非均匀边界对相变储热过程特性及性能强化的影响机制研究

Latent heat storage system (LHSS) has many advantages, such as having high heat-storage density and maintaining a constant temperature. Therefore, it can be used to alleviate or even eliminate the negative influence of time/space-varying energy supply on the energy-utilization efficiency. However, the performance of LHSS is interactively affected by solid-liquid phase change, heat conduction and natural convection, which leads to an unsteady conjugate heat-transfer problem. Low heat-transfer capability and nonlinear dynamic behavior are two main heat-transfer characteristics of LHSS. More researches should be carried out to reveal the effects of non-uniform boundary on the LHSS and its performance enhancement. In this project, a model of LHSS under non-uniform heat flux condition will be constructed. The non-uniform heat flux condition is generated by the concentrated solar technology. The effects of the solar parameters, the concentrated characteristic, and the gravity on the performance of LHSS will be revealed. Furthermore, with respect to the poor heat transfer conductivity of phase change material (PCM), high-conductivity material is used to enhance the heat-transfer performance of PCM. An optimization algorithm will be built to optimize the layout of high-conductivity material with the consideration of the effects of non-uniform boundary and natural convection. Finally, a universal method will be developed for fast predicting the nonlinear dynamic characteristic of the solid-liquid phase-change heat transfer. A half-empirical and half-theoretical correlation will be built for the time-averaged dimensionless heat transfer coefficient. From the point of engineering thermo-physics, this project has some theoretical and practical values on providing guidance for perfecting the mechanism of LHSS, improving the performance of LHSS, and accelerating the exploitation and design of LHSS.

相变储热具有储热密度高和恒温等特点，可以缓解或消除能源供应中经常出现的时空非均匀性对能源利用效率产生的不利影响；然而，相变储热过程涉及相变、导热和自然对流相耦合的动态传热，具有传热效果差以及动态非线性两大特征；非均匀边界对于动态储热过程特性及性能强化的影响机制还有待深入研究。本项目以聚光太阳能系统产生的非均匀边界入手，建立非均匀边界下的相变储热模型，揭示太阳参数、聚光特性以及重力等参数对相变储热过程的影响规律；并采用高导材料作为强化换热措施，综合考虑非均匀边界和自然对流的影响，开发适用于动态非线性问题的高导材料布局主动优化算法，获得与非均匀边界相匹配的高导材料布置方式；最后，构建动态非线性相变传热特性的通用快速预测方法，建立无量纲时均换热系数的半理论半经验关联式。本项目从工程热物理角度，为完善相变储热机制，提升相变储热性能，推进相变储热器的开发与设计提供参考与指导，具有理论和实际应用价值。

相变储热具有储热密度高和恒温等特点，可以缓解或消除能源供应中经常出现的时空非均匀性对能源利用效率产生的不利影响；然而，相变储热过程涉及相变、导热和自然对流相耦合的动态传热，具有传热效果差以及动态非线性两大特征；复杂非均匀边界对于动态储热过程特性及性能强化的影响机制还有待深入研究。本项目以聚光太阳能产生的非均匀边界入手，提出了直接耦合相变储热材料的新型槽式聚光集热器。重点揭示非均匀边界和自然对流影响下的非均匀相变储热过程机制。通过解析方法计算槽式聚光吸热管外表面的非均匀热流分布，并实现与数值模拟方法的耦合；搭建测试相变储热单元充放热特性的实验台，验证非均匀边界加剧非均匀相变储热过程，验证高导多孔介质对非均匀相变过程的改善作用，验证复杂非均匀边界条件影响下的相变储热模型的准确性。采用泡沫金属作为强化传热措施，提出与非均匀热流相匹配的部分填充泡沫金属结构，以及二维梯级泡沫金属结构；进一步提出与非均匀热流相匹配的非均匀翅片布置方式和偏心结构的异形储热单元结构；开发出适用于动态非线性相变储热优化问题的遗传算法与计算流体力学（CFD）耦合优化算法，提出应对巨量计算的并行计算和数据库理念，成功开发出非均结构参数的翅片设计方案；针对非均匀相变导致的储热速率下降问题，提出温度场均匀性原则以及定量化指标（温度场均匀性因子及其随时间的积分因子），并指导开发出轴向梯级多孔复合相变材料；进一步在均匀相变原则指导下，提出以最短路径作为优化指标的翅片快速优化算法，并通过将计算域网格化，以及引入蒙特卡洛面积统计方法解决了复杂翅片结构优化的难点，开发出箭头形翅片结构；最后，提出实际时间与理论参考时间的对比无量纲完全储热时间的概念，降低其与影响参数之间的非线性关系，并建立新的预测关联式。本项目从工程热物理角度，为完善相变储热机制，提升相变储热性能，推进相变储热器的开发与设计提供参考与指导，具有理论和实际应用价值。