【摘要】：目前,科技發(fā)展日新月異,工業(yè)產(chǎn)品設(shè)備對換熱特性的要求越來越高。熱管作為具有優(yōu)異的換熱性能的元件,在換熱領(lǐng)域具有很廣的應(yīng)用范圍。它通過內(nèi)部工質(zhì)相變帶走熱量,換熱工質(zhì)對于熱管的換熱性能起到主導(dǎo)作用。對熱管的研究又主要集中在兩個(gè)方面:一個(gè)是針對管內(nèi)換熱工質(zhì)納米流體的研究;另一方面是對多孔介質(zhì)的研究。本文基于格子Boltzmann方法(LBM)研究了多孔介質(zhì)內(nèi)納米流體沸騰換熱相變過程氣液兩相換熱機(jī)理;并基于離散元素法對三維多孔介質(zhì)進(jìn)行了重構(gòu)。具體的研究工作如下:(1)根據(jù)Shan-Chen提出的偽勢格子Boltzmann模型,考察納米顆粒直徑變化引起的納米流體的布朗力變化對沸騰換熱過程的影響,建立能夠描述納米流體相變分離的格子Boltzmann模型。通過比對純液體、納米顆粒直徑為5nm、10nm和20nm的納米流體沸騰過程的氣液兩相流型圖,考察氣液兩相質(zhì)量隨納米顆粒直徑的變化曲線,分析得出納米顆粒直徑越小,納米流體氣液兩相分離越快,沸騰換熱能力越強(qiáng)。另外,本文還發(fā)現(xiàn)過熱度對氣液兩相相變速度的影響變化,研究表明過熱度越大,納米流體氣泡生成加快,同一時(shí)間步長氣相質(zhì)量越大,說明可以通過控制熱負(fù)荷的大小來控制沸騰過程的相變快慢。(2)通過隨機(jī)配置的方法構(gòu)造了部分填充二維多孔介質(zhì)結(jié)構(gòu)模型,結(jié)合Zhao的相變模型。將納米流體相變格子Boltzmann模型與二維多孔介質(zhì)模型相結(jié)合,考慮固體的浸潤和非浸潤性質(zhì),得到部分填充多孔介質(zhì)內(nèi)的氣泡生成、長大、相互融合以及與固體壁面的碰撞,反彈等現(xiàn)象。說明相變格子Boltzmann模型與隨機(jī)多孔介質(zhì)相結(jié)合的可行性。考察了多孔介質(zhì)的孔隙率對多孔介質(zhì)的相變過程的影響,得出在一定范圍內(nèi),孔隙率越大,多孔介質(zhì)內(nèi)的相變速度加快,多孔介質(zhì)模型的換熱能力越強(qiáng)。(3)基于離散元素法堆積構(gòu)造了三維多孔介質(zhì)模型,探究了球形顆粒在不同相對邊界尺度下的孔隙率和孔密度的影響變化。研究發(fā)現(xiàn)多孔介質(zhì)模型的孔隙率隨著相對邊界尺度的增大而增大,孔密度則是隨著相對邊界尺度的增大而減小。考察了球形顆粒和不同長細(xì)比的針狀顆粒因形狀變化對多孔介質(zhì)模型孔隙率的變化影響,結(jié)果表明在直徑相同的情況下,針狀顆粒堆積模型的孔隙率要大于球形顆粒堆積模型的孔隙率,并且針狀顆粒隨著長細(xì)比的增大,模型的孔隙率也相應(yīng)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.
【學(xué)位授予單位】：江蘇科技大學(xué)
【學(xué)位級別】：碩士
【學(xué)位授予年份】：2017
【分類號】：TG139.8

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