Thermocapillary-enhanced Melting of Different Phase-change Materials in Microgravity

A numerical analysis of the thermocapillary-driven melting of phase change materials (PCMs) in weightlessness is presented. The phase change is explored for different PCMs with moderate melting temperatures, due to their potential for thermal control in space applications. We consider three different alkanes — n-octadecane, n-nonadecane, and n-eicosane — and gallium. Results are discussed in terms of the dimensionless Stefan (Ste) and Marangoni (Ma) numbers, which quantify the importance of the latent heat and the thermocapillary effect during the phase change process, respectively, and the container aspect ratio Γ\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\Gamma$$\end{document}. For alkanes, similar results are obtained with melting rate enhancements that depend on Γ\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\Gamma$$\end{document}. In short (deep) containers, the thermocapillary effect accelerates melting — with respect to the conduction-driven case — by a factor of as much as 4 depending on Ma, while in large (shallow) containers, this enhancement factor can take values up to 20. The best performance is featured by n-eicosane, followed closely by n-octadecane. For gallium, results differ substantially due to its high thermal diffusivity, leading to a significant reduction of the enhancement up to a value of approximately 1.2 at large Ma and Γ\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\Gamma$$\end{document}.