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Heat transfer transitions of natural convection flows in a differentially heated square enclosure filled with nanofluids

机译:自然对流的热传递转换在充满纳米流体的差热方形外壳中流动

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Numerical investigations are carried out for steady natural convection of Al_2O_3 - H_2O nanofluids having temperature-dependent properties inside a differentially heated square enclosure by using Buongiorno's two-phase mixture model, where Brownian diffusion and thermophoresis are regarded as the primary slip mechanisms between solid and liquid phases. Influence of the Rayleigh number on the heat transfer behavior with increasing nanoparticle volume fraction to 6%, are systematically investigated for various physical conditions, including averaged temperature, temperature difference and nanoparticle diameter. There are three scenarios in general, except the type 1 (heat transfer rate enhances continuously) and the type 3 (optimum volume fraction exists) mentioned in previous references, the transition between these types, namely, type 2 (heat transfer rate becomes constant) is detected. By increasing the Rayleigh number, the heat transfer behavior of rising volume fraction experiences transition from type 1, type 2 and then to type 3 in sequence, and the transition is valid for low Ra only. The corresponding critical Rayleigh number is found to increase in pace with the rise of averaged temperature and temperature difference, but is found to decrease with the growth of the nanoparticle diameter. Finally, one correlation equation for reproducing the critical Rayleigh number with averaged temperature ranging from 300 K to 320 K is presented.
机译:通过使用Buongiorno的两相混合物模型,对差热方形罩中具有温度依赖性的Al_2O_3-H_2O纳米流体的稳态自然对流进行了数值研究,其中布朗扩散和热泳被视为固液之间的主要滑动机理阶段。对于包括平均温度,温差和纳米颗粒直径在内的各种物理条件,系统地研究了瑞利数对纳米颗粒体积分数增加到6%时传热行为的影响。通常存在三种情况,除了先前参考文献中提到的类型1(传热速率不断提高)和类型3(存在最佳体积分数)以外,这两种类型之间的过渡即类型2(传热速率变得恒定)。被检测到。通过增加瑞利数,体积分数上升的传热行为依次从类型1,类型2到类型3过渡,且该过渡仅对低Ra有效。发现相应的临界瑞利数随平均温度和温度差的增加而增加,但随纳米粒子直径的增加而减少。最后,给出了一个相关方程,用于再现平均温度范围为300 K至320 K的临界瑞利数。

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