Abstract

The nanofluids characterization is of fundamental importance to predict their applicability in a real heat transfer situation. In addition to the transport properties, the nanofluid stability is an important factor to be studied, due to the long pumping, heating, cooling and resting cycles the fluids are usually subjected to. Carbon nanotubes have great potential to be used in nanofluids, due to their high thermal conductivity and low density. However, for aqueous medium, stability requires the use of stabilization methods. This work carried out the study of the transport properties and stability of nanofluids with multiwalled carbon nanotubes (MWCNT), using three stabilization methods: Triton X-100 (TrX–MWCNT) and Arabic gum (AG–MWCNT) as surfactants and functionalized carbon nanotubes with COOH group (COOH–MWCNT). These three nanofluids were compared to investigate the best configuration for heat transfer processes. The thermal conductivity was studied using the hot wire method; rheology was studied through three curves in rotational rheometer of concentric cylinders; specific heat was studied by the differential scanning calorimetry method; density was measured by the Coriolis Effect method; nanoparticles size and zeta potential were measured by the dynamic light scattering method. The COOH–MWCNT nanofluids present the highest thermal conductivity increase reaching 15% at 60 °C and 12% at 30 °C with 1wt% while the A.G–MWCNT and TrX–MWCNT reach 11% and 7.5%, respectively, at 30 °C. A.G–MWCNT nanofluids showed greater stability after long sonication times, while COOH–MWCNT presents best dispersion at 0.125 and 0.25 wt%, from 40 to 160 min of sonication. The specific heat increases with temperature and decreases with MWCNT concentration; however, it presents a peculiar increase in the concentration of 0.25 wt%, increasing about 0.5% and 4.5% for A.G–MWCNT and COOH–MWCNT, respectively at 60 °C. COOH–MWCNT nanofluids showed a thixotropic behavior for 0.5 and 1.0 wt% and large viscosity increase reaching 3.75 and 5.5 times the base fluid viscosity at 5 °C and 60 °C, respectively, with 1 wt%. The nanofluids A.G–MWCNT and COOH–MWCNT present good stability, as well as an increase in transport properties, optimized at a concentration close to 0.25 wt% and the COOH–MWCNT nanofluids can be stabilized simply by changing the water pH.

Graphic abstract

中文翻译：

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MWCNT和COOH–MWCNT水性纳米流体：热物理和流变学表征

摘要

纳米流体的表征对于预测其在实际传热情况下的适用性至关重要。除了传输特性之外，由于流体通常要经历较长的泵送，加热，冷却和静置循环，因此纳米流体的稳定性也是要研究的重要因素。碳纳米管由于其高导热性和低密度而具有在纳米流体中使用的巨大潜力。但是，对于水性介质，稳定性需要使用稳定化方法。这项工作使用三种稳定方法：Triton X-100（TrX–MWCNT）和阿拉伯胶（AG–MWCNT）作为表面活性剂和功能化的碳纳米管，研究了具有多壁碳纳米管（MWCNT）的纳米流体的传输特性和稳定性。与COOH基团（COOH–MWCNT）。比较了这三种纳米流体，以研究传热过程的最佳配置。使用热线法研究热导率；通过同心圆柱体旋转流变仪中的三个曲线研究流变学。通过差示扫描量热法研究比热。密度通过科里奥利效应法测量；通过动态光散射法测量纳米颗粒的尺寸和ζ电位。COOH–MWCNT纳米流体的导热率最高，在60°C时达到15％，在30°C时达到12％，1wt％，而在30°CAG时AG–MWCNT和TrX–MWCNT分别达到11％和7.5％。在长时间超声处理后，–MWCNT纳米流体显示出更高的稳定性，而在超声处理40至160分钟时，COOH–MWCNT在0.125和0.25 wt％时表现出最佳的分散性。比热随温度而增加，随MWCNT浓度而降低；然而，在60°C时，浓度却显着增加了0.25 wt％，AG-MWCNT和COOH-MWCNT分别增加了约0.5％和4.5％。COOH-MWCNT纳米流体的触变性为0.5和1.0 wt％，并且在5°C和60°C时，粘度的大增分别达到基础流体粘度的3.75和5.5倍，为1 wt％。纳米流体AG–MWCNT和COOH–MWCNT表现出良好的稳定性以及传输性能的提高，在接近0.25 wt％的浓度下进行了优化，而COOH–MWCNT纳米流体可以简单地通过改变水的pH值来稳定。AG–MWCNT和COOH–MWCNT在60°C时分别为5％和4.5％。COOH-MWCNT纳米流体的触变性为0.5和1.0 wt％，并且在5°C和60°C时，粘度的大幅度增加分别达到基础流体粘度的3.75和5.5倍，为1 wt％。纳米流体AG–MWCNT和COOH–MWCNT表现出良好的稳定性以及传输性能的提高，在接近0.25 wt％的浓度下进行了优化，而COOH–MWCNT纳米流体可以简单地通过改变水的pH值来稳定。AG–MWCNT和COOH–MWCNT在60°C时分别为5％和4.5％。COOH-MWCNT纳米流体的触变性为0.5和1.0 wt％，并且在5°C和60°C时，粘度的大幅度增加分别达到基础流体粘度的3.75和5.5倍，为1 wt％。纳米流体AG–MWCNT和COOH–MWCNT表现出良好的稳定性以及传输性能的提高，在接近0.25 wt％的浓度下进行了优化，而COOH–MWCNT纳米流体可以简单地通过改变水的pH值来稳定。

图形摘要