Abstract:

Mesoscale structures such as clusters are common in gas-solid two-phase flows. These mesoscale heterogeneous structures directly affect the gas-solid flow characteristics and gas-solid contact efficiency, and then affect the gas-solid interphase heat transfer, mass transfer and chemical reaction process. In the coarse grid method which is more suitable for industrial large-scale gas-solid heat transfer simulation, there is a lack of heterogeneous heat transfer model with high accuracy, simple and easy to use, and can consider the influence of mesoscale non-uniform structure. Computational fluid dynamics-discrete element method (CFD-DEM) is used to study the interphase heat transfer of gas-solid two-phase flow. In order to ensure the continuous heat transfer between gas and solid, two methods to maintain the temperature difference between gas and solid during the heat transfer procedure are adopted, and the advantages and disadvantages of the two methods are discussed. Method 1: add a heat source term to the gas phase energy equation; Method 2: reset the gas phase temperature at intervals and free heat transfer between the gas-solid two phases after resetting the temperature. The solid-phase temperature remains unchanged in both methods. The results show that the local gas-solid heat transfer per unit volume at the cluster interface is the largest. The ratio of local gas-solid heat transfer per unit volume to the total gas-solid heat transfer per unit volume at the dilute phase and the interface of the clusters in the testing temperature method is greater than that of the heat source term method, while the ratio of local gas-solid heat transfer per unit volume to the total gas-solid heat transfer per unit volume at the dense phase is less than that of the heat source term method. By filtering the CFD-DEM calculation data, a two parameter (filtered solid volume fraction and filter size) heat transfer coefficient correction factor model is constructed for the reset temperature method. The performance of the model is evaluated through a priori analysis. The results show that the proposed model is better than the existing two parameter model in the literature when the filter size in the range of 5 to 40 times the particle diameter.

Key words: mesoscale, heat transfer, gas-solid two-phase flow, computational fluid dynamics-discrete element method, filter technique, reset temperature

摘要：

颗粒聚团等介尺度结构在气固两相流中普遍存在，这些介尺度非均匀结构直接影响气固流动特性及气固接触效率，进而影响气固相间传热、传质及化学反应过程。在更适合工业大尺度气固传热模拟的粗网格方法中缺乏准确度高、简单易用且可以考虑介尺度非均匀结构影响的传热模型。采用计算流体力学-离散单元法（CFD-DEM）研究了气固两相流相间传热，为了保证气固相间持续传热，采用了两种维持气固相间传热温差的方法，并讨论了两种方法的优缺点。方法一：给气相能量方程添加热源项；方法二：每间隔一段时间重置气相温度，重置温度后气固两相自由传热，两种方法中均保持固相温度不变。结果表明聚团界面位置的局部单位体积气固传热量最大，重置温度方法在稀相和界面位置的局部单位体积传热量与总单位体积传热量之比大于热源项方法，而在浓相位置的局部单位体积传热量与总单位体积传热量之比小于热源项方法。通过过滤CFD-DEM计算数据，为重置温度方法构建了双参数（过滤固含率、过滤尺度）传热系数修正因子模型，通过先验分析评价了模型的表现，研究表明所构建模型在过滤网格尺度为5~40倍颗粒直径范围内优于文献中已有的双参数模型。

关键词: 介尺度, 传热, 气固两相流, 计算流体力学-离散单元法, 过滤技术, 重置温度