Optimization Of The Capillary Structure In Loop Heat Pipe Evaporator And Research On Its Performance

With the rapid development of electronic technology, great pressure has been placed on the researchers to realize high efficient heat management of the electronic products. Considering that most of the traditional single-phase heat transfer methods cannot meet the requirements anymore, two-phase heat transfer technologies such as heat pipes are considered as the best choice for achieving more efficient heat transfer. Loop heat pipe is a novel heat transfer technology based on a separate heat pipe mechanism. Evaporator is the key component of an LHP system and a porous capillary wick is the key component of an evaporator. The capillary wick provides the main force to circulate the working fluid and is also the most important component to manage the heat and mass transfer process in the LHP system. As a result, the capillary wick in the evaporator is considered as the heart of an LHP system.This paper studied the relationship between the heat and mass transfer performance of capillary wicks and the overall heat transfer performance of LHP system by improving the fabrication and performance of porous wicks. Firstly, this paper researched a salt-leaching pore forming method to fabricate capillary wicks and then studied the relationship between porous parameters (i.e. porosity, pore size and its distribution, etc.) and pore-forming agents (PFA). Then, the effect of the porous parameters (i.e. porosity, pore size and its distribution, etc.) on the capillary pumping performance and thermophysical parameters were analyzed. The results indicate that LHP wicks with excellent biporous structure were successfully fabricated by using salt-leaching pore forming method. The porosity of sintered wicks increases linearly with increasing the amount of soluble salt (PFA). The pore size is decreased and the distribution is centralized when decreasing the particle size of the PFA, but the porosity is not changed obviously. The capillary pumping performance of the wicks is influenced not only by porosity but also the pore size distribution. Wicks with higher porosity, smaller pore size and more centralized pore size distribution showed more excellent capillary pumping performance. Moreover, the thermophysical parameters are influenced not only by porosity but also pore size distribution. Specifically, the thermal conductivity decreases with increasing porosity. With the same porosity, wicks with smaller mean pore size and more centralized distribution show lower thermal conductivity. The thermal diffusivity of salt-leaching pore forming sintered wicks first increase and then decrease with the increase of porosity. With decreasing the pore size and centralizing the pore size distribution, the thermal diffusivity decreases. Moreover, the thermal conductivities of the wicks in wet mode are larger than those in dry mode, while the thermal diffusivities show a reverse trend.This paper studied the relationship between material properties of the LHP wicks and the key performances and parameters. On the one hand, high porosity biporous Ni-Ti capillary wicks were successfully fabricated. The influences of NaCl and Ti addition on the microstructure characteristics, the capillary pumping performance and the thermal conductivity of sintered wicks were investigated. The results indicate that Ni-Ti wicks not only keep the merits of traditional nickel wicks, such as good pore-forming ability, easy sintering process and good working fluid compatibility, but also exhibit much lower thermal conductivity and excellent thermal conductivity controllability. On the other hand, this paper also successfully fabricated two kinds of MAX phase ceramic wicks (211family-Ti2AlC and312family-Ti3AIC2) and studied the optimization of their fabricating processes. The results indicate that various factors such as the selection and the particle size of sintered powders, the selection of PFA, sealing of the sintering mould, the sintering parameters, salt leaching processes, etc., can directly or indirectly influence the fabrication of MAX phase ceramic wicks.The thermal behaviors of LHP systems were tested and the relationship between the performance of capillary wicks and LHP systems were analyzed later in this paper. This study successfully designed an LHP system with a cylinder evaporator and a MAX phase ceramic wick in it. The thermal behavior analysis results indicate that the biporous MAX phase ceramic wicks fabricated by salt-leaching pore forming method show better capillary pumping performance and higher permeability and porosity than those of the monoporous wicks fabricated without any PFA. At the same heating power, the LHP with biporous MAX phase ceramic wick (30%NaCl) started up successfully and operated reliably compared with that employing monoporous MAX phase ceramic wick. Moreover, a direct shaping technology of LHP wicks was introduced to avoid further damage of the porous structures during the second manufacturing process. By employing the direct shaping technology and using PFA with different particle sizes, this paper also studied two-layer composite wicks with controllable pore parameters and then analyzed their holistic capillary pumping performance. Considering the arrangement of thermal conductivity, the holistic capillary pumping performance and the vapor permeability, the composite wicks with fine pores in the inner layer and large pores in the outer layer (both layers have biporous strucure) show the best overall performance. In this study, an LHP with a two-layer composite wick was prepared and its thermal behaviors were analyzed. The results show that this LHP operated reliably not only at a stable heating power of20W but also at different heating powers (10W-15W-20W-25W-20W-15W-10W). The temperature fluctuation phenomenon in the LHP system was analyzed and the results indicate that the details of temperature fluctuation in different heating power ranges are different, and this can be attributed to the heat leaking phenomenon in the evaporator. The affecting factors of the LHP performance were analyzed and the results indicate that the LHP show poor startup performance and thermal behavior when it has insufficient working fluid (e.g.,45%), relatively high condensing temperature or low startup power (e.g.,5W).