基于LBM的泡沫金属内纳米流体气液两相传热机理研究

发布时间：2018-10-25 07:21

【摘要】：目前,科技发展日新月异,工业产品设备对换热特性的要求越来越高。热管作为具有优异的换热性能的元件,在换热领域具有很广的应用范围。它通过内部工质相变带走热量,换热工质对于热管的换热性能起到主导作用。对热管的研究又主要集中在两个方面:一个是针对管内换热工质纳米流体的研究;另一方面是对多孔介质的研究。本文基于格子Boltzmann方法(LBM)研究了多孔介质内纳米流体沸腾换热相变过程气液两相换热机理;并基于离散元素法对三维多孔介质进行了重构。具体的研究工作如下:(1)根据Shan-Chen提出的伪势格子Boltzmann模型,考察纳米颗粒直径变化引起的纳米流体的布朗力变化对沸腾换热过程的影响,建立能够描述纳米流体相变分离的格子Boltzmann模型。通过比对纯液体、纳米颗粒直径为5nm、10nm和20nm的纳米流体沸腾过程的气液两相流型图,考察气液两相质量随纳米颗粒直径的变化曲线,分析得出纳米颗粒直径越小,纳米流体气液两相分离越快,沸腾换热能力越强。另外,本文还发现过热度对气液两相相变速度的影响变化,研究表明过热度越大,纳米流体气泡生成加快,同一时间步长气相质量越大,说明可以通过控制热负荷的大小来控制沸腾过程的相变快慢。(2)通过随机配置的方法构造了部分填充二维多孔介质结构模型,结合Zhao的相变模型。将纳米流体相变格子Boltzmann模型与二维多孔介质模型相结合,考虑固体的浸润和非浸润性质,得到部分填充多孔介质内的气泡生成、长大、相互融合以及与固体壁面的碰撞,反弹等现象。说明相变格子Boltzmann模型与随机多孔介质相结合的可行性。考察了多孔介质的孔隙率对多孔介质的相变过程的影响,得出在一定范围内,孔隙率越大,多孔介质内的相变速度加快,多孔介质模型的换热能力越强。(3)基于离散元素法堆积构造了三维多孔介质模型,探究了球形颗粒在不同相对边界尺度下的孔隙率和孔密度的影响变化。研究发现多孔介质模型的孔隙率随着相对边界尺度的增大而增大,孔密度则是随着相对边界尺度的增大而减小。考察了球形颗粒和不同长细比的针状颗粒因形状变化对多孔介质模型孔隙率的变化影响,结果表明在直径相同的情况下,针状颗粒堆积模型的孔隙率要大于球形颗粒堆积模型的孔隙率,并且针状颗粒随着长细比的增大,模型的孔隙率也相应增大。
[Abstract]:At present, with the rapid development of science and technology, industrial products and equipment require higher and higher heat transfer characteristics. Heat pipe, as a component with excellent heat transfer performance, has a wide range of applications in the field of heat transfer. It takes away the heat through the internal phase transformation of the working fluid, which plays a leading role in the heat transfer performance of the heat pipe. The research on heat pipe is mainly focused on two aspects: one is the study of the heat transfer fluid nanoscale fluid in the tube, the other is the study of porous media. In this paper, the gas-liquid two-phase heat transfer mechanism during boiling phase transition of nano-fluid in porous media is studied based on lattice Boltzmann method (LBM), and the three-dimensional porous medium is reconstructed based on discrete element method. The specific research works are as follows: (1) according to the pseudo-potential lattice Boltzmann model proposed by Shan-Chen, the effect of Brownian force change on the boiling heat transfer process caused by the change of nanoparticles diameter is investigated. A lattice Boltzmann model was established to describe the phase change separation of nanoscale fluids. By comparing the gas-liquid two-phase flow pattern of the boiling process of pure liquid, 5 nm or 10 nm nanocrystalline particles with 20nm, the variation curve of gas-liquid two-phase mass with the diameter of nano-particles was investigated. The smaller the particle diameter was, the smaller the particle diameter was. The faster the gas-liquid separation, the stronger the boiling heat transfer ability. In addition, the influence of superheat on the gas-liquid phase transition velocity is also found. It is shown that the higher the superheat degree, the faster the bubble formation and the larger the gas phase mass at the same time step. It is shown that the boiling process can be controlled by controlling the heat load. (2) the two-dimension porous media structure model with partial filling is constructed by random collocation, and the phase transition model of Zhao is combined. By combining the Boltzmann model of nano-fluid phase transition lattice with the two-dimensional porous media model and considering the wetting and non-wetting properties of solids, the formation, growth, fusion and collision with solid wall of partially filled porous media are obtained. Rebound, etc The feasibility of combining phase change lattice Boltzmann model with random porous media is demonstrated. The effect of porosity on the phase change process of porous media is investigated. It is concluded that the larger the porosity is, the faster the phase transition rate in porous media is. The heat transfer ability of porous media model is stronger. (3) the three-dimensional porous media model is constructed based on the discrete element method, and the effect of spherical particles on porosity and pore density at different boundary scales is investigated. It is found that the porosity of the porous media model increases with the increase of the relative boundary scale, while the pore density decreases with the increase of the relative boundary scale. The effects of spherical particles and needle-shaped particles with different aspect ratios on the porosity of porous media model were investigated. The porosity of the model is larger than that of the spherical model, and the porosity of the model increases with the increase of the slenderness ratio.
【学位授予单位】：江苏科技大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TG139.8

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