Model development and validation of structural two-fluid model for heat transfer in a gas-solid bubbling fluidized bed

doi:10.11949/0438-1157.20231332

Abstract

Abstract:

The mesoscale structures such as clusters and bubbles have a significant impact on the flow, transfer, and reaction process of heterogeneous gas-solid systems. This article proposes a structural two-fluid model for simulating the hydrodynamics and heat transfer in a complex gas-solid system. Based on the different flow control mechanisms, the bubbling fluidized bed system is treated as two interpenetrating fluids that are the fluids of gas-dominated bubble phase and the particle-dominated emulsion phase. With this fundamental idea, we establish the governing equations and constitutive relationships considering the influence of mesoscale structure. Reasonable empirical correlations are utilized to close the interphase drag force, the emulsion phase viscosity, the interphase heat transfer coefficient, and the thermal conductivity of each fluid. Present structural two-fluid model in conjunction with an explicit resolution of hydrodynamic and thermal boundary layer is employed to simulate a bubbling fluidized bed system with a vertical heating tube. The simulation results demonstrate that the structural two-fluid model accurately predicts the axial distribution of solid holdup as well as the bed-to-wall heat transfer coefficient, specifically, the relative error between the simulated and experimental values of the wall heat transfer coefficient in the dense phase regime is less than 10%, and the simulated wall heat transfer coefficient in the dilute phase regime is in the same order of magnitude as the experimental value. It shows that the structural two-fluid heat transfer model can accurately describe the flow heat transfer characteristics of the gas-solid two-phase in the bubbling bed system.

Key words: multiphase flow, bubbling fluidized bed, mesoscale structure, continuum model, heat transfer

摘要：

介尺度结构显著影响非均匀气固复杂系统内的流动传递反应特性。构建了结构双流体传热模型用于模拟复杂气固系统内的流动传热过程，按照流动控制机制将鼓泡床系统划分成由气体主导的气泡相和颗粒主导的乳化相两个相互渗透的连续流体，进而确立考虑介尺度结构影响的控制方程及本构关系，其中相间曳力、乳化相黏度、相间传热系数及各相热导率均采用合理的经验关联式进行封闭。利用双流体传热模型与显式解析壁面流动和传热边界层方法相结合，对安装加热管的鼓泡床系统进行了模拟。结果表明：结构双流体传热模型可成功预测鼓泡床系统内固含率的轴向分布及床层壁面传热系数的变化规律；模拟结果与实验数据吻合较好，密相区壁面传热系数模拟值与实验值的相对误差小于10%，稀相区壁面传热系数的模拟值与实验值在同一量级，表明结构双流体传热模型可准确刻画鼓泡床系统内气固两相的流动传热特性。

关键词: 多相流, 鼓泡床, 介尺度结构, 连续介质模型, 传热

CLC Number:

TQ 021.1

Jinpeng ZHAO, Yongmin ZHANG, Bin LAN, Jiewen LUO, Bidan ZHAO, Junwu WANG. Model development and validation of structural two-fluid model for heat transfer in a gas-solid bubbling fluidized bed[J]. CIESC Journal, 2024, 75(4): 1497-1507.

赵金鹏, 张永民, 兰斌, 罗节文, 赵碧丹, 王军武. 气固鼓泡床结构双流体传热模型及其模拟验证[J]. 化工学报, 2024, 75(4): 1497-1507.

Figures/Tables 5

Table 1 Summary of the used parameters

参数	数值
颗粒粒径 $d p / m$	$76 × 10 - 6$
颗粒密度 $ρ s / (k g ∙ m - 3)$	$1500$
气体密度 $ρ g / (k g ∙ m - 3)$	$1.17285$
气体黏度 $μ g / (P a ∙ s)$	$1.84 × 10 - 5$
时间步长 Δ $t$ /s	$1.0 × 10 - 4$
起始流化空隙率 $ε m f$	$0.45$
进口表观气速 u $/ (m ∙ s - 1)$	$0.1, 0.2, 0.3, 0.4$
初始堆料高度 $H s / m$	$0.76$
整体模拟时间/s	20
统计时长/s	15~20

Table 1 Summary of the used parameters

参数	数值
颗粒粒径 $d p / m$	$76 × 10 - 6$
颗粒密度 $ρ s / (k g ∙ m - 3)$	$1500$
气体密度 $ρ g / (k g ∙ m - 3)$	$1.17285$
气体黏度 $μ g / (P a ∙ s)$	$1.84 × 10 - 5$
时间步长 Δ $t$ /s	$1.0 × 10 - 4$
起始流化空隙率 $ε m f$	$0.45$
进口表观气速 u $/ (m ∙ s - 1)$	$0.1, 0.2, 0.3, 0.4$
初始堆料高度 $H s / m$	$0.76$
整体模拟时间/s	20
统计时长/s	15~20

Fig.1 Schematic diagram of the bubbling fluidized bed[37]

Fig.2 The circumferentially bed-to-wall heat transfer coefficient with different mesh sizes in the thermal boundary layer

Fig.3 Comparison of the axial distribution of solid concentration between the simulated and experimental results at different inlet gas velocities

Fig.4 Comparison of the average wall-to-bed heat transfer coefficients at different heights predicted by experimental, structural two-fluid model, CPFD (Wei’s simulation[38]) with different inlet gas velocities

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