Optimal configuration for fiber-type-wick heat pipe with top heat mode

Abstract

Heat pipes are a one of the heat transfer devices which are popularly used in the cooling system of electronic devices to prevent the decreasing of work performance. The evaporator section of heat pipe which is located at one end side of the heat pipe receives the dissipating heat from the processing unit of electronic devices. Then, that heat causes to evaporate phenomenon of working fluid inside the heat pipe. After that the vapor of working fluid moves to the other end side of the heat pipe which has low temperature or the condenser section. Then, it is condensed to fluid phase. The working fluid in the fluid phase is returned to evaporator section by capillary pressure from porous material. Therefore, porous materials which provide the high capillary pressure and high permeability allow rapid flow back of working fluid to the evaporator section. In case of heat pipes using for smart phones, these have a small sizing due to the limit space of working space and almost of these were constructed by using the fiber as a porous because that provide the high of capillary pressure and permeability. As per the characteristic of smart phone using, evaporator section of heat pipe is equipped with higher than condenser section which is called the top heat mode heat pipe. As mentioned above, this makes it difficult for the working fluid to flow back through the porous material to the evaporator portion because the flow direction is against earth's gravity. According to the section on the manufacture of heat pipes, the diameters of the fiber porous material used to produce heat pipes are 30 μm and 50 μm respectively. Each type of it is gathered along its length to set as the fiber bundle wick. After that, it is twisted to maintain the bundle shape. As mentioned, that causes to varies of internal pore structures of fiber bundle wick. So, suitable configuration of fiber bundle wick for top heat mode heat pipe were studied in this research. Moreover, the proportion of 30 μm and 50 μm of diameter of fiber bundle wick and pitch of twisting which were affected to porosity and thermal performance of heat pipe were studied by comparing the results between numerical method and experimental method. Numerical method was used to predict the porosity of twist porous material by using the packing model method. As mentioned, each type of twist fiber bundle wick was simulated to demonstrate the cross-section area. Moreover, mathematical models were found to predict the thermal performance of top heat mode heat pipe by using finite element method. The simulation results were compared to the experimental results. The numerical method consisted of three domains as vapor core, porous material structure and wall domain. The tetrahedral element in cartesian coordinate was selected to simulate the character of the heat transfer of fluid which was steady state, laminar and incompressible flow of saturated wick. The proportions of mixing between 30-micrometer and 50-micrometer fiber sizes, expressed as the percentage of the number of 30-micrometer fibers to the number of 50-micrometer fibers, as well as the pitch distance of 10, 15, and 20 millimeters, were studied to determine the optimal configuration of fiber bundle wick. This was done by considering the heat performance of the heat pipe and evaluating the production costs, along with capillary pressure and permeability values that indicate the fluid circulation capability within the heat pipe. These factors directly affect the heat performance of the heat pipe. Therefore, this research consists of three main parts: Part 1 focuses on the modeling of the arrangement of mixed-size fiber materials and twisted configuration to determine the porosity. Part 2 involves modeling the heat performance of the heat pipe using the mixed-size fiber materials and twisted configuration. Lastly, Part 3 involves evaluating and determining the most suitable configuration of mixed-size fiber materials and twisted configuration for the heat pipe with top heat mode. From the investigation of the fiber size mixing ratios, it was found that there were no significant differences in the experimental values of porosity. The average porosity values were 28.75%, 29.32%, 30.92%, and 29.85% for pitch distances of 10, 15, 20 millimeters, and non-twisted, respectively. The model for arranging the fiber materials, which incorporated both twisted and untwisted configurations, was developed based on the model that simulated the arrangement of circular cross-sectional fibers within the designated boundary as a representative of the porous material. In the case of the twisted configuration, the cross-sectional shape of each fiber was assumed to be a ellipse due to the helical twisting of the fibers. The models, both with and without twisting, were capable of predicting the porosity values of the fiber materials effectively. The maximum percentage of the root mean square error (RMSE) was found to be 8.80%. Furthermore, the investigation of the pitch distances revealed no significant differences in the experimental porosity values. However, the predicted porosity values increased as the pitch distance decreased (indicating a tighter twist). This is because the twisting of the fibers transformed the cross-sectional shape from circular to ring-shaped, resulting in larger areas and sizes of the porous material as the pitch distance decreased. The maximum percentage of the root mean square error (RMSE) was found to be 29.21%, occurring at a mixing ratio of 90%:10%. In the experimental study of the thermal resistance of fiber bundle wick heat pipe with top heat mode, it was observed that the wall temperature, particularly in the evaporator section, reached its maximum value. The wall temperature distribution exhibited a rapid initial decrease followed by a slower decrease in the later stages. The pitch distance of the twisted fibers did not significantly affect the thermal performance of the tubes. However, the untwisted fiber material with a mixing ratio of 90%:10% demonstrated a significant increase in thermal resistance. The thermal performance model of the heat pipe was able to predict the temperature distribution and thermal resistance more accurately. When considering the liquid film formation in the condenser section due to the loss of evaporating liquid at evaporator section, it was observed that the excessive heat absorbed by the porous material caused it to be unable to transport the liquid to the evaporator section in a timely manner. This receding or excess liquid within the porous material significantly reduced the effective thermal conductivity of the material, leading to a deterioration in heat transfer. Consequently, the wall temperature distribution predicted by the model for the tube wall exhibited a similar trend and values to the experimental results. The investigation of the optimal configuration of fiber bundle wick used in heat pipes with top heat mode considers capillary pressure, permeability, thermal resistance, and production cost. An increase in the mixing ratio of fiber's diameter 30μm leads to an increase in capillary pressure, enhancing capillary action. Permeability shows slight variation with the change in mixing ratio, while different pitch lengths in the twisted configuration do not significantly affect permeability or capillary pressure. The analysis focuses on the heat input of 4W, representing the maximum heat transfer requirement in the miniature heat pipe manufacturing industry. It was found that the twisting process increases the production cost of heat pipes, resulting in longer production time and additional costs. Moreover, using a higher proportion of fiber's diameter 30μm beyond a mixing ratio of 70%:30% significantly increases production cost due to the more challenging and time-consuming production process. Considering thermal resistance and production cost, the optimal configuration for the heat pipe is a non-twisted fiber bundle wick with a mixing ratio of 60%:40%. This configuration exhibits the lowest thermal resistance and a smaller percentage change in production cost compared to thermal resistance. The choice of this configuration is influenced by thermal resistance considerations and offers higher capillary pressure and reduced production time by eliminating the twisting process.

Keyword: Mixing ratio, Pitch length, Porosity, Fiber bundle wick, Heat pipe, Top heat mode, Thermal performance.