【摘要】：熱能作為生產(chǎn)生活中最普遍且可以直接利用的能源形式之一,常常伴隨著各種能源轉(zhuǎn)換和利用過(guò)程。熱能存儲(chǔ)可以有效解決能源在時(shí)間和空間上的供求不匹配,因此成為了提高能源利用效率的關(guān)鍵手段。相變儲(chǔ)熱是利用相變材料在物態(tài)變化的過(guò)程中,吸收或釋放大量潛熱而進(jìn)行的,具有儲(chǔ)能密度高、吸放熱過(guò)程中溫度幾乎不變等優(yōu)點(diǎn)。常用的有機(jī)相變材料雖然具有較大的比熱容和相變潛熱,但是其導(dǎo)熱系數(shù)低、換熱性能差等缺點(diǎn)成為了制約實(shí)際應(yīng)用的主要因素。為了提高潛熱儲(chǔ)能系統(tǒng)的換熱效率,在相變材料中添加具有高導(dǎo)熱系數(shù)的納米顆粒,是一種常見(jiàn)的強(qiáng)化傳熱手段。在實(shí)際應(yīng)用中,球形容器常被用于堆積床式潛熱儲(chǔ)能系統(tǒng)的儲(chǔ)熱單元。為了研究堆積床式的潛熱儲(chǔ)能系統(tǒng)中能量的吸收和釋放,研究單個(gè)球形容器內(nèi)的熔化和凝固換熱過(guò)程則顯得尤其重要。因此,本文針對(duì)相變材料向內(nèi)凝固過(guò)程難以實(shí)現(xiàn)可視化的問(wèn)題,提出了一種基于體積收縮定量研究球形容器內(nèi)相變材料凝固換熱過(guò)程的實(shí)驗(yàn)方法,同時(shí)通過(guò)制備不同質(zhì)量分?jǐn)?shù)的納米復(fù)合相變材料,研究了高導(dǎo)熱系數(shù)的納米顆粒對(duì)相變材料的強(qiáng)化傳熱影響。本文以十四醇(熔點(diǎn)37℃)為相變材料,采用"兩步法"制備了石墨烯納米片質(zhì)量分?jǐn)?shù)分別為0%、0.5%、1.0%、3.0%的復(fù)合相變材料,并對(duì)納米復(fù)合相變材料的熱物性,包括導(dǎo)熱系數(shù)、相變焓、相變溫度、比熱容、密度以及粘度等進(jìn)行了測(cè)試。本文提出的間接實(shí)驗(yàn)方法,通過(guò)在球形容器內(nèi)部插入下降管,將凝固過(guò)程中體積收縮產(chǎn)生的空隙轉(zhuǎn)化為下降管內(nèi)液態(tài)相變材料高度的變化,從而進(jìn)一步計(jì)算相變材料的瞬態(tài)液相質(zhì)量分?jǐn)?shù)。實(shí)驗(yàn)結(jié)果顯示,三組平行試驗(yàn)的溫度曲線最大實(shí)驗(yàn)偏差百分比約為3.7%,下降高度的相對(duì)偏差不超過(guò)6.0%,說(shuō)明實(shí)驗(yàn)重復(fù)性良好。同時(shí),將實(shí)驗(yàn)測(cè)試的的總下降高度與基于固液相密度差的理論高度進(jìn)行對(duì)比,相對(duì)偏差在10%-15%;將液相質(zhì)量分?jǐn)?shù)曲線擬合成基于FoSte的無(wú)量綱關(guān)聯(lián)式,考慮到材料、幾何尺寸以及工況的不同,實(shí)驗(yàn)曲線與前人的理論曲線趨勢(shì)一致,說(shuō)明實(shí)驗(yàn)具有良好的可靠性。本文采用體積收縮法,還進(jìn)一步研究了納米顆粒對(duì)球形容器內(nèi)相變材料凝固換熱過(guò)程的影響。研究結(jié)果表明,球形容器內(nèi)納米復(fù)合相變材料的總凝固時(shí)間隨著納米顆粒質(zhì)量分?jǐn)?shù)的增加而減少,這是因?yàn)樘砑蛹{米顆粒增強(qiáng)了復(fù)合相變材料的導(dǎo)熱系數(shù)。此外,三種復(fù)合材料的液相質(zhì)量分?jǐn)?shù)曲線存在交叉點(diǎn),這可能是因?yàn)橐后w相變材料的粘度隨著質(zhì)量分?jǐn)?shù)的添加而大幅度提高,導(dǎo)致相變材料流動(dòng)性下降,延緩了下降管內(nèi)液體高度的變化。因此,更高質(zhì)量分?jǐn)?shù)樣品的下降高度在實(shí)驗(yàn)初期存在時(shí)間滯后,導(dǎo)致初始凝固速率被低估。對(duì)實(shí)驗(yàn)數(shù)據(jù)進(jìn)行擬合,得出了液相質(zhì)量分?jǐn)?shù)和Nu基于FoSte的無(wú)量綱關(guān)聯(lián)式,對(duì)工程應(yīng)用具有一定的指導(dǎo)意義。
[Abstract]:Thermal energy, as one of the most common and direct energy forms in production and life, is often accompanied by various energy conversion and utilization processes. Thermal energy storage can effectively solve the mismatch of energy supply and demand in time and space, so it has become a key means to improve energy efficiency. In the process of state change, a large amount of latent heat is absorbed or released, which has the advantages of high energy storage density and almost constant temperature in the process of heat absorption and release. In order to improve the heat transfer efficiency of latent heat storage system, adding nanoparticles with high thermal conductivity into the phase change material is a common means to enhance heat transfer. Therefore, in view of the difficulty in visualizing the inward solidification process of PCM, an experimental method based on volume shrinkage is proposed to quantitatively study the solidification heat transfer process of PCM in a spherical vessel, and different mass is prepared. The effect of nano-particles with high thermal conductivity on the heat transfer enhancement of phase change materials was studied. The composite phase change materials with 0%, 0.5%, 1.0% and 3.0% graphene Nano-sheets were prepared by two-step method using tetradecanol (melting point 37 C) as phase change materials. Physical properties, including thermal conductivity, phase change enthalpy, phase change temperature, specific heat capacity, density and viscosity, were measured. The indirect experimental method proposed in this paper is to convert the void caused by volume shrinkage into the change of the height of liquid phase change material in the falling tube by inserting a falling tube into the spherical vessel. The experimental results show that the maximum deviation percentage of the temperature curves is about 3.7%, and the relative deviation of the drop height is less than 6.0%. The experimental repeatability is good. For the deviation between 10% and 15%, the liquid phase mass fraction curve is fitted into a dimensionless FoSte-based correlation. Considering the different materials, geometric sizes and working conditions, the experimental curves are consistent with the previous theoretical curves, indicating that the experiment has good reliability. The results show that the total solidification time of nanocomposite phase change materials in spherical vessels decreases with the increase of nano-particle mass fraction, which is attributed to the enhancement of thermal conductivity of composite phase change materials by adding nano-particles. At the crossing point, this may be because the viscosity of the liquid phase change material increases greatly with the addition of the mass fraction, resulting in the decrease of the flowability of the phase change material and delaying the change of the liquid height in the descending tube. Fitting the experimental data, the dimensionless correlation between the liquid mass fraction and Nu based on FoSte is obtained, which has a certain guiding significance for engineering application.
【學(xué)位授予單位】：浙江大學(xué)
【學(xué)位級(jí)別】：碩士
【學(xué)位授予年份】：2017
【分類號(hào)】：TB34

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