氢氧燃料电池三维流道结构中液态水的液滴动力学与变形

这篇文章涉及水动力学相关知识。

Droplet dynamics in a proton exchange membrane fuel cell flow field design with 3D geometry

Zijun Li

Shubo Wang

Weiwei Li

Tong Zhu

Zhaohu Fan
Xiaofeng Xie

Abstract

Water management is crucial to achieve both high－performance and durability of proton exchange membrane fuel cell （PEMFC）． Therefore， it is necessary to investigate the dynamic behavior of droplets in PEMFC channel for water management． In this paper， we explore the kinetics of droplets in a 3D flow field by experimental and theoretical analysis． More specifically， we examine the following four perspectives： 1） the movement and falling of droplets， and their force and deformation， 2） the superiority of 3D flow field drainage， 3） the pressure and viscous force under different scenarios including varying droplet sizes and velocities， and 4） the expression describing the shape change of droplets． The results show that the 3D flow field has a greater driving force on droplets and that their deformation affects the discharge of liquid water． Throughout the study， we provide better understanding of droplet dynamic in PEMFC gas channels． It enables to optimize the design and working conditions of these channels．

Fig． 1 e Schematic of the transparent PEMFC （a） is the photo of a PEMFC used for this study． （b） is the assembly drawing． （c） is
the internal structure of the 3D flow field．

Fig． 2 e The computational domains of VOF model for （a） droplet in flow field and （b） water film in flow field．

Fig． 3 e The computational domains of droplet－fixed model． The location of the fixed－droplet is in the （a）inlet， （b）middle， and
（c）narrow of channel．

Fig． 4 e The grid independency test of the VOF model．

选用不同类型的单元，或选同相同的单元，但网格的结构发生变化，会对计算结果产生一定的影响。如果单元类型选择不合适，或单元形态不良，可能会对计算结果产生严重的影响。

Fig． 5 e Water droplet at the gas channel．

Fig． 6 e Distributions of the PEMFC voltage and power density against the current density for 3D and 2D flow fields．

氢氧燃料电池，不是氢空燃料电池。

性能出现差异在3．5A／cm2以上。

Fig． 7 e Pressure responses at the inlet of channels with various droplet diameters and inlet velocities． （aeb） are the different
inlet velocities for 3D and 2D flow fields． （ced） are the different droplet diameters for the 3D flow field at inlet velocity 5 m／s
and 3 m／s．

2D和3D的流体阻力特性差异大

the channel width and height were set to 0．8 and 1 mm， respectively． The maximum and minimum heights in the 3D flow field were 0．8 and
0．4 mm， respectively．

0．3mm液滴在0．4－0．8mm高的3D流道内运动，速度为2m／s就会产生流阻的波动。最大的流阻波动是1．8kPa＠5m／s＠0．6mm液滴。

Table 1 e Simulation cases for different conditions．

Fig． 8 e The position of droplet in the gas channel at 11．8 ms for different inlet velocities．

Fig． 9 e Water coverage ratio of GDL in case 5．

缩颈能促进液态水膜的排出，减少气体扩散层表面液态水的覆盖度。

Fig． 10 e Liquid water flow rate at the outlet for case 5．

Fig． 11 e Comparison of forces between the 3D flow field and 2D flow field（Case 6，9，12，15）．

Fig． 12 e Comparison of forces of 3D flow field at different positions and droplet diamaters （Case 6－17）．

Fig． 13 e Velocity and pressure distribution for 3D and 2D flow field． （a－f） are the case for 15e17 with an inlet velocity of 5 m／s．

Conclusions

In this paper， the VOF method， droplet fixed model， and
experimental method were used to explain why PEMFC with a
3D wave channel flow field have a better drainage performance．
In addition， the droplet dynamics in a PEMFC gas flow
channel were studied． Performed experiments reveal that the
3D flow field outperforms traditional 2D flow field in a high
current density area with more water． Based on this， the
mechanism of the 3D flow field with higher drainage performance
was studied further． The results showed that the 3D
flow field had a better drainage capacity for spherical droplets
and a water film， and for the removal for larger diameter
droplets． The gas in the 3D flow field had a greater shear force
and pressure force for the droplets， and the stress of the
droplets in the narrow region of the channel was increased by
an order of magnitude， which was more conducive to the
discharge of the droplets with a smaller radius in the middle
region of the channel． To study the change degree of the
droplet shape in the two channels under different conditions，
the change in the Weber number and the radius of curvature
of the droplets were calculated． The results showed that the
deformation of the droplets was affected by the Weber number，
and that the influence of the wave channel on the change
in droplet curvature was greater than that of the 2D flow field．
The basic knowledge obtained in this paper is of great significance
to understand the droplet dynamics in PEMFC gas
channels and to optimize the design and working conditions
of these channels．