纳米流体液滴撞击壁面铺展动力学特性研究

纳米流体液滴撞击固体壁面的铺展动力学特性是基于液滴沉积实现高效传热传质过程的关键因素,然而由于纳米流体的非牛顿流变特性及液滴内微流动与纳米颗粒的耦合作用,目前对纳米流体液滴撞击固体壁面的铺展动力学行为缺乏足够的认识.本研究利用了两步法分别配制了分散有3种纳米颗粒的均匀稳定纳米流体(碳纳米管、石墨烯、纳米石墨粉),并对流体的流变特性进行了测量分析.利用显微高速数码摄像技术捕捉了液滴撞击固体壁面的动态过程,通过图像处理技术分析铺展过程中液滴的无量纲高度、铺展因子及动态接触角,探究了液滴在韦伯数约为200及800时撞击壁面后铺展沉积形态的演变规律.研究表明,3种不同纳米颗粒的加入均使基液表现出明显的剪切变稀特性,在液滴撞击壁面的铺展过程中,流体的剪切黏度起重要作用,液滴的无量纲高度和铺展因子的变化幅度随着纳米流体剪切黏度的增大而减小.纳米流体液滴撞击疏水表面时能更快的达到平衡状态,液滴的惯性力主导着液滴的初始铺展阶段,液滴的铺展范围和速度随撞击速度的增大而增大.开展该研究能够为基于液滴沉积的增益冷却技术以及微型高导热及导电材料的制造提供理论依据和技术指导. 

STUDY ON SPREADING CHARACTERISTICS OF NANOFLUIDS DROPLET IMPINGING ON SOLID SURFACE1)

The spreading characteristics of a nanofluids droplet impinging on the solid surface are the key factors in efficient heat and mass transfer technology which based on droplet deposition. However, the dynamic behaviors and characteristics of the nanofluid droplet haven't been fully understood since the presence of the non-Newtonian fluid behavior and the interaction between the microstructure of nanoparticle and micro-flow field in the droplet which complicates the spreading process. In this study, we prepared homogeneous and stable nanofluids by dispersing different nanoparticles (multiwall carbon nanotube (MWCNT), graphene and nano-graphite powder) to epoxy resin with two-step method. The rheological behaviors of these nanofluids have been measured and analyzed. The evolution of droplet morphology during the spreading process has been captured by means of high speed camera visualization technique. Based on the image processing technique, the transient dimensionless height, transient spreading factor and dynamic contact angle (DCA) of the droplet have been studied. The results show that the nanoparticles bring the base fluid non-Newtonian shear thinning property. The shear viscosity of test fluid plays important role during the spreading phase and the nanofluid with a lower shear viscosity over the entire range of the shear rate results inlarger variations of the spreading factor and dimensionless height. Nanofluids droplet impacting on the hydrophobic surface could be faster to reach equilibrium condition. The inertial force of impacting droplet dominants the initial spreading process, the spreading variation and velocity are proportional to the impact velocity. This study can provide theoretical basis and specific guidance for the development of gain cooling technology and the manufacture of micro high thermal and electrical conductivity materials.