【作者】 张良；

【导师】 胡亚才； 樊建人； 俞自涛；

【作者基本信息】 浙江大学 ， 工程热物理， 2013， 博士

【摘要】 光热太阳能技术在解决全球清洁能源生产问题上具有重要的战略意义。然而,成本和经济性问题是当前光热太阳能技术发展面临的主要瓶颈,因此,提高系统效率降低成本是光热太阳能技术发展的必然趋势。自然循环集热系统以利用气-液重力压差为动力实现循环换热为最大特点,为槽式太阳能集热系统的安全、稳定、高效运行提供了新的选择。此外,纳米流体作为一种新型的强化传热工质,其强化传热机理及其在工业换热系统中的应用研究一直是强化传热领域的热点。本论文课题研究以“节能环保”和“新能源”两大战略新兴领域为导向,以自然循环槽式太阳能集热系统为对象,以非能动自然循环集热(换热)系统中流动沸腾传热特性以及过程中的强化传热机理为主要研究内容。论文首先对自然循环槽式太阳能中高温集热系统的结构特点进行了优化分析,并对本课题提出和首次建立的50kW自然循环槽式太阳能中高温集热系统的关键性能参数(集热效率、散热损失、热阻特性以及运行最低辐照条件)进行了实验分析。得到50kW自然循环集热槽式太阳能集热系统的集热效率为0.3852,通过工业保温后可达0.439～0.458；集热管热效率为0.662～0.792；热阻于加热功率之间的关系为RHP=155.116q-1.523；系统运行最低辐照条件为Ic=254-272W/m2。在此基础上,建立了基于等效热流密度关系的室内自然循环集热系统实验台,得到对应的加热功率范围为0.45kW～1.35kW。为了更好的理解和掌握自然循环槽式太阳能集热系统流动传热特性,本文利用室内实验系统对自然循环槽式太阳能集热系统典型流动传热特征和流动不稳定特征以及加热功率、充液率和加热方式三种因素对集热系统的流动传热特性的影响规律进行了完整的实验研究。研究结果表明：自然循环集热系统在流动特征上存在逆流和双向并行流两种不稳定性流动特征；随着加热功率的增加,逆流特征逐渐减弱,管内流动循环最终转变为顺时单向流动。在同一流型特征下,集热管内的对热系数随加热功率增加而增加。然而,当逆流出现转变时(1.0kW,0.15MPa排汽工况),集热管内的对流换热系数出现最高值,为285.86W/m2K。现场系统与室内系统在热管热阻与加热功率之间的关系可以很好的吻合,得到系统逆流流型发生转变的热管热阻参数为0.83kW,34.37K/kW。在0.8kW加热功率条件下,集热系统的最佳充液率为FR=0.5；集热系统的最低充液率为FR=0.2。半管加热方式增强了管内逆流特征导致集热系统效率较低；但从系统安全运行和高效吸热角度来讲,下半管加热方式最有利于自然循环集热系统的高效、稳定安全运行。为了研究氧化石墨烯(GONs)纳米流体沸腾强化传热特性及其强化传热调控机理,探索其在然循环循环集热系统中的应用。本文利用瞬态淬火沸腾实验对不同浓度的氧化石墨烯(GONs)纳米流体的沸腾强化传热特性及其强化传热调控机理展开了系统的实验研究。研究结果表明,氧化石墨烯(GONs)纳米流体瞬态淬火沸腾强化传热特性主要体现在过渡沸腾和临界热流密度(CHF)上,其受到纳米颗粒沉积表面和悬浮液中纳米颗粒两者因素的共同影响。更重要的是,氧化石墨烯(GONs)纳米颗粒沉积形成的沸腾表面对浸润性的改变是强化CHF的决定因素。值得指出的是,本文在浓度为0.0002wt.%氧化石墨烯(GONs)纳米流体淬火结果中得到一种特别的“花状”由鳞片状石墨烯纳米片叠成的“突起”结构沉积表面；在该浓度条件下得到本文在CHF上的最高强化效果,为25.0%。此外,氧化石墨烯(GONs)纳米流体对循环启动前热管的加热速率提升了近15.15%；但随着循环流动的开始,受管内流型特征的影响,集热系统的逆流效果得到减弱,热效率提高了18.0%,但热管热阻增加了6.7%,对流换热系数也降低7.85%。更多还原

【Abstract】 Solar thermal technology plays an important and strategic role in addressing the issue of global clean energy production. However, the bottlenecks in cost and eco-nomics have hindered the development of solar thermal technology. Therefore, ther-mal efficiency improvement and system cost reduction become the inevitable devel-oping trend of solar thermal technology. Natural circulation solar steam generation systems, driven passively by gas-liquid gravitational difference, provide a new choice for parabolic trough solar steam generation featuring safe, stable, and highly-efficient operation.Moreover, as a new kind of working fluid for heat transfer enhancement, study of the mechanisms of nanofluids on enhanced heat transfer and their applications in in-dustrial heat exchangers have been a hot spot for decades.The main objective of this dissertation, which is oriented by two strategic emerg-ing fields of "energy conservation and environment protection" and "new energy", is focused on the characteristics of flow boiling heat transfer as well as the underlying mechanisms of enhanced heat transfer in parabolic trough collector (PTC)-based nat-ural circulation steam generation systems.Firstly, an analytical and optimization work on the structure of the the PTC based medium-high temperature natural circulation steam generation system was carried out. Moreover, an experimental analysis was done on heat transfer characteristics, such as thermal efficiency, heat losses, thermal resistance and the minimum solar radiation for operation, etc., for the firstly proposed and constructed50kW medium-high temper-ature natural circulation steam generation system. The result show that the thermal efficiency was0.3852which could reached to0.439～0.458after industrial thermal improvement, thermal efficiency of the receiver was0.662～0.792, the relationship between thermal resistance and heat power was RHP=155.116q-1523and the mini- mum solar radiation for operation of the system was Ic=254～272W/m2.Based on this field setup, an indoor experimental system was design and con-structed based on equivalent heat flux model. The heat load of the indoor test rig was ranged from0.45kW to1.35kW.In order to better understand the thermal performance of natural circulation PTC systems, both flow and heat transfer instabilities and the effects of heat load, filling ratio and heating condition on thermal performance of the system was studied exper-imentally in this dissertation.The results showed that two types of flow instabilities, i.e., backflow and bi-directional flow, were observed in the system. The effects of backflow were de-creased as the heat load was raised and the flow pattern was finally transformed to be a clockwise unidirectional flow at the end. The two phase heat transfer coefficient in the receiver was increased with heat load for the same flow pattern conditions. As the transition of flow patterns happened, however, a highest heat transfer coefficient,285.86W/m2K, was obtained for the0.15MPa steam discharging process at the heat load of1.0kW. Moreover, the relationship between the heat pipe thermal resistance and heat load has a good agreement with the equivalent heat flux. The critical heat pipe thermal resistance for the flow pattern transition was found to be0.83kW,34.37K/kW. Additionally, the best filling ratio for the heat load of0.8kW was found to be0.5. The limited filling ratio for the system operation was found to be0.2. Moreover, an enhanced backflow effect was observed by the half circular heating method, which finally resulted in a lower thermal efficiency. The lower half heating method, however, was identified to be the optimized heating way in view of safe operation and efficient heat transfer.In studying the mechanisms of enhanced boiling heat transfer of graphene oxide nanosheets (GONs) nanofluid and their thermal performance in natural circulation system generation system, transient boiling experiments for nanofluids with different concentrations were carried out by quenching method. The results showed that the enhanced boiling heat transfer was mainly happened in the transient boiling and criti-cal heat flux (CHF) region, which are, however, both affected by the dynamic deposi-tion process and the suspended GONs nanopartilces. Moreover, it was concluded that the enhancement of CHF is primarily resulted from surface deposition of GONs. It is important to point out that the formation of unique discrete flower-like circular bumps consisting of self-assembly fish-scale-shaped GONs was only seen for the nanofluid sample of0.0002wt.%, which was the case of the greatest CHF enhancement of25.0%.As the nanofluid was present in the natural circulation steam generation system, the heating rate was increased by nearly15.15%before the starting of the circulation. As the circualiton was started, however, backflow effects were found to be weakened in the presence of nanolfuids. Accordingly, thermal efficiency was increased by18.0%while the heat pipe thermal resistance was increased by6.7%. What’s worse, the heat transfer coefficient in the receiver was decreased by7.85%in the presence of nanofluids as well.更多还原