Numerical Study On Phase Transition Heat Transfer Characteristics Of Heat Storage Units With Eccentric Fractal Fins

Phase change heat storage technology solves the intermittency and instability of solar energy,but the low thermal conductivity of phase change materials and the uneven distribution of temperature field reduce the heat storage and release rate,which limits the wide use of solar phase change heat storage.Therefore,the enhancement of phase change heat storage has become a hotspot of extensive research.Based on the enthalpy—porous medium model,the melting and solidification process of paraffin wax in the eccentric fractal finned element was studied numerically,and the partial strengthening and parameter optimization of the eccentric fractal finned element were carried out.The main work is as follows:(1)The paraffin melting process in the optical tube unit was simulated based on the enthalpy — porous medium model,and the liquid ratio was compared with the literature results.The maximum error was 9.33%,which verified the accuracy of the calculated results.The numerical analysis and comparison of phase transformation process of paraffin wax in concentric and eccentric units without fins showed that the total melting and solidification time of paraffin wax in eccentric optical tube unit was shorter.(2)The eccentric optical tube element was strengthened by adding rectangular fins and fractal fins respectively.It was concluded that during the melting process,the natural convection enhancement of the eccentric fractal fin element was higher than that of the eccentric rectangular fin element.However,the bottom area range of the fractal fin was lower than that of the rectangular fin element,resulting in the later melting speed decrease.During the solidification process,the solidification rate of eccentric fractal finned element was significantly higher.Compared with the whole process,the heat transfer rate of the eccentric fractal fin unit was higher.When the base of the eccentric fractal finned element was locally strengthened,the melting rate of the whole process was increased obviously,and the total melting time was reduced by 70%.The solidification rate of the early and middle stages increased obviously,but the solidification rate of the late stage did not increase obviously,and the total solidification time was reduced by 15%.With the heat storage and heat release power per unit mass as the performance evaluation index,the heat storage and heat release performance of partially strengthened eccentric fractal fin element was the best.(3)The geometric parameters of partially strengthened eccentric fractal fin element were studied,and it was concluded that the ratio of length to width of fractal fin,the secondary branch angle and the eccentricity of inner tube all had great influence on the melting and solidification process of paraffin wax.When comparing length-width ratio,γ=1.3,the melting and solidification rates were the fastest.The melting time increases with the increase of α,and the solidification time decreases first and then increases.The optimal α was 60°.When the eccentricity was compared at γ=1.3,α=60°,ε=0.38,the melting rate was the fastest.Whenε=0.3,the solidification rate was the fastest,and the final optimal ε was 0.3.Compared with the local strengthened heat storage unit model,the total melting and solidification time of the optimized heat storage unit model was reduced by 12%,and the average power was increased by 71.7%.