【作者】 刘辉；

【导师】 钱晨；

【作者基本信息】 兰州理工大学 ， 流体机械及工程， 2025， 硕士

【摘要】 在光热发电系统的运行体系中,吸热塔通过定日镜聚光实现对二元储热熔盐（60%NaNO₃-40%NaNO₂）的加热,加热后温度处于400℃-700℃的熔盐,由高温熔盐泵输送至蒸汽发生器用于发电,其中,高温熔盐泵系统主要在0.15Q_d-Q_d工况下运行。根据现有研究,熔盐在450℃-600℃时会发生热分解反应,分解气体主要以氮气和氧气为主。热分解气体游离在熔盐泵流道中,形成复杂的不同尺度气泡/气团结构会改变流向波长,使近壁低速区发卡涡的剪切应力和流向排列模式发生改变,进而对熔盐泵的内外特性产生多维度影响。基于上述现象,本研究以敦煌100MW太阳能光热发机组所用16ENH-2型高温熔盐泵为研究对象,结合Fact Sage热力学计算与热分解实验,分析了二元储热熔盐的热分解机理;同时,基于CFD-PBM耦合模型研究了热分解气体对熔盐泵内气液两相流场的影响规律;此外,通过动态模态分解方法对非定常条件下熔盐泵内的速度场进行了模态分析,获得了速度场的动态变化特征及内在规律,主要研究结论如下:（1）根据旋转效应下的二元储热熔盐热分解试验,发现试验温度升高至455℃时,熔盐表面有明显的气泡析出,气相产物中N₂成分较少。伴随温度上升,热分解气体生成速率呈现出成倍增长。当温度升高至565℃时,O₂的生成速率为1.84mol/m³·s,N₂的生成速率为0.51mol/m³·s。熔盐泵转速对熔盐热分解有一定的激励作用,在较低的工作温度下,这种激励作用更显著。基于热力学计算得到的吉布斯最小自由能与反应平衡常数确定了熔盐的热分解主要以NaNO₃分解成NaNO₂和O₂为主;此外,随着温度的增高,NaNO₂进一步分解会分解产生N₂。与实验结果一致,验证了热力学方法的准确性。（2）基于CFD-PBM耦合模型计算了热分解条件下高温熔盐泵的内外特性,对比单相流计算结果,熔盐的含气率对外特性变化规律有显著影响,当氧气含量增加59.4%、氮气含量增加17倍时,1.0Q_d下的效率降低了7%,而0.15Q_d流量下,效率下降了11%。小流量工况下的液相流量对气泡阻力较小,而滞留在流道中的气泡会导致能量耗散增加,降低熔盐泵的水力性能。根据不同热分解气体生成速率下的数值计算,发现气体生成速率、叶轮流道含气量和湍动能分布受液相流量的影响显著,在0.4Q_d和0.2Q_d流量工况下,流道内出现大尺度漩涡,导致流道内流体质点的压力分布不均,流动分离现象越发明显。随着转速的增加,泵内流体的湍流强度和离心力逐渐增大,流场内气泡的聚并现象加剧,进而导致流道堵塞、流场涡带区明显增加,熔盐泵的水力性能下降。（3）动态模态分解方法可以捕捉到熔盐泵内气液两相流动的核心流动特征,基于前四阶模态叠加得到的液相与气相速度重构流场相对误差分别为4.11%和5.83%。不同热分解气体生成速率下,液相速度场与气相速度场都是稳定或微衰减的,且1阶模态对应频率为0的基本模态,在流场中始终占主导地位;热分解气体生成速率的增大会导致大尺度漩涡的形成,由于气液相互作用的增强还会引起湍流脉动增强,故液相速度场的3阶模态频率降低,4阶模态频率升高;而气相速度场对热分解气体生成速率的变化不敏感,流动频率主要由流道形状和叶轮的周期性旋转决定。更多还原

【Abstract】 In concentrated solar power systems,the receiver tower heats solar salt（60%NaNO₃-40%NaNO₂）through heliostat-focused sunlight.High-temperature molten salt pumps then transport the solar salt at 400℃-700℃to steam generators for power generation,primarily operating at 0.15Q_d-Q_d conditions.However,thermal decomposition occurs within 450℃-600℃,producing nitrogen and oxygen gases.These free gas bubbles/clusters in pump flow channels alter the flow wavelength,modifying the shear stress and streamwise arrangement of hairpin vortices in near-wall low speed regions,ultimately affecting the pump’s internal and external performance characteristics.To investigate this phenomenon,this study examines the16ENH-2 high-temperature molten salt pump from the Dunhuang 100MW concentrated solar power plant.Combining Fact Sage thermodynamic calculations with thermal decomposition experiments,we reveal the decomposition mechanism of solar salt.Using a CFD-PBM coupled model,we analyze the effects of flow conditions,rotational speed,and gas generation rates on single-phase and gas-liquid two-phase flow characteristics under thermal decomposition.Furthermore,dynamic mode decomposition is applied to analyze velocity field modes under unsteady conditions.The main research conclusions are as follows:（1）According to the thermal decomposition test of binary molten salt energy storage medium under rotational effects,it was found that when the test temperature rose to 455℃,obvious bubbles precipitated on the surface of the molten salt,and the N₂component in the gas-phase products was less.With the increase of temperature,the generation rate of thermal decomposition gas showed exponential growth.When the temperature rose to 565℃,the generation rate of O₂was 1.84 mol/m³·s,and the generation rate of N₂was 0.51 mol/m³·s.The rotational speed of the molten salt pump has an exciting effect on the thermal decomposition of the molten salt,which is more significant at lower working temperatures.Based on the Gibbs minimum free energy and reaction equilibrium constant obtained from thermodynamic calculations,it was determined that the thermal decomposition of the molten salt mainly involves the decomposition of NaNO₃into NaNO₂and O₂.Additionally,with the increase of temperature,the further decomposition of NaNO₂produces N₂,which is consistent with the experimental results and verifies the accuracy of the thermodynamic method.（2）Based on the CFD-PBM coupling model,the internal and external characteristics of high-temperature molten salt pumps under thermal decomposition conditions were calculated.Compared with single-phase flow results,the gas holdup of molten salt significantly influences the variation law of external characteristics.When the oxygen content increases by 59.4%and the nitrogen content increases by 17 times,the efficiency at 1.0Q_d decreases by 7%,while at0.15Q_d flow rate,the efficiency drops by 11%.In small flow conditions,the liquid phase flow has low resistance to bubbles,but bubbles stagnant in the flow passage increase energy dissipation and degrade the hydraulic performance of the molten salt pump.Numerical calculations under different thermal decomposition gas generation rates show that the gas generation rate,gas holdup in the impeller passage,and turbulent kinetic energy distribution are significantly affected by the liquid phase flow rate.At 0.4Q_d and 0.2Q_d flow conditions,large scale vortices appear in the flow passage,causing uneven pressure distribution of fluid particles and more obvious flow separation.As the rotational speed increases,the turbulence intensity and centrifugal force of the fluid in the pump gradually increase,intensifying bubble coalescence in the flow field,which in turn leads to flow passage blockage,significant expansion of the vortex zone,and degradation of the molten salt pump’s hydraulic performance.（3）The dynamic mode decomposition method can capture the core flow characteristics of gas-liquid two-phase flow in the molten salt pump.The relative errors of the reconstructed flow fields of the liquid phase and gas phase velocities based on the superposition of the first four modes are 4.11%and 5.83%,respectively.Under different thermal decomposition gas generation rates,both the liquid phase velocity field and the gas phase velocity field are stable or slightly decaying,and the first order mode corresponding to a frequency of 0（the basic mode）always dominates the flow field.An increase in the thermal decomposition gas generation rate will lead to the formation of large scale vortices,and the enhancement of gas-liquid interaction will also cause an increase in turbulent fluctuations,resulting in a decrease in the frequency of the third order mode and an increase in the frequency of the fourth order mode of the liquid phase velocity field.However,the gas phase velocity field is not sensitive to the change in the thermal decomposition gas generation rate,and its flow frequency is mainly determined by the flow channel shape and the periodic rotation of the impeller.更多还原