柱形生物质和石英砂传热实验与模拟研究

Experiment and Improved Simulation on Heat Transfer for Cylindrical Biomass and Quartz Sand

从实验和模拟两方面,利用虚拟球元法改进传统的计算流体力学耦合离散单元法(DEM-CFD),对流化床内的柱形生物质和石英砂的流动传热过程进行了研究。建立了混合流动与传热的实验系统,并进行了模拟和分析,结果表明:双组分颗粒混合指数(MI)经过迅速增加、缓慢增加以及稳定期3个阶段。随着表观气速的增加,颗粒混合指数升高,一定范围内的高气速有利于柱形生物质和石英砂的混合。加热后的柱形生物质和石英砂的初始温度为120℃,流化空气为常温25℃。流化过程中,柱形生物质颗粒温度普遍大于石英砂的温度。当表观气速比较小时,双组分颗粒的混合受到限制,流化床内不同高度的颗粒平均温度差较大;表观气速较大时,流化床颗粒平均温度差不明显。虚拟球元解决了传统DEM-CFD计算模型不适用于大尺寸柱形颗粒系统的问题。对比红外热图像和模拟结果,实验值和模拟结果比较接近,改进的DEM-CFD方法可以较好地模拟柱形生物质和石英砂的流动传热。

From the two aspects of experiment and simulation, virtual spherical method was used to improve the numerical simulation of the traditional computational fluid dynamics coupling discrete element method （ CFD - DEM）. The cylindrical particle was regarded as aggregation of small virtual ball, and particle volume factor concept was also introduced to amend the gas-solid force. The gas force acting on the cylindrical particle was calculated through the calculation between gas phase and virtual sphere. The experimental system of flow and heat transfer was established, and the simulation and comparative analysis were also carried out. The mixing index （MI） of the special binary particles was followed by three stages： rapid increase, slow increase and stable phase. With the increase of superficial gas velocity, the mixing index of particles was increased, and the higher gas velocity was favorable for the mixing of the cylindrical biomass and quartz sand. In the experiment of heat transfer, the temperature of cylindrical biomass particles was generally higher than that of quartz sand. When the superficial gas velocity was small, the fluidization of particles was restricted, and the average temperature difference at different heights of fluidized bed was larger. At large gas velocity, the average temperature difference was not obvious. The fictitious element method was employed to solve the incompatibility of the traditional DEM - CFD model. Compared with the infrared thermal images and the simulation results, they were very close and the improved DEM - CFD method can simulate the flow and heat transfer of cylindrical biomass and quartz sand.