STUDY ON SPREADING CHARACTERISTICS OF NANOFLUIDS DROPLET IMPINGING ON SOLID SURFACE1)
LiuHailong, ShenXuefeng, WangRui, CaoYu, WangJunfeng2), School of Energy and Power Engineering, Jiangsu University, Zhenjiang 212013, Jiangsu, China 中图分类号:O359.1 文献标识码:A
关键词:纳米流体;剪切变稀;液滴;撞击;铺展 Abstract 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.
新窗口打开 显示原图|下载原图ZIP|生成PPT 图 4纳米流体剪切黏度曲线 -->Fig. 4The shear viscosity of test nanofluids -->
2 可视化实验结果与分析
用来刻画液滴撞击壁面动力学行为的无量纲参数包括$We$、$Re$、$Oh$和$Ca$.然而,除 $We$之外的无量纲参数中均包含黏性力项,而本研究的纳米流体是剪切黏度是随着剪切速率动态变化的非牛顿流体,因此只采用$We$来表征撞击时的条件,图5表示3种不同液滴撞击亲水壁面的动态过程.在不发生溅射的情况下,液滴撞击固体壁面后的变形过程可分为4个过程[12]:下落、铺展、回缩以及平衡.从图中可以看出, 液滴在撞击20ms后达到平衡状态,在液滴撞击过程,纳米颗粒的加入改变了环氧树脂液滴的动力学行为,在质量分数为0.1%时,石墨粉/环氧树脂液滴的铺展和回缩程度大于多壁碳纳米管/环氧树脂液滴.下文将会详细讨论影响纳米流体液滴撞击壁面动态铺展特性的关键因素. 显示原图|下载原图ZIP|生成PPT 图 5液滴以3.43m/s撞击亲水壁面过程($We$$\approx$800),(a)环氧树脂、(b) 0.1%纳米石墨粉、(c) 0.1%多壁碳纳米管液滴. 撞击速度为3.43 m/s -->Fig. 5Impact sequence of droplet impact on hydrophilic surface at a velocity of 3.43 m/s ($We$$ \approx $800), (a) pure resin,(b)0.1%Nano powder, (c) 0.1%MWNTs -->
图9表示撞击速度为3.43 m/s($We$$ \approx$800)的情况下,不同纳米流体液滴撞击亲水表面(玻璃板)和疏水表面(Teflon基板)时,铺展因子随时间的变化曲线.从图中能够发现,在1$\sim$100ms的时间段内,相同的纳米流体液滴在撞击到玻璃板表面时其铺展因子变化缓慢,在100ms后,其铺展因子显著上升. 液滴撞击疏水表面时,当时间大于100ms时液滴的铺展因子保持不变.撞击疏水表面时纳米流体液滴的铺展过程被抑制的更明显,这使得纳米流体液滴达到最终平衡状态的速度更快. 显示原图|下载原图ZIP|生成PPT 图 9不同纳米流体撞击不同润湿性表面时铺展因子变化曲线 -->Fig. 9The spreading factor of droplets impact on hydrophilic(glass) and hydrophobic (Teflon) surface -->
2.3 液滴惯性力的影响
图10为不同撞击速度下纳米流体液滴的无量纲高度随时间的变化曲线.当液滴以1.71 m/s 的速度撞击固体壁面时,不同液滴的无量纲高度在前1ms的变化趋势并无明显差异,而在1$\sim$100ms范围内,不同液滴的无量纲高度变化显著,纯环氧树脂液滴的相对高度在此范围内存在明显的先下降后上升的趋势,纳米流体液滴的相对高度并无明显变化.当液滴以3.43 m/s 的速度撞击固体壁面时,不同液滴的无量纲高度在前0.3ms的变化趋势并无明显差异,在0.3$\sim$100ms范围内,纯环氧树脂液滴内表现出明显的先下降后上升的趋势,而纳米流体液滴的相对高度变化幅度较小.通过图10能够发现,在较高的撞击速度下,纳米流体液滴初始铺展时的无量纲高度变化范围更大,这说明液滴的惯性力在其初始的铺展阶段起着主导作用. 显示原图|下载原图ZIP|生成PPT 图 10纳米流体无量纲高度($We$$ \approx $200, $We$$ \approx $800) -->Fig. 10The dimensionless height of nanofluids ($We$$ \approx$200,$We$$ \approx $800) -->
3 结 论
本文采用两步法和超声波分散技术配制出均匀稳定不同分散相的纳米流体,基于高速摄像及图像后处理技术,对不同种类的纳米流体液滴撞击固体壁面行为进行可视化研究,得出如下结论. (1)流变学分析表明纳米颗粒的加入使得环氧树脂基液表现出剪切变稀行为,且添加了不同纳米分散相的基液表现出的剪切变稀程度不同. (2)纳米分散相的加入有效的抑制了液滴撞击固体壁面时的铺展和回缩行为,在相同的碰撞$We$及纳米颗粒质量分数的情况下,稠度系数$K$较大的纳米流体如碳纳米管/环氧树脂纳米流体,其铺展因子和无量纲高度的变化范围最小,即其液滴在壁面上的铺展行为被抑制的最为显著. (3)比较液滴撞击不同润湿性表面的过程,撞击疏水表面时纳米流体液滴的铺展过程被抑制的更明显,这使得纳米流体液滴能更快达到最终平衡状态.液滴的惯性力主导者液滴的初始铺展阶段,不同液滴在在惯性力相同时其初始铺展阶段($We$$\approx $200时为前1 ms, $We$$ \approx $800时为前0.3ms)的变化趋势并无明显差异.随着液滴撞击惯性力的增大,液滴的铺展及回缩的速度更快,变化幅度也更大. The authors have declared that no competing interests exist.
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