Study Of Bionic Vapor Chamber Based On The Plant Leaf Structure

As two primary elements of microelectronic and photoelectricity, the developments of CPU and LED are confronted with big challenge in heat dissipation,, as the electronic equipment will be weakened or even be damaged by the accumulation of large heat. As a high-efficiency element for heat transfer, vapor chamber could obtain higher cooling efficiency than heat pipe by changing the heat transfer from one dimension to two dimension. Besides, its flatten structure is better for the packaging of the electronic equipment. The plant, especially those surviving in the unaltered environment, is exposed to the high temperature and fast cooling is urgent. As the most important cooling way, plant dissipates heat by evaporation, which is also known as the phase change. Leaf vein system, as an optimized structure for heat and mass transfer, is the results of nature evolution for millions of years. Therefore, the investigation of the performance of the leaf vein system for fluid flow and heat transfer is very important for the design of the wick of the vapor camber. Based on the leaf vein system, the paper focuses on the performance of fluid flow and heat transfer of the leaf vein, and explores the application of the leaf vein network in the wick structure. A conceptual structure based on leaf vein system is proposed in this paper. The main contents are as follows:(1) The performance in fluid flow and heat transfer of the leaf veinAfter the tertiary vein was removed, the influence of branching angle between the main vein and the second vein on the fluid flow and heat transfer of the leaf vein was analyzed. Voronoi graph was used to create the polygonal network to simulate the real leaf vein network. In order to obtain the topology of the real leaf vein, the structural parameters were extracted from the image obtained by electronic scanning of a real leaf picked from a living Osmanthus. The numerical model of the real leaf vein and simulation were established and compared to reveal the distributional principle of the polygon loop of the leaf vein network.(2)Application of the leaf vein network in the wick structureThe leaf vein network model was designed based on the leaf vein structure in fractal pattern, which was composed of many microchannels with rectangular cross section, and the capillary force of the microchanel was calculated. The flow resistance of the leaf vein fractal network model was established when the length ratio, l=1 and the length ratio, 0<l<1 respectively, and itwas compared with the flow resistance and capillary force of the parallel structure. The number of parallel channels and the practical feasible microchannels are obtained under the condition that the two structures could provide the same driving force and occupy the same area. The permeable model of the conceptual structure was established to obtain the permeability, and found that the leaf vein fractal network could improve the permeability. Through the comparison between the models of the leaf vein fractal network and the parallel structure, it was found that the leaf vein network could improve the ability of self-suction and avoid local blocking.(3) Conceptual structure based on leaf vein system Because of the porous structure of plants, the real leaf vein network has permeable wall, so a conceptual structure composed of permeable leaf vein fractal network with porous structure was proposed. The permeable leaf vein fractal network was used to simulate the real leaf vein network and the porous structure composed of micro-fin-pins and porous-channel was used to simulate the mesophyll tissue. The flow model of the microchannel with permeable wall was established to calculate the permeability of the conceptual structure.The permeability of the conceptual structure was dimensionless by utilizing the permeability of the porous structure to obtain the related structural parameters with the conceptual structure. The models of the conceptual structure and conventional structure were both established to obtain the temperature and velocity, which were compared to reveal the advantages of the conceptual structure in heat and mass transfer.(4) The conceptual wickThe conceptual wick was composed of many microchannls with T-shaped cross section made by chemical etching, and the model was established to calculate the capillary pressure of the microchnnel. An experimental platform was established to measure the temperature on the top surface of the condenser and the temperature of the center point on the bottom surface of the evaporator, which was used to calculate the thermal resistance of the vapor chamber. The thermal resistances of the vapor chamber under different input power, filling ratio, working fluid and wind speed were calculated. The experimental device was set up to measure the pressure drop under different mass flow rate, and the result was used to make a linear fit to obtain the permeability of the conceptual wick.(5) The bionic vapor chamberThe wick of the condenser with different fractal angles, such as 30°, 40°, 50°, 60°, 70° were manufactured, and the wick of the evaporator was sintered by dendritic copper powder, in which the particle size was 48μm. The vapor chamber was assembled with bolts for easy disassembling. The performance of the vapor chamber was analyzed under different fractal angle, filling amount, input power and heat density. In order to analyze the performance of temperature uniformity and heat transfer, the rectangular wick of the condenser was manufactured by chemical etching and mechanical engraving, and the wick of the evaporator was sintered with spherical powder in which the piratical size was 96~270μm for packaging by diffusion welding.