使用分子動力學模擬分析奈米流體之熱傳導係數

Thermal Conductivity of Nanofluids Diagnosis by Molecular Dynamics Simulation

鄧國良(Kuo-liang Teng)

指導教授 ： 錢景常 ； 蕭百沂

國立清華大學/原子科學院/工程與系統科學系/碩士(2007年)

https://doi.org/10.6843/NTHU.2007.00307

摘要

奈米流體由奈米尺寸的奈米球及基礎流體構成。在眾多的實驗顯示奈米球與基礎流體的體積比非常低，奈米流體的熱傳導係數也會顯著增加。在能源緊張的今日，使用奈米流體做節能是個引人矚目之方法。本文利用分子動力學模擬計算奈米流體之熱傳導係數，並了解奈米流體隻熱傳導係數比基礎流體高許多的基本機制，並設計有效之奈米流體作為熱傳之工作流體，來實行節能。分子動力學模擬法與每一個原子的位置、動能、及相互作用力息息相關。其所建構出來的系統或物質，可展現特定之物理係數，如熔點、沸點、黏滯係數或熱傳導係數。熱傳導係數的獲得，在這些物理係數中是公認最困難的常數。在系統達到平衡態後，本文採取非平衡法(NEMD)及葛因古博法(Green-Kubo法)來計算熱傳導係數。當奈米球包含的原子數大於10以上，兩個方法所得的值並不相同，本文也仔細探討原因，並尋求合理的解釋。此外，由於奈米流體在低體積比時所需的分子數十分龐大，若要與實驗中奈米流體動輒為數十奈米尺寸之奈米球相比，我們必須設計一套方法，做實際的估計，本文也提出可行之法。

關鍵字

奈米流體 ； 熱傳導係數 ； 分子動力學模擬

並列摘要

Nanofluid is defined as a nanoparticle suspension in the base fluid. Tremendous enhancement of thermal conductivity of nanofluids has been observed in the experiment, which leads to the applications for energy saving. This paper aims to calculate the thermal conductivity of nanofluids, and to identify the physical mechanism of enhancement by Molecular Dynamics simulation. Position and velocity of each atom, and the interaction between atoms are obtained by Molecular Dynamic simulation, and thus the physical coefficients, such as melting point, boiling point, viscosity, thermal conductivity of a system of substance can be modeled by these quantities. Among these physical properties, thermal conductivity is the most difficult quantity to calculate by Molecular Dynamics simulation. Two approaches of calculating thermal conductivity by molecular dynamic method, non-equilibrium method (NEMD) and Green-Kubo method, are explored and compared after system reaching equilibrium state. The results show that different values are obtained if the number of atoms in the nanoparticle is larger than 10 atoms. The physics will be identified and discussed in detail to reach reasonable conclusion. Furthermore, the low volume fraction nanofluids are composed of a large number of atoms, which implies the impossible computer resource required, therefore, a simplified shell model is derived to estimate the thermal conductivity of nanofluids, so that the comparisons with the experimental results can be conducted.

並列關鍵字

nanofluids ； thermal conductivity ； molecular dynamics simulation ； md