Shen Jian, Liburdy James A, Pence Deborah V, Narayanan Vinod
Mechanical Engineering, Oregon State University, Corvallis, OR 97331-6001, USA.
J Phys Condens Matter. 2009 Nov 18;21(46):464133. doi: 10.1088/0953-8984/21/46/464133. Epub 2009 Oct 29.
This study investigates the hydrodynamic characteristics of droplet impingement on heated surfaces and compares the effect of surface temperature when using water and a nanofluid on a polished and nanostructured surface. Results are obtained for an impact Reynolds number and Weber number of approximately 1700 and 25, respectively. Three discs are used: polished silicon, nanostructured porous silicon and gold-coated polished silicon. Seven surface temperatures, including single-phase (non-boiling) and two-phase (boiling) conditions, are included. Droplet impact velocity, transient spreading diameter and dynamic contact angle are measured. Results of water and a water-based single-wall carbon-nanotube nanofluid impinging on a polished silicon surface are compared to determine the effects of nanoparticles on impinging dynamics. The nanofluid results in larger spreading velocities, larger spreading diameters and an increase in early-stage dynamic contact angle. Results of water impinging on both polished silicon and nanostructured silicon show that the nanostructured surface enhances the heat transfer for evaporative cooling at lower surface temperatures, which is indicated by a shorter evaporation time. Using a nanofluid or a nanostructured surface can reduce the total evaporation time up to 20% and 37%, respectively. Experimental data are compared with models that predict dynamic contact angle and non-dimensional maximum spreading diameter. Results show that the molecular-kinetic theory's dynamic contact angle model agrees well with current experimental data for later times, but over-predicts at early times. Predictions of maximum spreading diameter based on surface energy analyses indicate that these models over-predict unless empirical coefficients are adjusted to fit the test conditions. This is a consequence of underestimates of the dissipative energy for the conditions studied.
本研究调查了液滴撞击加热表面的流体动力学特性，并比较了在抛光表面和纳米结构表面使用水和纳米流体时表面温度的影响。分别在冲击雷诺数和约1700、韦伯数约25的条件下获得了结果。使用了三种圆盘：抛光硅、纳米结构多孔硅和镀金抛光硅。研究包括七种表面温度，涵盖单相（非沸腾）和两相（沸腾）条件。测量了液滴冲击速度、瞬态扩展直径和动态接触角。比较了水和水基单壁碳纳米管纳米流体撞击抛光硅表面的结果，以确定纳米颗粒对撞击动力学的影响。纳米流体导致更大的扩展速度、更大的扩展直径以及早期动态接触角增大。水撞击抛光硅和纳米结构硅表面的结果表明，纳米结构表面在较低表面温度下增强了蒸发冷却的传热，这表现为蒸发时间更短。使用纳米流体或纳米结构表面可分别将总蒸发时间减少多达20%和37%。将实验数据与预测动态接触角和无量纲最大扩展直径的模型进行了比较。结果表明，分子动力学理论的动态接触角模型在后期与当前实验数据吻合良好，但在早期存在过度预测的情况。基于表面能分析的最大扩展直径预测表明，除非调整经验系数以适应测试条件，否则这些模型会过度预测。这是在所研究条件下对耗散能量估计不足的结果。
