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An improved Capillary Breakup Extensional Rheometer to characterize weakly rate-thickening fluids: Applications in synthetic automotive oils
Journal of Non-Newtonian Fluid Mechanics ( IF 3.1 ) Pub Date : 2021-02-15 , DOI: 10.1016/j.jnnfm.2021.104496
Jianyi Du , Hiroko Ohtani , Crystal E. Owens , Lenan Zhang , Kevin Ellwood , Gareth H. McKinley

We describe a customized Capillary Breakup Extensional Rheometer (CaBER) with improved dynamic performance and added features for temperature control over the range from room temperature up to 250 °C. The system is aimed at characterizing the extensional rheological behavior of weakly rate-thickening fluids that are widely utilized in the automotive industry. We examine the shear rheology and filament-thinning dynamics of two commercially available automotive fluids with the same viscosity index. Comparisons of the rheological properties of the two samples reveal that although they have identical shear viscosities, they exhibit significant and distinct rate-thickening behavior in the strong extensional flow that is generated close to filament breakup. For the more elastic sample, the exponential filament-thinning dynamics are well-described by the Oldroyd-B model; however, this viscoelastic model poorly describes the response of the more weakly rate-thickening fluid. To address this limitation, we propose a simple Inelastic Rate-Thickening (IRT) model that more robustly describes the measured material response. The two constitutive parameters of the model represent the zero-shear-rate viscosity of the fluid and the rate of extensional thickening in the fluid. Numerical calculations with the IRT model show that the radii of thinning fluid filaments deviate from a linear decay in time and approach a quadratic dependence very close to breakup. By carefully fitting the measured temporal evolution of the mid-plane radius we can therefore systematically differentiate the extensional rheological response of the two oils. More generally, we show that the weakly rate-thickening regime can be distinguished from the well-known elasto-capillary response predicted by the Oldroyd-B model, via a constraint on the relaxation time (or more specifically the elasto-capillary number) of the fluid. The weakly rate-thickening behavior documented in these oils is representative of the relatively unstudied extensional rheology of many industrial fluids at large extensional strain rates (100-1000 s-1) and influences many complex industrial processes such as jetting, coating and stamping.



中文翻译:

改进的毛细管破裂延伸流变仪,用于表征低速率增稠的流体:在合成汽车油中的应用

我们介绍了一种定制的毛细管破裂延伸流变仪(CaBER),它具有改进的动态性能,并提供了从室温到250°C范围内的温度控制功能。该系统旨在表征在汽车工业中广泛使用的弱速稠化流体的扩展流变行为。我们研究了两种具有相同粘度指数的市售汽车油的剪切流变学和细丝稀化动力学。比较这两个样品的流变特性,发现尽管它们具有相同的剪切粘度,但它们在细丝断裂附近产生的强延伸流中表现出明显且截然不同的速率增厚行为。对于更具弹性的样品,Oldroyd-B模型很好地描述了指数细丝稀疏动力学。但是,这种粘弹性模型很难描述速率较弱的流体的响应。为了解决此限制,我们提出了一个简单的非弹性增厚(IRT)模型,该模型可以更可靠地描述所测量的材料响应。模型的两个本构参数表示流体的零剪切速率粘度和流体中的拉伸增稠速率。用IRT模型进行的数值计算表明,细化流体细丝的半径随时间线性变化而偏离,并接近于非常接近破裂的二次相关性。通过仔细拟合所测量的中平面半径的时间演化,我们可以系统地区分两种油的拉伸流变响应。更广泛地说,我们显示弱速率增厚机制可以通过限制松弛时间(或更具体地讲,弹性毛细管数)与Oldroyd-B模型预测的众所周知的弹性毛细管反应有所区别。流体。这些油中记录的弱速率增稠行为代表了许多工业流体在大拉伸应变率(100-1000 s)下相对未被研究的拉伸流变学。-1)并影响许多复杂的工业过程,例如喷射,涂层和冲压。

更新日期:2021-02-15
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