Thermocapillary effects on heat transfer structure in buoyancy suppressed thin film melting

초록

This study investigates thermocapillary convection in a thin liquid film formed during melting in a direct-contact latent heat thermal energy storage system under buoyancy-suppressed conditions. In the top-heating configuration, stable density stratification suppresses buoyancy-driven natural convection, making interfacial thermocapillary forcing the primary flow-driving mechanism. A temperature-dependent surface tension model was implemented in a VOF-based multiphase solver by introducing tangential Marangoni stresses while retaining the curvature-based continuum surface force formulation. The coupled thermocapillary-phase-change formulation was verified against a Stefan limiting case, and buoyancy suppression was quantified using a signed film-based Rayleigh number and a buoyancy-to-thermocapillary ratio. The Rayleigh number remained negative and far below the critical value for Rayleigh-Benard convection, confirming stable stratification. In the absence of thermocapillary effects, both interfacial temperature variation and flow intensity remained negligible, indicating conduction-dominated heat transfer. In contrast, temperature-dependent surface tension induced interfacial temperature non-uniformity and cellular recirculating flows within the liquid film. In the strongest thermocapillary case, TS-8.0, the peak Marangoni number reached approximately 108, and the heat-flux non-uniformity index at the solid surface increased from 1.00 to 2.48 relative to the constant-surface-tension case. However, the reduction in residual solid fraction remained limited to approximately 3.8%. These findings indicate that thermocapillary convection primarily redistributes local heat transfer and modifies melting morphology, rather than substantially enhancing the overall melting rate.

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