thermal conductivities and porosities of the porous wicks prepared by the porosity control method were the same. This is useful for guiding the optimization of porous wicks for LHP.Some important matters in the design and the preparation of LHP were finally studied, and some LHP samples were fabricated and their heat transfer characteristics were investigated. The experimental result of the whole LHP shows that the system starts quicker and the working temperature increases as the increasing heat source power. The experimental study on the open loop heat pipe evaporator (just like a loop heat pipe without linking the vapor line) was carried out wisely without coupling both the effects of the circulation of working fluid and the parameters of condenser. The result shows that the temperature of the evaporator fluctuates in some cases during the startup of the system. The heat source power making the temperature fluctuate decreases with the increasing porosity of the porous wick. When porous wick is the same, temperature is easier to fluctuate and it is much serious in the cases of bigger heat source power. The reason for the generation of the temperature fluctuation is that the parameter (such as porosity) of capillary wick is not matching with the external parameters of the system (such as heat source power).In a word, a series of solutions can be found to improve the thermal performance of LHP in this thesis. In general, firstly choose working fluid with good latent heat of vaporization according to the requirement of working temperature. Secondly, determine the needed capillary pumping force of the capillary wick according to the heat transfer distance and the against gravity height. Finally, select the capillary wicks with better capillary pumping performance and make sure that the capillary force meets the above requirement. The capillary pumping performance of porous structure, together with the heat transfer analysis model of LHP, was studied in detail in this thesis. The optimization of porosity, thickness, pore diameter, pore size distribution of capillary wick and its matching with the external parameters of the LHP such as the heat course power, as well as the preparation technologies for achieving the detail optimization results were performed.