Highly rarefied gas flows in rough channels of finite length

IF 2.9 3区 工程技术 Q2 ENGINEERING, MECHANICAL Advances in Aerodynamics Pub Date : 2023-11-22 DOI:10.1186/s42774-023-00159-9
Zheng Shi, Yulong Zhao, Wei Su, Lei Wu
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Abstract

Highly rarefied gas flows through a rough channel of finite length with small bumps appended to its surfaces are investigated, by varying the accommodation coefficient $$\alpha$$ in Maxwell’s diffuse-specular boundary condition, the characteristic size and position of the bumps, and the channel length. First, we study the influence of the surface bumps and consider the rarefied gas flow in a unit channel with periodic boundary conditions to remove the end effect. It is found that the surface bumps have a significant impact on the flow permeability. When $$\alpha$$ is very small (i.e., nearly specular reflection of gas molecules at the channel surface), the apparent gas permeability is dramatically reduced, even in the presence of small bumps, to a value that is almost comparable to the one when fully diffuse gas-surface scattering is assumed. This impact can be taken into account through an effective accommodation coefficient, i.e., the permeability of the rough channel is taken equivalently as that of a smooth channel without bumps but having gas-surface scattering under the effective accommodation coefficient. Second, we study the end effect by connecting a smooth channel of length $$L_0$$ to two huge gas reservoirs. It is found that (i) the end correction length is large at small $$\alpha$$ . Consequently, the mass flow rate barely reduces with increasing $$L_0$$ rather than scales down by a factor of $$1/L_0$$ as predicted by the classical Knudsen diffusion theory; and (ii) the end correction is related to the channel’s aspect ratio. Finally, based on the effective accommodation coefficient and end correction, we explain the exotic flow enhancement in graphene angstrom-scale channels observed by Geim’s research group (Keerthi et al, Nature 558:420–424, 2018).
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高度稀薄的气体在有限长度的粗糙通道中流动
通过改变麦克斯韦漫射-镜面边界条件下的调节系数$$\alpha$$、凸起的特征尺寸和位置以及通道长度,研究了高稀薄气体在具有小凸起的有限长度粗糙通道中的流动。首先,我们研究了表面凸起的影响,并考虑了具有周期性边界条件的单元通道中的稀薄气体流动,以消除末端效应。研究发现,表面凸起对渗透率有显著影响。当$$\alpha$$非常小时(即气体分子在通道表面的近镜面反射),即使存在小凸起,表观气体渗透率也会急剧降低,其值几乎与假设完全漫射气体表面散射时的值相当。这种影响可以通过有效调节系数来考虑,即粗糙通道的渗透率等同于在有效调节系数下没有凸起但存在气面散射的光滑通道的渗透率。其次,通过将长度为$$L_0$$的光滑通道连接到两个大型气藏,研究了末端效应。发现(i)末端修正长度在小$$\alpha$$处较大。因此,质量流率几乎不随$$L_0$$的增加而减小,而不是像经典Knudsen扩散理论预测的那样按$$1/L_0$$的比例减小;(ii)终端校正与信道的宽高比有关。最后,基于有效调节系数和末端校正,我们解释了Geim研究小组观察到的石墨烯埃级通道中的外来流动增强(Keerthi et al ., Nature 558:420-424, 2018)。
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来源期刊
CiteScore
4.50
自引率
4.30%
发文量
35
审稿时长
11 weeks
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