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This work presents the preliminary characterization of the vibrational environment of the DCMIX4 thermodiffusion experiment conducted onboard the ISS from December 2018 to March 2019. Given the long duration of each one of the 58 runs of the campaign and to ensure a correct interpretation of the results, an accurate analysis of acceleration levels all along the experiment is advisable. Digital signals coming from the nearest sensor, es09006, located in the Microgravity Science Glovebox (Destiny module) were downloaded from the PIMS NASA website. The techniques used to identify the main disturbances during the experiments were defined both in time and frequency domains. It is expected that the results obtained can help the experimentalist to have an overview of the possible sources of disturbances that may affect their experiments. To visualize the possible impact of the accelerometric environment on the experiment a numerical simulation has been performed. Four signals have been chosen, one considering the ideal case g = 0, one coming from the OSS raw sensor (outside the Microgravity Science Glovebox) and the other two, coming from the es09006 sensor that needed to be mathematical manipulated for considering only the low frequency range. Independent of the location of the sensor, numerical simulations do not detect, in any case, appreciable flow disturbances if quiescent periods are considered. Therefore, in case the reference sensor is not available one can use other sensors placed in the same module. [ABSTRACT FROM AUTHOR]

• Analysis of the heat transport achieved by thermocapillary effects during the melting of PCMs in microgravity. • For large containers, heat transport can be substantially enhanced by a factor of up to 20, depending on Ma. • For short containers, the enhancement can be on the order of 5 when oscillatory thermocapillary flow is present. • Existence of an optimal curve Γ(Ma) that maximises the thermocapillary contribution. An extensive numerical investigation of the heat transport enhancement achieved by the thermocapillary effect during the melting of Phase Change Materials in microgravity is presented. The phase change transition is analysed for the high Prandtl number paraffin n-octadecane in a two-dimensional rectangular container subjected to isothermal conditions along the lateral boundaries. The thermocapillary effect, supported by a free surface created as the solid/liquid front progresses, acts driving convection and as an effective mechanism for heat transport. For reference, the melting is first analysed under purely diffusive transport (conduction) and the heat transfer rate is shown to depend only on the Stefan number. Thermocapillary effects are then examined in the limiting cases of large and short containers. Results are presented for the representative aspect ratios Γ = 12 , 2.25 , where the thermocapillary flow is characterised by the appearance of oscillatory convection through the hydrothermal travelling wave and oscillatory standing wave modes, respectively, at a critical Marangoni number (Ma). The contribution of the thermocapillary effect to the heat transfer rate is quantified by the ratio of melting times between reference and thermocapillary simulations. For large Γ, heat transport can be greatly enhanced by a factor of up to 20, depending on Ma. For small Γ, this factor can be on the order of 5 when oscillatory flow is present. The analysis of the melting over a wide range of the governing parameters discovers the existence of an optimal curve Γ(Ma) that maximises the thermocapillary contribution. [ABSTRACT FROM AUTHOR]

• First microgravity experiments investigating enhancement of PCM melting due to thermocapillary convection. • Heat transport measured by observing the cross-sectional liquid area over time. • Phase change modelled numerically using an enthalpy-porosity based formulation. • Thermocapillary convection enhances heat transport by approximately a factor of two, depending on geometry. Experiments on the melting of n-octadecane in the presence of a free surface were performed under the reduced gravity conditions of a parabolic flight, where natural gravitational convection is negligible. The results constitute the first systematic experimental investigation of the capacity of thermocapillary (Marangoni) convection to enhance heat transfer during the melting of organic Phase Change Materials (PCMs) in microgravity, where their low thermal conductivity leads to significant lag (long response times) that, in some applications, constrains their utility as passive thermal control devices. Rectangular containers of three different aspect ratios holding n-octadecane and a layer of air were heated from one side in order to observe the melting process. During the relatively short microgravity periods, which lasted approximately 20 s, the effect of thermocapillary convection remained concentrated in a layer near the free surface that is thinner than the PCM height H for the aspect ratios considered. The weak dependence of this layer on the lower region, where heat transport is dominated by thermal diffusion, means that the relative contribution of thermocapillary convection decreases with H. The experimental results are supported and complemented by a series of numerical simulations modelling the phase change and liquid dynamics with an enthalpy-porosity-based formulation of the Navier-Stokes equations. Very good agreement with the experimental observations is found. [ABSTRACT FROM AUTHOR]

The Thermocapillary Effects in Phase Change Materials in Microgravity (TEPiM) experiment investigated the potential of Marangoni convection for heat transfer enhancement in Phase Change Materials (PCMs) in weightlessness, where the large melting/solidification timescale compromises their performance as thermal control devices. The TEPiM experiment, which was conducted during the 65th ESA Parabolic Flight campaign, represents the first microgravity research of this nature. We present here the main scientific outcome of the experimental campaign by measuring the precise contribution of thermal Marangoni convection to the heat transfer rate of the system. The test cells, holding the experimental PCM and air, were heated to drive controlled solid/liquid phase transitions during the microgravity lapses provided in the flight. Careful observation of the melting front confirms the enhancing effect of thermocapillary convection on heat transport, able to increase the phase change rate by a factor of 1.88. This result strongly supports that, as natural convection is exploited on ground, the thermal Marangoni effect can be used in weightless conditions to improve PCMs performance. • Microgravity experiments of Phase Change Materials melting with Marangoni effect. • Experimental evidence of Thermocapillarity in PCMs in reduced gravity. • First measurement of the Marangoni effect in PCMs phase change in microgravity. • Marangoni convection enhances heat transport by 1.93 in microgravity. [ABSTRACT FROM AUTHOR]

• Analysis of heat transport due to the thermocapillary effect during PCM melting on ground. • In long containers, heat transport may be significantly enhanced, by a factor of up to 20. • In intermediate containers, the enhancement factor is typically between 1–3 and depends weakly on Ma • In short containers, the thermocapillary effect is detrimental on average and increases the total melting time. • The presence of normal vertical gravity delays the appearance of oscillatory convection. The results of numerical simulations investigating the influence of natural and thermocapillary convection on the solid/liquid phase transition of n-octadecane in rectangular containers are presented. The melting process is modelled using an enthalpy-porosity formulation of the Navier–Stokes equations. A systematic analysis is performed by varying key dimensionless parameters including the container aspect ratio (Γ), the Rayleigh (Ra) and Marangoni (Ma) numbers, which quantify the strength of natural and thermocapillary convection, and the dynamic Bond number (Bo dyn), which measures their relative importance. In large containers with Γ ≫ 1 and Bo dyn ≪ 1 , thermocapillary convection is shown to significantly accelerate the melting process, enhancing the heat transfer rate of the system by as much as a factor of 20 at large applied Ma. For Γ ≲ 8 , this enhancement factor takes reduced values of 1–3 and exhibits relatively weak dependence on Ma. In short containers with Γ ≲ 3 and Bo dyn ∼ O (1) , the thermocapillary effect is detrimental on average and increases the total melting time. The presence of normal vertical gravity is seen to stabilise the dynamics of the flow, delaying the appearance of oscillatory convection beyond the range of parameters considered here. By reducing Bo dyn , we examine the transition to microgravity and determine the critical value Bo dyn cr for oscillatory flow at large Ma for the representative aspect ratios 1.5 and 12. [ABSTRACT FROM AUTHOR]