Multiscale Heat-fluid-structure Of Heat-resistant Plant Leaves With Netted Venation And Design Of The Bionic Vapor Chamber

Natural design and evolutionary mechanism often bring inspiration and unexpected surprises to engineering design.Heat-resistant plants with netted venation grow in high temperature environment,relying on their near-perfect self-adaptive thermal structures,they can resist the influence of high temperature on their normal physiological activities by heat dissipation through the transpiration within the leaves.This heat dissipation mechanism is very significant for the structure design of a novel bionic vapor chamber which requires efficient heat dissipation.In this paper,the structures and functions of the heat-resistant plant leaves with netted venation were investigated under different scales?macroscopic,mesoscopic and microscopic?.Bionics designs of different scales were carried out based on their morphologies and structures,to improve the structure of the leaf-vein-inspired fractal vapor chamber and further improve its flow and heat transfer performances.Furthermore,an effective and convenient processing method was developed for the microchannels of the wick on the bionic vapor chamber.The main research works of this paper are as follows:?1?Investigation on the water transport in leaf vein systems under macroscale and its bionics design characteristicsThree ordinary species of heat-resistant plants with netted venation were selected,and differences among the measured flow velocities of different leaf veins of the same leaf as well as the measured flow velocities of different species were investigated.The corresponding parameters of the leaf vein structures of the three selected plants were extracted respectively and the bionic microchannel models with different structural parameter combinations were established only considering about the midribs and secondary veins.Then the orthogonal simulation analysis was conducted to obtain the bionics design characteristics with superior flow performance.Considering both of the pressure drop p and the maximum flow velocity v_max,the best combination of factors is that the angle between the midrib and secondary veins is=55°,the diameter of the midrib is d₁=1.7 mm,the diameter of the secondary veins is d₂=0.9 mm,the length ratio between the midrib and secondary veins is=0.5.It indicated that the structure of the leaf vein systems of the heat-resistant plant with netted venation had great influence on the water transport performance.The optimized leaf vein structure of Ficus virens Ait.var.sublanceolata?Miq.?Corner could be a bionics design characteristic with superior flow performance.?2?Investigation on the heat and mass transfer characteristics of the anatomical structure of leaf veins under mesoscale and microscaleUnder mesoscale,the anatomical structure characteristics of different leaf veins of the three selected heat-resistant plant leaves with netted venation were measured and counted.Under microscale,three different models with wall thickening patterns were established based on the real microstructures of the xylem vascular bundles in the midribs of the three selected heat-resistant plant leaves with netted venation,and their heat and mass transfer characteristics were investigated through numerical simulation.The analysis results showed that the reticulate/pitted wall thickening patterns on the leaf veins of heat-resistant plant with netted venation could enhance the heat transfer characteristic,and the offset pattern had a better performance than the aligned pattern.Thus,the offset reticulate/pitted wall thickening patterns could be extracted as a bionics design characteristic under microscale.The larger inner diameter?d=25?with wider and higher wall thickening?a=4.8,h=4?could obtain better heat and mass transfer characteristics.?3?Multiscale bionics design based on the heat-resistant plant leaves with netted venationOn one hand,the offset reticulate/pitted wall thickening patterns was applied on the bionics design of the structure of microchannel heat sink and the heat and mass transfer characteristics among six different models were compared.The results indicated that the natural selected structure of the xylem vascular bundles in the leaf veins of heat-resistant plant with netted venation could provide excellent heat transfer characteristic with a smaller volume while sacrificing a little bit ability in mass transfer.On the other hand,a novel leaf-vein-inspired gradient porous wick structure was designed for the condenser of vapor chamber and this design was compared and verified on a 3D model.The simulated result indicated that the design had superior fluid and thermal performances with a relatively equitable pressure distribution.?4?Multiscale modeling of the leaf-vein-inspired gradient porous wick structure for the condenser of vapor chamberA connected 2D random porous wick was generated by the expanded quartet structure generation set method.Then a complete set of MATLAB codes for the Lattice Boltzmann Method?LBM?was developed and the comparisons between LBM and FLUENT verified the the effectiveness of the algorithm.Using the developed LBM algorithm,the microscopic flow behaviors of the fluid on the 2D random porous wick model were analyzed.Observing from the entire flowing process from the inlet to outlet,it explained the phenomenon in the macroscopical simulation of the designed leaf-vein-inspired gradient porous wick structure for the condenser of vapor chamber well,i.e.the temperature of the gradient porous wick structure could reach balanced state faster and its temperature uniformity was better.For the porous media,the existence of isolated small solid structures should be reduced artificially during the manufacturing process,and the distribution uniformity of the porous media structures near the two higher level microchannels at the bifurcate position should be guaranteed as much as possible.?5?Experimental investigation on the bionic vapor chamberThrough setting up the experimental setup for the performance tests,the temperature of the central point on the bottom surface of the evaporator and the temperatures of the upper surface of the condenser of the manufactured bionic vapor chamber samples were measured respectively under different heating powers?60 W,80 W,100 W,120 W,140 W?,to compare and analyze the influence of different structures on its temperature uniformity of the upper surface of the condenser and overall thermal resistance.Through the comparison of the performances between the bionic vapor chamber and the existing vapor chambers,it was found that under the condition that the working fluid was deionized water,the minimum maximum temperature difference of the upper surface of the condenser was only 1.21,and the minimum overall thermal resistance was only 0.094/W,which were both smaller than most existing vapor chambers.