【作者】 叶为标；

【导师】 朱冬生；

【作者基本信息】 华南理工大学 ， 化学工程， 2012， 博士

【摘要】 在工业生产和日常生活中，用于热水供应的低温热能需求量巨大。热泵技术以节能和环保的优势在制取生活热水中的应用逐渐受到关注。然而，可作为热泵热源的空气能、太阳能、地热能和工业余热等低温热能由于能量供求在时间和强度上不匹配的矛盾难以直接满足实际需求。从提高能源利用效率和环境保护的角度，基于相变储能的热泵技术能够缓解这一矛盾。相变储能有储能密度大、使用过程中温度波动小、容易控制等优点，特别是能够缩小能量产生和消耗之间的差距，被认为是热能工程应用领域重要的能源技术。以相变储能式热泵热水器的储能换热器为研究对象，基于传热学、流体力学、热力学和相变储能理论对储能换热器的流体流动和传热特性进行了深入系统的研究，主要包括了以下几个方面的研究工作：(1)首先介绍了固-液相变的微观理论，详细论述了影响相变传热过程的主要因素——成核和晶体生长。讨论了模拟相变问题的热力学第一定理模型法，建立了对固-液相变适用的数学模型和数值方法。然后讨论了热力学第二定理评估相变储能换热器的性能，用火用分析来评价储能换热器储/放能过程中能量利用的品质。最后通过实验数据严格验证了数值模拟固-液相变和自然对流问题的可靠性，数值计算结果通过与公开出版的文献实验数据对比得到检验。(2)数值研究了在潜热能单元内不同相变材料(Phase Change Material, PCM)腔体积分数对流体流动和传热特性的影响，计算流体动力学(Computational Fluid Dynamics,CFD)代码用于数值求解基于瞬态的非线性耦合传热。研究了PCM腔体积分数为35%~95%时的体积膨胀率、完成热储存的时间、热流、液相分数以及速度和温度场变化情况。结果表明，对PCM腔体积分数为85%时，接近加热板壁面处产生漩涡(作为换热强化因子)。当PCM腔体积分数增加时，体积膨胀率减小，然而完成储能时间增加。此外，建立了体积膨胀率和完成储能时间分别与PCM腔体积分数之间的函数关系式。界面换热率的详细研究对固-液相变传热机理提供了较深入的理解。(3)研究了特征长度为2mm快速储/放能的板-翅单元内的传热和流动现象。使用基于有限体积法(Finite Volume Method, FVM)数值模拟相变传热的非稳态过程。分析了热能储存/释放体系内温差对流动和传热的影响。研究表明，当温差小于20℃时，温差对热储存时的性能影响较大。在放能过程中有部分未凝固的PCM存在，另外还有一个旋涡在空气区域形成。对不同变量和研究的参数拟合出相应的数学关联式，这些关联式可用于今后的组件设计和系统优化。(4)应用热力学第一定理以及第二定理的火用分析对储能单元在不同结构尺寸下的储/放热性能进行了研究。在储能过程中，较大的PCM及翅片宽度和小的储能温差有利于自然对流的产生而增强换热，较高翅片的换热效果更佳。高翅片、大的PCM及翅片宽度和低翅片、小的PCM及翅片宽度收集热量的效果一致。在放能过程中，不同结构尺寸对固相分数的影响很大，然而相同尺寸下，放能温差对固相分数的影响不大，小温差有利于热传导的进行。获得的基准数据可用于该储能单元结构优化和系统设计。火用分析表明换热流体(Heat Transfer Fluid, HTF)的速度越大，火用效率相对较小。(5)以PCM和高效换热器为基础，对板-翅式换热器用作热泵相变储热器进行了实验研究。探讨了板-翅式热泵相变储热器中板式换热器与储能材料界面的温度变化、不同HTF流量下出口水温及出水量。结果显示：处于同一横截面不同板侧上的热电偶所测温度差别较小，温度测量曲线存在较平缓的一段。在HTF的温降相同下，HTF流量越大，放热持续时间越短。保持出口水温为40℃时，HTF流量应采用0.3m3/h且对能量的利用品质高（即火用效率高）。增大HTF流量，储能换热器的换热率也增加且系统的COP值都达到5.0以上，具有很好的节能以及因相变储能的储能密度大表现出节地的效果。研究结果为板-翅式换热器作为热泵相变储热器奠定实验基础，对板-翅式换热器广泛用于热泵相变储能装置具有一定的参考价值和实际意义。更多还原

【Abstract】 The huge quantity of thermal energy is demanded for hot water in industrial productionand daily life. Therefore, heat pump technology is growing concern in the preparation of hotwater for energy saving and environmental advantages. However, the heat source of heatpump, such as, air energy, solar energy, geothermal energy, industrial waste energy and otherlow-temperature energy, is difficult to meet the actual demand because the supply of thermalenergy can not fulfill such a demand in terms of the mismatch both in time and intersity. Inview of the efficiency of the energy utilization and environment protection, a potential way isto use the heat pump with phase change thermal energy storage, which is considered to beimportant energy technology of thermal engineering applications, because it provides a highenergy storage density, small temperature fluctuations and is easy to control, especially itbridges the gap between energy generation and consumption.Energy storage heat exchanger for heat pump water heater with phase change thermalenergy storage is studied. Based on theory of heat transfer, fluid dynamics, thermodynamicsand theory of phase change energy storage, some researches are carried out in the following:(1) Firstly, the microscopic theory of solid-liquid phase change is introduced, the main factorsthat influence the phase change heat transfer process, namely, nucleation and crystal growth,is discoursed in detail. Further, the first law model of thermodynamics that simulation thephase change problems is discussed, the mathematical models and numerical methods forsolid-liquid phase change are established. Secondly, the second law of thermodynamic usedfor assessment the performance of heat exchanger with phase change energy storage isdiscussed, the quality of energy utilization of energy storage heat exchanger during energystorage and release is evaluated by exergy analysis. At last, the validation of numericalsimulation solid-liquid phase change and natural convection is rigouous verification, andexperimental data taken from the literatures are conducted to validate the model. Thenumerical results show a good agreement with the experimental ones.(2) In present work, the effects of different cavity volume fractions of phase change material(PCM) on fluid flow and heat transfer behavior in a latent thermal unit are studied numerically.The commercial Computational Fluid Dynamics (CFD) code is used for the numericalsolution based on transient nonlincer conjugate heat transfer. The volume expansion ratio, thetime of complete thermal storage, heat flux, liquid fraction, velocity and temperature fieldsare investigated for the range of PCM cavity volume fractions from35%to95%. It is notedthat a vortex (as a heat transfer enhancers) is present near the heating plate wall for the PCM cavity volume equal to85%. It is found that the volume expansion ratio decreases as PCMcavity volume fractions increasing, whereas the time for complete energy storage increases.Further, the correlations of the volume expansion ratio and the time of complete thermalstorage are developed as a function of PCM cavity volume fractions. The detailed knowledgeregarding interface heat transfer rate provides a deeper understanding the heat transfermechanisms.(3) The fluid flow and heat transfer in a plate-fin unit with a characteristic length of2mmused for rapid heat storage/release by paraffin (PCM) are investigated numerically. Transientsimulations are performed based on the finite volume method. The effect of temperaturedifferences on the fluid flow and heat transfer in the energy storage/release system is analyzed.It is found that temperature differences play a key role in the performances of energy storagewhen temperature differences are less than20℃. It is noted that part of not solidified PCMcan be observed clearly during energy release, and a vortex in the air region is formedremarkably at the moment of complete thermal energy release. The correlations are developedas a function of the associated variables. The obtained correlations are useful for futurecomponent design and system optimization.(4) The performance of energy storage/release of energy storage unit with various dimensionsis investigated, based on the application of the first law of thermodynamics and the secondlaw of exergy analysis. During energy storage process, small temperature difference and largewidth on PCM and fin is benefiting the natural convection formed; therefore, the heat transferis enhanced. Moreover, the better heat transfer performance is obtained for higher fin. Heatcollection is identical between high fin, large width on PCM and fin, and short fin, smallwidth on PCM and fin. During energy release process, the influence of dimensions on solidfraction is remarkable. However, the effect of temperature difference of energy release onsolid fraction is small, small temperature difference is beneficial for heat conduction.Furthermore, the obtained baseline dates are useful for energy storage unit structuraloptimization and system design. The exergy analysis shows that the larger of heat transferfluid (HTF) velocity, the smaller of exergy efficiency.(5) The plate-fin heat exchanger used for heat pumps phase change thermal storage is studiedby experiment, based on PCM and efficient heat exchanger. The interface temperaturechanges between plate heat exchanger and energy storage material, outlet temperature andflow in different HTF flow rates on heat pump phase change thermal storage device ofplate-fin are explored. The experimental results show that the temperature difference is smallin the same side of the plate at different cross-section by thermocouple measured, and there is a gentler section of the temperature measurement curve. The greater HTF flow rates, theshorter of exothermic under the same temperature drop of HTF. The flow rate of HTF shoulduse0.3m3/h when outlet temperature is40℃. Moreover, the high quality of using energy,namely, high exergy efficiency, is obtained for the flow rate0.3m3/h. The heat transfer rate ofenergy storage heat exchanger is increases as HTF flow rate increases. Further, the coefficientof performance (COP) of system reached5.0or above. Therefore, it presents good energyconservation and land saving due to high energy storage density. The results providedexperimental basis for the plate-fin heat exchanger used for heat pump phase change thermalstorage. The results have practical significance and reference value on plate-fin heatexchanger, which is widely used in heat pump phase change thermal storage device.更多还原