This paper contains natural convection of Ag–MgO/water micropolar hybrid nanofluid in a hollow hot square enclosure equipped by four cold obstacles on the walls. The simulations were performed by the lattice Boltzmann method (LBM). The influences of Rayleigh number and volume fraction of nanoparticle on the fluid flow and heat transfer performance were studied. Moreover, the effects of some geometric parameters, such as cold obstacle height and aspect ratio, were also considered in this study. The results showed that when the aspect ratio is not large (AR=0.2 or 0.4), at low Rayleigh number (10³), the two secondary vortices are established in each main vortex and this kind of secondary vortex does not form at high Rayleigh number (10⁶). However, at Ra=106, these secondary vortices occur again in the middle two vortices at AR=0.6, which is similar to that at Ra=103. At AR=0.2, the critical Rayleigh number, when the dominated mechanism of heat transfer changes from conduction to convection, is 10⁴. However, the critical Rayleigh number becomes 10⁵ at AR=0.4 or 0.6. When the cold obstacle height increases, the shape of the vortices inside the enclosure changes due to the different spaces. Besides, at Ra=106, for different cold obstacle heights, the location of the thermal plume is different, owing to the different shapes of vortices. Accordingly, the average Nusselt number increases by increment of the Rayleigh number, nanoparticle volume fraction, cold obstacle height and aspect ratio.

中文翻译：

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障碍物高度对空心方形外壳内纳米流体对流模式的影响

本文包含 Ag-MgO/水微极性混合纳米流体在一个空心的热方形外壳中的自然对流，该外壳在墙壁上配备了四个冷障碍物。通过格子玻尔兹曼方法（LBM）进行模拟。研究了纳米粒子的瑞利数和体积分数对流体流动和传热性能的影响。此外，本研究还考虑了一些几何参数的影响，如冷障碍物高度和纵横比。结果表明，当纵横比不大时（增强现实=0.2或0.4），在低瑞利数（10 ^{3 ）时，每个主涡都建立了两个次涡，在高瑞利数（10 6}）下不会形成这种次涡。然而，在拉=106，这些次级漩涡再次出现在中间的两个漩涡中增强现实=0.6, 这与在拉=103. 在增强现实=0.2，当传热的主要机制从传导变为对流时，临界瑞利数为10 ⁴。然而，临界瑞利数变为 10 ⁵在增强现实=0.4或 0.6。当冷障碍物高度增加时，由于空间的不同，围墙内的涡流形状发生变化。此外，在拉=106，对于不同的冷障碍物高度，由于涡流的形状不同，热羽流的位置也不同。因此，平均努塞尔数随着瑞利数、纳米粒子体积分数、冷障碍物高度和纵横比的增加而增加。