Discharging process and thermal evaluation in the thermal energy storage system with fractal tree-like fins.

• Fractal tree-like fins are used to accelerate melting under microgravity. • Thermocapillary convection and gravity effects on melting are analyzed. • Melting acceleration degree due to fins increases with increasing bifurcation angle. • Effect of thermocapillarity on melting increases with increasing bifurcation angle. Under microgravity environment for spacecraft operation, the melting rate of phase change material is restricted by its low heat transfer performance and disappearance of natural convection. To accelerate the melting under microgravity, the insertion of fractal tree-like fins into a latent heat thermal energy storage unit is proposed. The dynamic melting behavior and heat transfer performance in cavities with fractal tree-like fins having bifurcation angle of 30°, 60° and 90° are investigated through a transient model based on the enthalpy-porosity method. The relationship between the effects of tree-like fins and thermocapillary convection on melting is disclosed. The results show that during melting driven by combined heat conduction and thermocapillary convection, the presence of tree-like fins significantly accelerates melting, and the increment degree of melting rate increases with the increase of bifurcation angle. Compared with cavity without fin, 30°, 60° and 90° fins reduce melting time by 47.7%, 57.2% and 64.3%, respectively. The fractal tree-like fins also enhance the increment speed of energy storage capacity, while enhancing and slightly deteriorating heat transfer at the beginning and end of melting, respectively. The thermocapillary convection accelerates the melting under microgravity, and the acceleration effect degree increases with the increase of bifurcation angle. With the presence of natural convection, compared with the cavity without fin, 30°, 60° and 90° fins reduce the melting time by 57.8%, 60.6% and 65.3%, respectively; the acceleration effect of fins on melting is enhanced, and the effect of bifurcation angle on melting becomes weaker. The present design innovatively introduces tree-like fins into thermal energy storage under microgravity condition, and can be used for temperature control of spacecraft electronic devices. [ABSTRACT FROM AUTHOR]