| Thermal energy storage technology plays a key role in solving the discontinuity and instability problems in the utilization of renewable energy during the transformation of energy industry structure in China and the imbalance between the energy supply and demand.Thermal storage and heat exchanger with high storage density,low cost and high reliability are challenges for the development and application of thermal energy storage technology.The thesis takes a high-temperature solid sensible heat thermal energy storage steam generation system as the research object and conducts research on the flow boiling instability in the charging and discharging process of the system.The work focuses on two crucial issues of the special thermal boundary characteristics caused by thermal storage:(i)the occurrence and evolution of dynamic instabilities in gas-liquid stratified flow boiling;(ii)the coupling effects of thermal storage boundary and mass flux on dynamic instabilities.The research is dedicated to clarify the mechanism of flow boiling instability in the tube under the high-temperature sensible thermal storage condition,and provide guidance for the design,development and application of novel high-temperature sensible heat thermal energy storage heat exchangers.A visualized experimental test rig of flow boiling in high temperature thermal storage module using graphite as thermal storage material was first presented.A series of four thermal storage temperatures(200～340℃),nine inlet subcooling degrees(5～45℃)and mass flow rates(0～593.5 kg/m~2s)were applied to investigate the oscillation characteristics at the onset of flow boiling dynamic instability.Combined with the outlet flow pattern development,the work studied the influence mechanism of thermal storage boundary on the onset of dynamic instabilities.The results show that compared with the condition of constant heat flux,the high-temperature thermal storage boundary enhances subcooled boiling and the regulation of heat load distribution,which in turn lead to the early occurrence of dynamic instability.At the same time,different heat load and local subcooled nucleating boiling characteristics caused by different thermal storage temperatures contribute to different characteristics of the development of gas-liquid two-phase flow pattern in the tube under the cases of different subcooling and mass flux conditions.Moreover,both pressure drop oscillations(PDO)and density wave oscillations(DWO)were observed at the onset of the dynamic instability region.It is found that when dynamic instability occurs in the form of PDO,the outlet flow pattern is characterized by periodic evolution of bubbly-slug-stratified/wavy-annular flow.When dynamic instability occurs in the form of DWO,the flow pattern is characterized by periodic evolution of bubbly-slug-stratified flow.On that basis,the thesis conducts research on the coupling effects of thermal storage temperature and mass flux on the dynamic instability during gas-liquid stratified flow boiling by controlling thermal storage temperature and mass flux.The result shows that the increase of thermal storage temperature and mass flux has opposite effects on the void fraction.Under the same thermal storage temperature,the decrease of mass flux promotes the evolution of dynamic instability and flow pattern towards oscillation characteristics with higher void fraction.The specific evolution path is related to initial oscillation type.As a result,in the actual process of charging and discharging,different conversion characteristics of dynamic instability will occur due to the difference in mass flux.By changing the mass flux and adjusting the thermal storage and release ratio,the conversion of instability characteristics can be effectively regulated.Meanwhile,the regulation of local heat flux during discharging process caused by thermal storage boundary also has an important impact on the flow pattern development and transition of oscillation type by changing the local heat transfer characteristics. |
PDF Full Text Request