The melting and solidification of phase change material (PCM) in a horizontal eccentric shell-and-tube latent heat storage (LHS) unit were numerically studied to design a high-efficiency LHS configuration.Firstly, the melting and solidification in a concentric configuration were studied to reveal the influence of natural convection.The results show that, because of natural convection, the solid PCM above the tube has a high melting rate during the melting process and the liquid PCM under the tube has a high solidification rate during the solidification process.Then the eccentric tube configuration was used to take the best advantage of natural convection.The results show that the eccentric tube with the tube moving to the downside can efficiently enhance the PCM melting rate.The best melting performance can be obtained when the eccentricity (ε) is -0.6,where the PCM melting time reduces about 29.8% compared with the concentric tube.However, the downward movement of the tube seriously weakens the PCM solidification rate and causes total performance deterioration of the entire melting and solidification cycle.For the whole melting and solidification cycle without rotation, the minimum total time is obtained at ε= 0.1 which is only 1.3% less than the total time at ε= 0. A new LHS unit with a rotation configuration was proposed to enhance PCM melting and solidification rates simultaneously.The validation results show the optimum heat transfer performance for the whole melting and solidification cycle can be obtained when ε = -0.2,where the total melting and solidification time reduces by 11.1% compared with concentric configuration.
为了强化相变储热单元的储热性能,基于(煅)耗散原理,数值分析了相变材料的熔点对储热单元的储热速率、(煅)耗散率及储热品质的影响.结果表明:对于单级相变储热单元,相变材料熔点越低,相变储热过程的传热速率就越高,而对应的(煅)耗散率就越大,储存热量的品质也越低.证明了(煅)耗散原理用于相变储热过程优化的可行性.对于两级相变储热单元的分析结果表明,合理的匹配两级相变材料的熔点,可以实现储热速率高于单一相变材料,且传热过程的不可逆性(煨耗散率)低于单一相变材料.可见,多级相变材料的合理匹配,可以同时提高储热速率、降低储热过程的不可逆性.
建立了考虑液态相变材料自然对流的壳管式相变蓄热单元的三维模型,数值分析了自然对流对相变蓄热过程的影响.对比研究了外侧强化传热管和双侧强化传热管对相变蓄热单元蓄热性能的强化效果.结果表明,液态相变材料的自然对流,会引起固-液界面分布不均匀现象,采用外翅片管可以有效削弱这一现象;采用外侧强化传热管和双侧强化传热管,都可以缩短相变材料完全熔化以及整个蓄热过程所需时间.与采用光管时相比,采用外侧强化传热管时,完全熔化时间减少了18.0％;采用双侧强化传热管时,完全熔化时间减少52.5％.可见,采用带有外翅片的强化传热管,不仅可以削弱自然对流引起的固-液界面不均匀性问题,而且可以强化相变蓄热单元的蓄热性能.
