| 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. |
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