颗粒团聚状态对纳米流体热导率的影响

Effect of aggregation state on the thermal conductivity of nanofluids

纳米流体由基液与纳米颗粒组成,因具有优良的热物性而受到广泛关注。颗粒团聚是纳米流体的主要特征之一,对热导率影响显著。目前,通过实验手段很难观察到纳米流体中颗粒由均匀分散到团聚这一过程的微观变化。以铜(Cu)/氩(Ar)纳米流体为研究对象,基于非平衡分子动力学(NEMD)方法对颗粒团聚过程中基液原子的布朗运动及界面层性质进行模拟研究,通过分析均方位移(MSD)及数密度等微观参数得到团聚状态对热导率的影响。结果表明,当颗粒由均匀分散到完全团聚时,热导率逐渐增大,并在团聚时呈现最大值。在完全团聚状态下,基液原子的MSD值比均匀分散状态提高了3%,说明基液原子的布朗运动逐渐增强;同时,界面层的MSD值下降了22%,表明界面层原子活跃程度降低,界面层趋于稳定。通过改变初始颗粒质心距离可将团聚结构分为反应限制团聚(21A,1A=0.1nm)与扩散限制团聚(22A)两种类型。当界面层的数密度由67nm^(-3)升高到72nm^(-3),反应限制团聚向扩散团聚转变;当从67nm^(-3)降至62nm^(-3),扩散限制团聚向反应限制团聚转变,因此界面层密度是影响纳米颗粒团聚结构的主要因素之一。

Nanofluids consist of basefluids and nanoparticles,which have attracted wide attention because of their favorable thermal properties.The thermal conductivity of nanofluids is greatly affected by particles aggregation which is one of the significant characteristics of nanofluids.Currently,it is difficult to observe the microscopic changes in nanofluids from uniform dispersion to agglomeration by experimental method.The non-equilibrium molecular dynamics(NEMD)method was performed to investigate the Brownian motion of basefluid atoms and interfacial layer properties during particle agglomeration of Cu/Ar nanofluid.The mechanism of the effect of agglomeration behavior on the variation of thermal conductivity was obtained by analyzing microscopic parameters such as mean square displacement(MSD)and number density.The results showed that the thermal conductivity of nanofluid gradually increased during the process of nanoparticles from uniform dispersion to complete agglomeration,and the thermal conductivity obtaind the maximum value when the particles were agglomerated.Comparing the MSDs in the two states of uniform dispersion and complete aggregation of the particles,the MSD value of the base fluid atoms of fully agglomerated state increased by about 3%compared to that of uniformly dispersed state,showing that the Brownian motion of the base fluid atoms of fully agglomerated state was enhanced.Meanwhile,the MSD of the interfacial layer of fully agglomerated state decreased by about 22%compared to that of uniformly dispersed state,indicating that the atoms in the interfacial layer were less active and the interfacial layer tended to be stable.Furthermore,the aggregation structure of the particles was investigated.The agglomerated structures can be classified into reaction-limited agglomerates(21Å,1Å=0.1nm)and diffusion-limited agglomerates(22Å)by varying the initial particle center-of-mass distance.After increasing the number density of the nanofluidic interfacial layer from the initial value of 67nm^(-3)to 72nm^(-3),the reaction-limited agglomeration shifted to diffusionlimited aggregation.After decreasing the number density of the nanofluidic interfacial layer from the initial value of 67nm^(-3)to 62nm^(-3),the diffusion-limited aggregation shifted to reaction-limited aggregation.The results showed that the density of interfacial layer was one of the main factors affecting the aggregation structure of nanoparticles.