Simulation and analysis of mass transfer and absorption process intensification by villi movement

doi:10.11949/0438-1157.20191101

Abstract

Abstract:

The inner wall of the digestive organ has a multi-level and multi-scale structure and complex movement patterns. Understanding its role in the process of digestion and absorption is of great significance to human health. From a chemical engineer s unique perspective, this work developed a multi-physics computational fluid dynamics (CFD) model to describe nutrient transfer and absorption in the small intestine driven by villi movement. The moving mesh method was successfully implemented to realize the cyclic back-and-forth movement of villi. Furthermore, data analysis methods were developed to quantify mass transfer and absorption performance. Numerical simulation results show that the back and forth movement of villi along the axial direction can generate two characteristic vortexes. The formation of vortexes can effectively reduce mass transfer resistance in the radial direction. The top part of villi plays critical role in nutrient absorption. A shorter movement cycle offers a higher villi velocity, which results in a higher value of the maximum enhancement factor and hence a higher absorption amount. The case with taller villi demonstrates higher mass-transfer enhancement factor. At the same time, taller villi offer larger absorption area. These two positive factors together contribute to a much higher absorption amount as compared with the case with shorter villi. In this specific study, the case with 900 μm high villi and a movement cycle of 6 s, the mass transfer performance can be improved by over 500% as compared to the case without villi movement (i.e., a mass-transfer enhancement factor reaching 6).

Key words: absorption, mass transfer, mathematical modeling, computational fluid dynamics, small intestine, villi

摘要：

消化器官内壁有着多级多尺度结构且运动方式复杂，理解其在消化吸收过程中的作用对人类健康具有重要意义。着眼于人体小肠绒毛，从化学工程师的独特视角出发，建立多物理场耦合模型描述小肠绒毛运动驱动下的营养物质传递与吸收。成功应用动网格方法实现绒毛往返周期运动。同时建立分析方法，量化传质与吸收效果。模拟结果显示绒毛沿着小肠管路轴向的往返运动，可以形成两个特征涡流，有效强化绒毛间流体和外部流体的交换，减小径向传质阻力。绒毛顶部在吸收中起到了关键作用。绒毛运动周期越小，最大传质增强因子越高，吸收量越大。绒毛越高，最大传质增强因子越高，再加上吸收面积越大，总吸收量会显著提升。对于900 μm高的绒毛，在运动周期为6 s情况下，传质效果比绒毛不运动时的传质效果提升超过500%。

关键词: 吸收, 传质, 数学模拟, 计算流体力学, 小肠, 绒毛

CLC Number:

TQ 027.1

Xiaolan HUA, Yanan ZHANG, Zhizhong DONG, Yong WANG, Xiao Dong CHEN, Jie XIAO. Simulation and analysis of mass transfer and absorption process intensification by villi movement[J]. CIESC Journal, 2020, 71(5): 2024-2034.

华晓蓝, 张亚南, 董志忠, 王勇, 陈晓东, 肖杰. 模拟分析绒毛运动对传质和吸收过程的强化[J]. 化工学报, 2020, 71(5): 2024-2034.

Figures/Tables 9

Fig.1 Construction of geometry of simulation system together with its mesh and boundary conditions(scheme of villi movement is taken from Ref.[2])

Table 1 Values of parameters and variables in model

变量及参数	取值	数据来源
模拟区域宽度L	1 mm	式(1)
模拟区域高度H	2 mm	—
绒毛高度h	300~900 μm	[32]
绒毛宽度D	160 μm	[33]
绒毛分布密度 $ω$	25 mm^-2	[3]
绒毛间初始距离w₀	8 μm	—
t=T/2时绒毛间距离w	40 μm	式(1)
绒毛运动周期T	4~10 s	[34]
体温下葡萄糖扩散系数D	8.97×10^-10 m²/s	—
葡萄糖源的浓度C_b	200 mol/m³	[35]

Table 1 Values of parameters and variables in model

变量及参数	取值	数据来源
模拟区域宽度L	1 mm	式(1)
模拟区域高度H	2 mm	—
绒毛高度h	300~900 μm	[32]
绒毛宽度D	160 μm	[33]
绒毛分布密度 $ω$	25 mm^-2	[3]
绒毛间初始距离w₀	8 μm	—
t=T/2时绒毛间距离w	40 μm	式(1)
绒毛运动周期T	4~10 s	[34]
体温下葡萄糖扩散系数D	8.97×10^-10 m²/s	—
葡萄糖源的浓度C_b	200 mol/m³	[35]

Fig.2 Flow field and concentration field at representative time instants during one movement cycle (velocity field is plotted using colored streamlines and white arrows, arrow size is proportional to magnitude of fluid velocity, color of streamline represents velocity)

Fig.3 Distribution of absorption flux along villi surface at representative time instants during one movement cycle plotted as solid lines (comparison with distribution at time 0, dashed lines)

Fig.4 Evolution of Peclet number, mass-transfer enhancement factor and amount of absorption accumulated over time

Fig.5 Distribution of absorption flux along villi surface at different time instants during one movement cycle (comparison among cases with different cycle period values)

Fig.6 Evolution of Peclet number, mass-transfer enhancement factor and absorption amount during one movement cycle (comparison among cases with different cycle period values)

Fig.7 Distribution of absorption flux along villi surface at different time instants during one movement cycle (comparison among cases with different villi lengths)

Fig.8 Evolution of Peclet number, mass-transfer enhancement factor and absorption amount during one movement cycle (comparison among cases with different villi lengths)

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