面向塔式太阳能热发电系统,研究空沙移动床换热器内稠密气固两相流动及换热的机理和特性,将格子Boltzmann方法（LBM）与离散单元法（DEM）结合,从介观尺度上建立LBM-DEM流动及传热模型。以不同空沙质量流量比为变量,对空沙移动床换热进行数值模拟。分析其中气相和颗粒相温度场变化,比较不同类型的传热热流量在换热过程中所占比例及变化。结果表明,床内紧密的颗粒对气相温度场影响较大,阻碍气体沿流动方向传热,在加热一段时间后,床内气体温度分布逐渐稳定,温度梯度呈对角线分布,符合空气-沙子交叉换热的典型特点。颗粒在床内不同位置的温度变化差别较大,初始阶段仅左侧颗粒被加热,在温度场稳定后,颗粒温度沿气流入射方向的梯度较大。空沙移动床内主要换热方式为对流换热,在500 ℃的加热温度下,对流传热热流量占比约98.50%;辐射传热热流量、导热传热热流量与空气入射速度正相关;空沙流量比与气相效能负相关,与固相效能正相关;空沙流量比与火用效率正相关。
Abstract
This study systematically examines the dense gas-solid two-phase flow behavior and conjugate heat transfer characteristics in an air-sand moving bed heat exchanger for tower-type solar thermal power generation systems. A mesoscopic-scale coupled lattice Boltzmann method-discrete element method （LBM-DEM） framework is developed to characterize particle-fluid hydrodynamic interactions and associated thermal transport phenomena under high solid-phase volume fraction conditions. Numerical simulations of heat transfer in the air-sand moving bed were conducted with different mass flow ratios of the air and sand. The paper studies the changes in the temperature fields of both the gas and solid phases and compares the proportions and the trends of different heat transfer modes throughout the process. The results indicate that the dense sand which hinders heat transfer along the airflow direction significantly affects the gas phase temperature field. After a period of heating, the gas temperature distribution in the bed stabilizes with a diagonal temperature gradient. It conforms to the characteristics of staggered-flow heat transfer in air-sand moving beds. The simulation reveals a significant variation in temperature changes of sands at different positions in the bed. At the initial stage, only the sand on the left side of the bed is heated. After the temperature field stabilized, the temperature gradient of sand along the direction of airflow incidence is substantial. The main mode of heat transfer in the air-sand moving bed is convective tranfer rate. At a heating air temperature of 500 ℃, the convective tranfer rate accounts for 98.50% of the total heat transfer. The radiation tranfer rate and conduction tranfer rate are positively correlated with the incident air velocity. The air-sand flow ratio is negatively correlated with the gas phase efficiency, but positively correlated with the solid phase efficiency. In addition, the air-sand flow ratio is positively correlated with the exergy efficiency.
关键词
太阳能热发电 /
两相流 /
传热特性 /
数值模拟 /
移动床换热器 /
LBM-DEM方法
Key words
solar thermal power generation /
two phase flow /
heat transfer performance /
numerical simulation /
moving bed heat exchanger /
LBM-DEM method
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