Performance enhancement of bionic latent heat storage units with novel sandwiched fractal tubes

Latent heat storage units (LHSUs) suffer from intrinsically low heat storage rates, a limitation that bionic LHSUs with sandwiched fractal tubes attempt to alleviate by enhancing the thermal accessibility across the entire domain. Advancing the concept, this work presents a comprehensive investigation of the fractal tube cross-sectional designs in bionic LHSUs. Through systematic evaluation of 12 distinct tube configurations using the enthalpy-porosity model, the temperature distribution, convective propagation, heat transfer efficiency, pressure drop, and melting time are analyzed. The findings reveal that while maintaining a constant phase change material (PCM) volume to tube volume ratio, circular and square tubes demonstrate superior performance. Designs with shape ratio (S_H) < 1 consistently underperform across all evaluated parameters, whereas configurations with S_H > 1 enhance performance by improving vertical thermal accessibility and conduction pathways. The optimal S_H of 3.5 decreases melting time by 15.7 % compared to baseline designs. However, further increases in S_H degrade performance due to declined horizontal thermal accessibility. Pressure drop analyses show unfavorable increases at both S_H extremes, with particularly excessive values for S_H < 1 designs. The study demonstrates that rectangular tubes with S_H between 3 and 3.5 optimally balance point-to-area flow and heat transfer access, achieving superior heat accessibility while effectively addressing the inherent limitations of conventional LHSUs.