With the accelerated advancement of lunar and Mars exploration programs, long-term human habitation on extraterrestrial bodies faces the severe challenge of extreme diurnal temperature variations. Phase change materials (PCMs), owing to their ability to store or release substantial latent heat during solid–liquid phase transitions while maintaining a nearly constant temperature, have become an ideal solution for space thermal control systems. However, in the microgravity environment of space, the ground-dominant natural convection tends to disappear, and thermocapillary convection may become the prevailing heat transfer mechanism; in low-gravity environments such as those on the Moon and Mars, the coexistence and competition between these two convective regimes remain unclear. Existing studies have predominantly focused on either Earth's gravity or pure microgravity conditions, with a notable scarcity of research on the coupled effects of thermocapillary and natural convection at intermediate gravity levels. The melting dynamics of two typical organic PCMs, succinonitrile and n-octadecane, under low-gravity conditions have not yet been systematically elucidated. Therefore, clarifying the roles of thermocapillary convection under varying gravity levels and container geometries holds significant engineering guidance for the design of efficient space thermal control systems.
In a recent study published in Space: Science & Technology, the research team led by Ruiz from the Universitat Rovira i Virgili in Spain systematically investigated the influence of thermocapillary convection on the melting process of PCMs through numerical simulations. The study focuses on succinonitrile and n-octadecane as model PCMs, subjecting them to a temperature difference of 40 K in two rectangular cavities of different aspect ratios to drive melting, while simulating four gravity environments: microgravity on the International Space Station, lunar gravity, Martian gravity, and Earth's gravity. The results indicate that the aspect ratio is the key factor determining the dominant convective regime. In a flat cavity with an aspect ratio of 10, thermocapillary effects dominate the melting process at all gravity levels, significantly accelerating melting and generating multi-cellular oscillatory flows. In contrast, in a cavity with an aspect ratio of 2, natural convection is substantially enhanced and, at higher gravity levels, can counteract thermocapillary effects and even reduce the melting rate. Succinonitrile exhibits a markedly faster melting rate than n-octadecane owing to its higher thermal conductivity; nevertheless, the two materials show consistent trends in the competition between thermocapillary and natural convection. The study points out that in low-gravity environments such as those on the Moon and Mars, the rational selection of cavities with a large aspect ratio allows thermocapillary and natural convection to act synergistically, significantly enhancing the melting rate of PCMs. This research provides important theoretical foundations and optimization directions for the design of thermal management systems for future lunar and Martian bases, as well as for space-based PCM experiments in orbit, offering significant engineering application value for supporting the development of thermal control technologies for long-term extraterrestrial habitation.
First, this study focuses on the melting behavior of phase change materials (PCMs) under different gravity environments, with particular attention to the coupled effects of thermocapillary and natural convection. With the advancement of lunar and Mars exploration programs, the extreme diurnal temperature variations on extraterrestrial bodies pose severe challenges to thermal control systems, and phase change materials, owing to their constant-temperature heat storage and release characteristics, have become an ideal solution. However, under microgravity conditions, natural convection is weakened or even eliminated, and thermocapillary convection may become the dominant heat transfer mechanism; yet the competition between these two convection modes in low-gravity environments such as those on the Moon and Mars remains unclear. The study selects two typical organic PCMs—succinonitrile (Pr=23) and n-octadecane (Pr=56)—the former possessing higher thermal conductivity and faster melting, while the latter serves as the working fluid for the MarPCM experiment on the International Space Station. Simulations are conducted in rectangular cavities with a fixed length of 8 cm and aspect ratios of 2 and 10, respectively; a temperature difference of 40 K is imposed on the left and right walls to drive melting, and the upper surface is set as a free boundary to apply thermocapillary forces (as shown in the computational domain setup of Fig. 1). The accuracy of the numerical method is validated by comparison with published results; the comparison shows that the flow field structure obtained in this study is in excellent agreement with that in the literature under identical conditions, confirming the reliability of the solver.
Second, the study reveals the significant influence of gravity level and container aspect ratio on the melting dynamics. The liquid fraction evolution curves (Fig. 2) show that succinonitrile consistently melts faster than n-octadecane owing to its higher thermal conductivity, and thermocapillary effects serve to accelerate or modulate the melting process in both materials. In the flat cavity with an aspect ratio of 10, thermocapillary effects accelerate melting at all gravity levels, enabling the solid–liquid interface to reach the cold wall more rapidly, while the characteristic time for the liquid fraction to reach 95% is also substantially shortened. In the cavity with an aspect ratio of 2, the situation is more complex: in the early stages of melting, thermocapillary flow accelerates interface advancement; however, as the melt grows, natural convection progressively strengthens and flows in the direction opposite to the surface flow, thereby reducing the melting rate in the later stages at higher gravity levels. Fig. 3 summarizes the specific values of the two characteristic times under the four gravity scenarios, clearly demonstrating that thermocapillary effects are most pronounced under microgravity and low-gravity conditions, with their relative contribution gradually diminishing as gravity increases.
Finally, the study provides an in-depth elucidation of the melting mechanisms under different conditions through visualization of the flow and temperature fields, as well as the temporal evolution of the convective contribution factor. In the cavity with an aspect ratio of 2, pure natural convection gives rise to stable single-vortex or double-vortex structures; pure thermocapillary convection, in contrast, generates complex multi-cellular flows; when both mechanisms coexist, a distinctive three-zone structure emerges—comprising a bottom natural-convection vortex, a surface thermocapillary vortex, and an intermediate transition zone—with natural convection gradually becoming dominant as gravity increases (Fig. 4). In the flat cavity with an aspect ratio of 10, natural convection is significantly weakened due to the restricted height, and thermocapillary effects still dominate in the mixed convection regime, resulting in substantial deformation of the melting front and sustained oscillations under low-gravity conditions (Fig. 5). Fig. 6 presents the temporal evolution of the thermocapillary factor, which reaches a peak rapidly in the early stage of melting and then gradually declines, with its value decreasing as gravity increases. The study concludes that in low-gravity environments such as those on the Moon and Mars, a rational choice of containers with a large aspect ratio enables thermocapillary and natural convection to act synergistically in accelerating melting. This finding offers important engineering guidance for the design of thermal management systems for future lunar and Martian bases, as well as for space-based phase-change material experiments in orbit.
