Thermocapillary effects during the melting of phase change materials in microgravity: Heat transport enhancement

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.