耦合EMMS曳力与简化双流体模型的气固流动模拟

提出了一种耦合EMMS曳力的简化双流体模型,该模型忽略固相黏度,用简单的经验关联式来计算固相压力,并且耦合考虑了介尺度结构的EMMS曳力模型来计算气固相间作用力。采用简化双流体模型成功模拟一个三维实验室尺度鼓泡流化床,数值模拟结果与完整双流体模型以及实验测量结果进行了比较,结果表明耦合EMMS曳力的简化双流体模型模拟结果与完整双流体模型耦合EMMS曳力的模拟结果基本相当,并且都与实验结果吻合良好,然而简化双流体模型的计算速度是完整双流体模型的两倍以上。这表明曳力模型在气固模拟中起着主导作用,而固相应力的作用是其次的,耦合EMMS曳力的简化双流体模型在实现工业规模气固反应器快速模拟中具有巨大潜力。     

耦合EMMS曳力与简化双流体模型的气固流动模拟

提出了一种耦合EMMS曳力的简化双流体模型,该模型忽略固相黏度,用简单的经验关联式来计算固相压力,并且耦合考虑了介尺度结构的EMMS曳力模型来计算气固相间作用力。采用简化双流体模型成功模拟一个三维实验室尺度鼓泡流化床,数值模拟结果与完整双流体模型以及实验测量结果进行了比较,结果表明耦合EMMS曳力的简化双流体模型模拟结果与完整双流体模型耦合EMMS曳力的模拟结果基本相当,并且都与实验结果吻合良好,然而简化双流体模型的计算速度是完整双流体模型的两倍以上。这表明曳力模型在气固模拟中起着主导作用,而固相应力的作用是其次的,耦合EMMS曳力的简化双流体模型在实现工业规模气固反应器快速模拟中具有巨大潜力。     

基于气泡EMMS模型的鼓泡床两相流动CFD模拟

采用两步法将气泡EMMS模型拓展到计算网格层次,用于模拟鼓泡床两相流动,改进的EMMS曳力模型不仅与空隙率有关,且与网格内的速度直接关联.求解是在操作条件下利用宏观稳定性约束条件寻优得到乳化相内空隙率关系式,再用计算网格层次上的微观守恒方程结合乳化相空隙率关系式封闭求解其余结构参数及对应的曳力系数,最后借助用户自定义函数将气泡EMMS曳力嵌入双流体模型中,对含Geldart A类颗粒的鼓泡床内气固两相流动行为进行模拟与验证.结果表明,该模型能较好地模拟鼓泡床内的非均匀流动结构,准确捕捉到上稀下浓的轴向颗粒浓度分布特征,轴向颗粒浓度分布结果与实验值较接近,平均相对偏差为6.4%.床层径向上颗粒分布均呈明显的环-核结构.在不同床层高度(0.6,0.8和1.1 m)处的径向颗粒浓度分布与实验值基本吻合,仅在底部(0.4 m)略低,相对偏差小于14%.而Gidaspow曳力模型在4个床层高度与实验数据差距甚远,相对偏差可达91%.     

循环流化床中颗粒聚团特性的模拟

考虑到循环流化床中分散颗粒和颗粒聚团同时存在的多尺度结构,确定了密相和稀相加速度与计算网格局部参数之间的关系,建立了多尺度曳力消耗能量最小的稳定性条件,基于双变量极值理论,构建了考虑颗粒团聚效应的多尺度气固相间曳力模型。结合双流体模型,对循环流化床内气固流动特性以及颗粒聚团特性进行了模拟研究。通过与实验值比较,考虑颗粒聚团影响的计算模型可以更好地贴近实验结果,颗粒聚团直径随颗粒浓度增大呈现先增大后减小的分布趋势,气体和颗粒的加速度在模拟中与重力加速度同处一个数量级,求解过程中不能被忽略。     

循环流化床中颗粒聚团特性的模拟

考虑到循环流化床中分散颗粒和颗粒聚团同时存在的多尺度结构,确定了密相和稀相加速度与计算网格局部参数之间的关系,建立了多尺度曳力消耗能量最小的稳定性条件,基于双变量极值理论,构建了考虑颗粒团聚效应的多尺度气固相间曳力模型。结合双流体模型,对循环流化床内气固流动特性以及颗粒聚团特性进行了模拟研究。通过与实验值比较,考虑颗粒聚团影响的计算模型可以更好地贴近实验结果,颗粒聚团直径随颗粒浓度增大呈现先增大后减小的分布趋势,气体和颗粒的加速度在模拟中与重力加速度同处一个数量级,求解过程中不能被忽略。     

EMMS曳力模型及其颗粒团模型的构建和检验

准确描述颗粒团聚特性是发展完善基于多尺度最小能量原理(EMMS)的曳力模型的重要方向之一。提出描述颗粒团聚特性的数学模型,不仅符合物理判断,而且与实验结果吻合。采用颗粒团模型,改进EMMS曳力模型,与实验及直接数值模拟结果吻合较好。改进的曳力模型与欧拉-欧拉双流体方法耦合,实现了不同工况下A、B类颗粒流化床流动特性的数值模拟。成功预测了颗粒非均匀分布特性、局部滑移速度、局部非均匀度以及噎塞状态。     

EMMS曳力模型及其颗粒团模型的构建和检验

准确描述颗粒团聚特性是发展完善基于多尺度最小能量原理（EMMS）的曳力模型的重要方向之一。提出描述颗粒团聚特性的数学模型,不仅符合物理判断,而且与实验结果吻合。采用颗粒团模型,改进EMMS曳力模型,与实验及直接数值模拟结果吻合较好。改进的曳力模型与欧拉-欧拉双流体方法耦合,实现了不同工况下A、B类颗粒流化床流动特性的数值模拟。成功预测了颗粒非均匀分布特性、局部滑移速度、局部非均匀度以及噎塞状态。     

颗粒相壁面条件对非球形颗粒流动影响的数值模拟

采用双流体模型对设置竖直隔板的气固密相流化床中非球形颗粒的运动进行了模拟,颗粒形状的影响由相间曳力模型考虑,重点考察壁面处颗粒边界条件的影响。同时进行了实验室规模三维流化床的流化实验,以验证模型的有效性。通过压降轴向分布、颗粒浓度径向分布以及物料出口处颗粒质量流率功率谱估计等定量分析,结果表明：对不设置内构件的自由床,壁面反射系数对系统宏观流动特性影响较小,而对壁面处局部颗粒运动影响较大;对壁面面积大幅增加的内构件床,壁面反射系数可显著改变气体和颗粒的运动特征,取值需控制在适当范围内。     

基于不同亚格子尺度过滤模型下提升管内颗粒流动的数值模拟

固相亚格子尺度过滤模型是在高精度的网格下,系统地过滤了基于结合颗粒动理学的双流体模型的模拟结果而得到的曳力和固相应力等本构关系的计算模型。今分别采用固相亚格子尺度过滤模型(Filtered Model I)、壁面修正固相亚格子尺度过滤模型(Filtered Model II)和改进的固相亚格子尺度过滤模型(Filtered Model III)模拟NETL/PSRI挑战问题中的提升管内的颗粒流动特性,得到了时均气体压力梯度和时均轴向颗粒速度等分布。亚格子尺度模型和均匀流动模型(Huilin-Gidaspow model)的研究结果相比,改进的固相亚格子尺度过滤模型(Filtered Model III)与实验值更接近,尤其是对于高颗粒浓度流动。壁面修正可以提高压力梯度,从时均轴向颗粒速度分布曲线,可以看出在提升管内颗粒流动结构呈现更为明显的环核流动结构;同时,研究了提升管内气体压缩性、壁面修正和计算网格对模拟结果的影响,分析表明气体的可压缩性对提升管内轴向气体压力梯度有影响,在模拟计算时考虑气体的压缩性,可以提高计算精度。     

颗粒相壁面条件对非球形颗粒流动影响的数值模拟

采用双流体模型对设置竖直隔板的气固密相流化床中非球形颗粒的运动进行了模拟,颗粒形状的影响由相间曳力模型考虑,重点考察壁面处颗粒边界条件的影响。同时进行了实验室规模三维流化床的流化实验,以验证模型的有效性。通过压降轴向分布、颗粒浓度径向分布以及物料出口处颗粒质量流率功率谱估计等定量分析,结果表明:对不设置内构件的自由床,壁面反射系数对系统宏观流动特性影响较小,而对壁面处局部颗粒运动影响较大;对壁面面积大幅增加的内构件床,壁面反射系数可显著改变气体和颗粒的运动特征,取值需控制在适当范围内。     

边界条件和结构尺寸对提升管内气固流动特性的影响

引言提升管是非均匀结构显著的气固两相流动体系,其流动特性主要表现为轴向空隙率的S形分布、径向的"环-核"结构以及团聚物的生成和破碎等。近年来,有关数值计算方法的研究增多,其中双流体模型的应用最为广泛,颗粒动理学则是目前封闭控制方程的最合理有效的方法。     

颗粒相壁面条件对提升管内气固流动模拟的影响

引言提升管是非均匀结构显著的气固两相流动体系,其流动特性主要表现为轴向空隙率的"S"形分布、径向的"环-核"结构以及团聚物的生成和破碎等。近年来,国内外学者致力于数值计算方法的研究增多,其中双流体模型的应用最为广泛;颗     

Searching for a mesh-independent sub-grid model for CFD simulation of gas-solid riser flows

This work aims to examine the effects of grid size in applying the two-fluid model (TFM), and thereby attempts to search for a mesh-independent sub-grid model for simulating gas-solid riser flows. To this end, we performed a series of TFM simulations over a periodic domain with various grid resolutions and drag closures. Of these drag models, EMMS/matrix model in its simplified version was chosen to be the focus of discussion. It was found that TFM simulation with a homogeneous drag model reaches its numerically asymptotic solution when the grid scale is as small as 10 times the particle diameter, but it still fails to capture the characteristic S-shaped axial voidage profile and highly over-predicts the solids flux. By comparison, EMMS/matrix model seems to reach a mesh-independent solution of the effect of sub-grid structures on the drag force, and predict successfully the axial voidage profile and the solids flux with even coarse grid. Therefore, the fine-grid TFM simulation is inadequate for gas-solid riser flows. We need sub-grid modeling of the heterogeneous structure.     

Acceleration of CFD simulation of gas-solid flow by coupling macro-/meso-scale EMMS model

Gas-solid flow features significant dynamic multi-scale structure; multi-scale modeling is therefore in order. In this article, the macro-scale EMMS model was coupled with a two-fluid method (TFM) elaborated by the meso-scale EMMS model resolving sub-grid scale heterogeneity to simulate the hydrodynamics of circulating fluidized bed (CFB) risers. The overall flow distribution under the steady state was approximately predicted by the macro-scale EMMS model, which serves as the initial condition for meso-scale TFM simulations reproducing the dynamic behavior of heterogeneous gas-solid flows. Using the solid circulation flux as criterion, it was shown that this coupling approach can significantly reduce the time required to reach the statistically steady state, as compared to the packed bed or homogeneously dispersed initial condition. It also suggests a general approach to speedup dynamic simulation in the multi-scale paradigm of computation.     

非均匀气固两相系统中多尺度传质模型

建立了适用于气固循环流化床的多尺度传质模型 .从过程与尺度的角度出发 ,将非均匀气固两相流中的传质过程分解为静态与动态的过程 ,并将前者分解为稀相内、密相内以及稀密相间 3个尺度下的传质 ;在用多尺度能量最小 (EMMS)模型求解已知物系性质和操作条件的非均匀气固两相流体动力学参数的基础上 ,借助于前人的研究结果 ,利用相对滑移速度、空隙率等参数求解传质系数 ,求得轴向的浓度场分布 ,并讨论非均匀两相流动结构对传质效率的影响     

快速流化床中颗粒浓度及颗粒速度的径向分布

本文在φ140mm的气固并流下行快速流化床中,研究了颗粒浓度及颗粒速度的径向分布随操作气速、颗粒循环速率以及床层轴向位置的变化规律。并探讨了颗粒质量通量的径向分布及颗粒流动的均匀性,表明在相同操作条件下,下行快速流化床中气固流动远比上行快速流化床均匀。     

GAS-SOLIDS FLOW PATTERNS IN A CONCURRENT DOWNFLOW FAST FLUIDIZED BED(CDFFB)

Radial profiles of solid concentration and velocity for concurrent downward gas-solid suspension in a140mm inside diameter fast fluidized bed were investigated.The influence of gas velocity,solid circulating rateand axial position on radial profiles of solid concentration and particle velocity has been examined.It hasbeen found that an annular region of high solid concentration exists at r/R=0.94.At both the center and wallregion,the solid concentration and the particle velocities are relatively low.The shape of radial solid con-centration profile curves is mainly dependent on the cross-section averaged voidage,and the shape of radialparticle velocity profile is mainly affected by the gas velocity and cross-section averaged voidage.Based on the radial profiles of solid concentration and particle velocity,the solid mass flux profile and thenonuniformity of solids flow are discussed in this paper.It is shown that solids flow in CDFFB is much moreuniform than that in UFFB.     

Experimental validation of the gas-solid flow in the CFB riser

In this paper, an experimental study is performed to investigate the flow structure in a circulating fluidized bed (CFB). The typical core-annulus structure and small amount of back-mixing of particles near the wall of the riser were observed. The axial solid concentration distributions contain a dilute region towards the up-middle zone and a dense region near the bottom and the top exit zones. Furthermore, the solid concentration decreases with the increase of the superficial gas velocity, and increases with the increment of the circulation rate at the same height position. The total pressure drop of the main bed represents a linear relationship with the solid flux rate. In the dense phase zone, the solid concentration increases linearly with the augmentation of the solid flux, however, the change of the solid concentration is slight, even unchangeable at the up zones. In addition, based on the Energy-Minimization Multi-Scale (EMMS) method, a revised drag force model is proposed, which is coupled in the Eulerian two-fluid model for simulating the flow structure in the riser. Numerical results are consistent with the experimental data, which indicate the revised drag force model is very successful in simulating flow structure of the dense gas-solid two-phase flow.     

Axial voidage profile in a cold model circulating fluidized bed

The axial voidage profile was measured in a cold model circulating fluidized bed (0.38 m in diameter and 9.1 m in height) of sand particles as bed materials. Voidage in the riser column increases along the height above the distributor plate with increasing the gas velocity. However, it decreases with an increase in solid circulation rate in the bed. Model correlations to predict the solid circulation rale and the axial voidage profile in the bed are proposed.