The thermal transpiration of molecular gas is investigated based on the model of Wu et al. [“A kinetic model of the Boltzmann equation for non-vibrating polyatomic gases,” J. Fluid Mech. 763, 24–50 (2015)], which is solved by a synthetic iterative scheme efficiently and accurately. A detailed investigation of the thermal slip coefficient, Knudsen layer function, and mass flow rate for molecular gas interacting with the inverse power-law potential is performed. It is found that (i) the thermal slip coefficient and Knudsen layer function increase with the viscosity index determined by the intermolecular potential. Therefore, at small Knudsen number, gas with a larger viscosity index has a larger mass flow rate; however, at late transition and free molecular flow regimes, this is reversed. (ii) The thermal slip coefficient is a linear function of the accommodation coefficient in Maxwell’s diffuse–specular boundary condition, while its variation with the tangential momentum accommodation coefficient is complicated in Cercignani–Lampis’s boundary condition. (iii) The ratio of the thermal slip coefficients between monatomic and molecular gases is roughly the ratio of their translational Eucken factors, and thus, molecular gas always has a lower normalized mass flow rate than monatomic gas. (iv) In the transition flow regime, the translational Eucken factor continues to affect the mass flow rate of thermal transpiration, but in the free molecular flow regime, the mass flow rate converges to that of monatomic gas. Based on these results, accommodation coefficients were extracted from thermal transpiration experiments of air and carbon dioxide, which are found to be 0.9 and 0.85, respectively, rather than unity used in the literature. The methodology and data presented in this paper are useful, e.g., in the pressure correction of capacitance diaphragm gauge when measuring low gas pressures.

中文翻译：

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分子气体中的热蒸发

基于Wu等人的模型研究了分子气体的热蒸发。[“非振动多原子气体的玻尔兹曼方程的动力学模型，” J。流体力学。763，24-50（2015）]，通过有效且准确的综合迭代方案解决了这一问题。进行了对分子气体与逆幂律势相互作用的热滑移系数，努森层函数和质量流率的详细研究。发现（i）热滑移系数和克努森层函数随着由分子间电势确定的粘度指数而增加。因此，在克努森数较小的情况下，粘度指数较大的气体的质量流量较大，反之亦然。但是，在后期过渡和自由分子流态下，这是相反的。（ii）热滑动系数是麦克斯韦扩散-镜面边界条件下调节系数的线性函数，在切尔西尼亚尼-兰皮斯的边界条件下，其随切向动量调节系数的变化变得复杂。（iii）单原子气体和分子气体之间的热滑系数之比大致等于它们的平移欧肯因数的比，因此，分子气体总是具有比单原子气体更低的归一化质量流量。（iv）在过渡流态中，平移Eucken因子继续影响热蒸腾的质量流率，但在自由分子流态中，质量流率收敛于单原子气体的质量流率。根据这些结果，从空气和二氧化碳的热蒸腾实验中提取的住宿系数分别为0.9和0.85，而不是文献中使用的统一系数。