空气源热泵变均匀度结霜除霜流动传热耦合作用机理研究

In existing literatures, few studies on an air source heat pump (ASHP) unit operating at different frosting/defrosting evenness values (FDEVs) are reported, although the FDEVs affect system operation performance directly and its defrosting control strategy indirectly. In the field of heat transfer, as well as frosting/defrosting, the refrigerant mass distribution (RMD) for a multi-circuit heat exchanger is also a fundamental problem, because its uneven distribution degrades the whole heat transfer performance. However, the refrigerant characteristic makes it hard to be clearly observed, accurately measured and exactly calculated, especially at two-phase state during heat transferring in a multi-circuit heat exchanger. Consequently, little progress in this field has been made in recent decades. In this proposed project, two mentioned problems are innovatively crosslinked in an ASHP unit with a multi-circuit outdoor coil. The refrigerant temperatures at the entrance and exit of each circuit are firstly detected, and then their temperature differences are seen as the index of adjusting the RMD. With this method, frost accumulations on each circuit’s surface could be quantitatively controlled, and thus the mechanism of RMD at static and dynamic heat transfer loads between two sides of heat exchanger could be obtained. As the second part of this study, based on the previous experimental results, a new dynamic defrosting model for this multi-circuit heat exchanger will be built. Different from previous models, the uneven frost accumulation at the start of defrosting and uneven RMD during defrosting will be comprehensively considered in this model. After it is validated with experimental results, more data could be collected. Based on the data analysis of system operation characteristics at different FDEVs of an ASHP unit, the mechanism of uneven heat transfer (UHT) between two sides of a multi-circuit heat exchanger is explored. Therefore, the two parts of the coupling effect mechanism of flow and heat transfer, mechanisms of RMD and UHT, in an ASHP unit operating at different FDEVs are reached. Thirdly, although many researchers pay their attentions to defrosting control study, few of them try to couple the control strategies of frosting and defrosting for an ASHP unit. Based on the validated defrosting model, a new control strategy will be explored, in which frosting and defrosting operation control strategies are coupled. System operation performance will be improved by the optimized control strategy under the guide of the coupling effect mechanism of flow and heat transfer. All the conclusions of this project will be useful for further optimizing heat exchangers and frosting/defrosting control strategy for ASHP units.

为提高空气源热泵结霜除霜工况运行能效，本项目旨在通过热泵变均匀度结霜除霜实现其室外多环路换热器外侧换热负荷的定量调配，揭示变均匀度结霜除霜过程中多环路换热器两侧介质间流动传热耦合作用机理。具体研究内容包括三个方面：（1）创新地将制冷剂分配纳入结霜除霜研究，基于热阻并联求和理论分析与结霜除霜对比实验研究，获得换热负荷分布动态变化时流动传热耦合作用下的两相流分配规律及系统运行特性；（2）基于实验结果构建变均匀度除霜动态数值模型，对换热器外侧介质非定常、多维度、有相变、移动边界的动态传热传质过程进行定量化、精确化探索，揭示多环路换热器两侧介质间不均匀换热机理及系统内能量迁移规律；（3）针对热泵结霜除霜周期循环运行，创新地将结霜与除霜的调控策略耦合，以流动传热耦合作用机理指导变均匀度除霜模型外推，获得结霜除霜耦合调控优化策略。本项目研究成果具有重要的学术价值和实际应用价值。

空气源热泵室外换热器表面结霜是造成系统低效运行的传统难题，多环路换热器表面的不均匀结霜除霜过程也是换热器两侧介质间不均匀换热的典型案例。为提高空气源热泵结霜除霜工况运行能效、确定通过变均匀度结霜除霜实现热泵室外多环路换热器外侧换热负荷定量调配的可能性，本项目展开了如下几方面的研究：（1）创新地将制冷剂分配纳入结霜除霜研究，基于结霜除霜循环工况对比实验研究，获得了换热负荷分布动态变化时流动传热耦合作用下的系统运行特性，实现了换热器外侧热负荷的“可视化”；（2）对热泵系统除霜过程中能量的来源项与消耗项进行了定量分析，确定了系统内部能量迁移规律，厘清了化霜水流动对系统除霜过程造成的影响，定性定量地评估了金属蓄热量对除霜能效的影响，为增设系统外部热源提供了基础理论支撑；（3）首次对双螺旋盘管相变蓄热罐在热泵系统结霜除霜工况下的储热释热过程进行了动态数值模拟与实验验证，确定了相变蓄热罐内部在储热释热过程中的能量迁移速率，完成了二元有机复合相变储能材料万次冷热循环后热物性漂移规律的测试，为建筑节能领域内相变储能材料的筛选方法提供了直接借鉴；（4）分别完成了变均匀度结霜除霜工况下热泵逆循环除霜基于时间控制的起始点与基于温度测量的除霜结束点的优化，该除霜控制策略可有效消除因“无霜除霜、有霜不除、除霜不尽、除霜不止”四种典型的误除霜现象导致的潜在能量浪费；（5）对增设制冷剂流量调节阀与消除化霜水作用托水盘两项改进技术，分别完成了调整后新型热泵的技术经济性分析，确定了额外投资与运行费用随运行时间之间的变化规律，为创新技术的市场化应用提供了参考；（6）基于热泵结霜除霜领域的研究成果，对热泵应用于建筑环境内的热湿耦合调控、低温金属平板表面液滴冷凝与结霜结冰机理等进行了深入探索。本项目研究成果具有重要的学术价值和实际应用价值。