Study On Enhanced Wetting And Heat Transfer Characteristics In Microgrooves Heat Sinks With Cupric Hydroxide Nanorods Structured Surfaces

The microgrooves heat sinks possess outstanding phase-change heat transfer performance,whose heat transfer characteristics are closely linked with the wetting characteristics.Constructing the micro/nano structured surfaces are effective approaches to further enhance the wetting and heat transfer characteristics in microgrooves heat sinks.However,current studies on enhanced wetting and heat transfer characteristics in microgrooves heat sinks with nano structured surfaces were less conducted,there were a few reports on enhanced flow boiling and pool boiling heat transfer in micro-nano hybrid heat sink combined microgrooves and nanostructured surfaces.Additionally,conventional fabricating technologies of nanostructured surfaces generally require high-cost and bulky manufacturing equipment,as well as involve relatively complicated and time-consuming procedures.It is still a challenge to prepare the micro-nano hybrid heat sink based on microgrooves with the capacity of large-scale engineering applications.The present dissertation successfully apply the efficient,facile and low-cost alkali assistant surface oxidation technique to microgrooves,resulting in the cupric hydroxide(Cu(OH)2)nanorods arrays generation on the copper microgrooves heat sink surface,to obtain novel micro-nano hybrid heat sinks combined microgrooves and Cu(OH)2 nanorods surfaces.The experimental and theoretical studies on the enhanced wetting and heat transfer characteristics in micro-nano hybrid heat sinks combined microgrooves and Cu(OH)2 nanorods surfaces are conducted,which can provide theoretical basis for the optimization design and performance improvement of micro/nano scale phase-change heat transfer heat sink based on microgrooves.It is of importance to solve the heat dissipation bottleneck in the high-power-density electronic device.Firstly,three heat sinks with same microgrooves machining dimensions(microgroove width is 0.3 mm,depth is 0.6 mm and pitch is 0.3 mm)are prepared by utilizing wire cutting process and alkali assistant surface oxidation technique.They are copper microgrooves heat sink,micro-nano hybrid heat sink combined microgrooves and hydrophilic Cu(OH)2 nanorods surfaces,and micro-nano hybrid heat sink combined microgrooves and superhydrophilic Cu(OH)2 nanorods surfaces.Based on the tests by scanning electron microscope and contact angle meter,it is found that the contact angle of copper microgrooves heat sink surface is 65.2°.When the reaction time is increased to a given time,the formations of Cu(OH)2 nanorods can be observed on the copper microgrooves surface and the contact angle is reduced to 45°,to obtain a micro-nano hybrid heat sink combined microgrooves and hydrophilic Cu(OH)2 nanorods surfaces.When the reaction time is increased to a given time,the copper microgrooves surface is uniformly covered by the slender and dense Cu(OH)2 nanorods arrays,the contact angle is reduced to 4.1°,to obtain a micro-nano hybrid heat sink combined microgrooves and superhydrophilic Cu(OH)2 nanorods surfaces.It indicates that with the increased reaction time,the size and amount of generated Cu(OH)2 nanorods increase and the surface hydrophily become better.Afterwards,the comparative visualization experiments on wetting length and evaporating flow characteristics in three heat sinks mentioned above are performed.The experimental results showed that Cu(OH)2 nanorods surface significantly improve the wetting characteristics in copper microgrooves heat sink,and the micro-nano hybrid heat sink combined microgrooves and superhydrophilic Cu(OH)2 nanorods surfaces possess the best wetting characteristics.Under the unheated and vertical conditions,compared to the copper microgrooves heat sink,the wetting length of distilled water in micro-nano hybrid heat sink combined microgrooves and hydrophilic Cu(OH)2 nanorods surfaces and micro-nano hybrid heat sink combined microgrooves and superhydrophilic Cu(OH)2 nanorods surfaces are increased by 150%and 300%,respectively.For the unheated micro-nano hybrid heat sink combined microgrooves and superhydrophilic Cu(OH)2 nanorods surfaces,the wetting length of distilled water and ethanol both decrease with the increased inclination angle with the horizontal.Under the input heat flux in the range from 12.5 to 75 kW·m-2,when the heat flux is same,the wetting length of distilled water in the micro-nano hybrid heat sink combined microgrooves and superhydrophilic Cu(OH)2 nanorods surfaces is maximum,indicating it can support the highest heat flux.Based on the theoretical analysis and experimental results of capillary rise tests,the capillary performance parameter(K/Reff)improvement of the micro-nano hybrid heat sink combined microgrooves and superhydrophilic Cu(OH)2 nanorods surfaces can be calculated,where the recalculated K/Reff considering the gravity effect is 261%higher than the case without a consideration of the gravity.Therefore the gravity effect can't be neglected.The K/Reff of the copper microgrooves heat sink and the micro-nano hybrid heat sink combined microgrooves and superhydrophilic Cu(OH)2 nanorods surfaces are 4.3?m and 25?m,respectively.The capillary performance incremental ratio is 481%,which is among the highest value ever reported.Based on the accommodation theory,a theoretical model is developed to illustrate the fundamental features of the axial flow in open rectangular microgroove-based structure under the evaporating heat transfer condition,where the inclination angles,the apparent contact angle,the influence of surface structures and input heat fluxes are comprehensively taken into considerations.By comparing the calculating results and experimental data,it is seen that current model can well predict the wetting characteristics of microgroove-based structures under evaporation conditions.The heat transfer characteristics of three heat sinks mentioned above are also compared.Compared with the copper microgrooves heat sink without nanostructure,at the same input heat flux,the temperature of the micro-nano hybrid heat sink combined microgrooves and superhydrophilic Cu(OH)2 nanorods surfaces are markedly reduced,61 It can be attributed to the significantly enhanced the area of evaporation thin film.Additionally,Cu(OH)2 nanorods array results in the formation of numerous micro/nano scale cavities and the increase of surface roughness.It makes the nucleation density of bubble increase,causing the better boiling heat transfer performance for the micro-nano hybrid heat sink combined microgrooves and superhydrophilic Cu(OH)2 nanorods surfaces.