Water Management Simulation of Proton Exchange Membrane Fuel Cells with Micro-ribs Based on Volume of Fluid Model

 

Haichao Liu,1 Lei Guo,2 Miaomiao Liu,4 Hongbo Chen,2 Wenwen Han,1 Huiguang Bian,1, 2 Xiaolong Tian,1, 2

Chuansheng Wang,1, 2,* Zhanhu Guo5,* and Jingyao Sun1, 3,*

 

1 National Engineering Laboratory of Advanced Tire Equipment and Key Materials, Qingdao University of Science & Technology, Qingdao 266061, China.

2 College of Electromechanical Engineering, Qingdao University of Science & Technology, Qingdao 266061, China.

3 College of Mechanical and Electrical Engineering, Beijing University of Chemical Technology, Beijing 100029, China.

4 FAW-Volkswagen Automotive Co. Ltd, Changchun 130011, China

5 Integrated Composites Laboratory (ICL), Department of Chemical & Biomolecular Engineering, University of Tennessee, Knoxville, TN 37996, USA.

*Email: wcsmta@qust.edu.cn (C. Wang); zguo10@utk.edu (Z. Guo); sunjingyao@mail.buct.edu.cn (J. Sun)

 

Abstract                                                                                              

 

Removal of liquid water from the surface of the gas diffusion layer (GDL) in the flow channel is an effective method for water management in a proton exchange membrane fuel cell (PEMFC). To enhance the water removal, we modify the conventional flow channel by adding a micro-rib in it. Numerical simulations are conducted to explore the water behaviors in the modified channel and the output performances of PEMFCs. The height and width of the micro-rib, as well as the wall contact angles, are investigated through the volume-of-fluid method to optimize the micro-rib. The results exhibit that the micro-rib can remove the liquid water from the GDL surface by the capillary effect when its height is 0.6 mm, width is 0.3 mm, and its contact angle is lower than the GDL surface. Besides, output performances of PEMFCs with the conventional channel and the modified channel are investigated through PEMFC simulations. The results exhibit that the modified channel can improve the output performance of PEMFC by enhancing the oxygen diffusion efficiency. Therefore, the modified channel is a superior alternative to the conventional channel for high output PEMFCs.

 

Table of Contents

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Keywords: Numerical simulation; PEMFC; Volume of fluid; Water management; Output performance.

 



1. Introduction

Proton exchange membrane fuel cell (PEMFC) has been regarded as one of the most promising energy devices attributed to its low emission, high efficiency, rapid startup, no noise, and high energy conversion efficiency.[1-4] However, it still has some insurmountable difficulties that need to be overcome before commercialization, one of which is water management.[5-7] In an operating PEMFC, electrochemical reaction at the cathode catalyst layer (CL) produces liquid water, which is conducive to maintain the water content in the proton exchange membrane (PEM). When the produced liquid water is excessive, it would diffuse through the gas diffusion layer (GDL) into the cathode flow channel.[8] The liquid water that emerges in the flow channel usually hangs on the GDL surface, thereby blocking the micro-channels of the GDL for reactant diffusion. Severe blocking would seriously decrease the local reactant concentration and thus suppress the output of PEMFC.

Since the flow channel is the last pathway for the drainage of liquid water,[9] water behavior in the flow channel is an important research object of water management.[10] Visualization techniques, including optical perspective,[11] neutron imaging,[12-15] and magnetic resonance imaging,[16] have been widely used in researches on water management, of which the optical perspective technique is the most common, direct, and effective. Using the optical perspective technique, Hussaini et al.[17] visualized water behaviors in the cathode flow channel and classified them into four categories, which are single-phase flow, droplet flow, film flow, and slug flow. These four flow patterns contain almost all the flow states of liquid water. Although the optical perspective technique provides a useful insight into the understanding of flow patterns in the flow channel, it still has some disadvantages, such as high cost and low efficiency for flow channel design. Compared with the visualization experiment, computational fluid dynamics (CFD) simulation is a more efficient and low-cost method. Common two-phase flow simulation techniques in this area of researches include multi-fluid model,[18] mixture model,[19] volume-of-fluid (VOF) model,[20] and lattice Boltzmann method (LBM).[21,22] Among them, the VOF model is the most appropriate method since it can simulate the flow conditions with two or more types of fluids and track the location and shape of the interfaces between different fluids. Quan et al.[20] used this method to simulate the two-phase flow in the flow channel of a PEMFC for the first time. From then on, the VOF model was widely used in this area.[23-26]

Water management in the flow channel usually includes two aspects: one is to accelerate the water transport in the flow channel, the other is to remove the liquid water from the GDL surface. To clear the liquid water on the GDL surface and improve the reactant diffusion efficiency, many visualization experiments and CFD simulations have been conducted. The main method for liquid water removal is designing the channel structures and the channel wall wettability.[27-29] Metz et al.[28] proposed a hydrophilic flow channel with a special section consisting of a trapezoid and a rectangle. With this structure, water droplets could be easily detached from the GDL surface by the capillary effect of the declining walls and then lifted to the channel bottom. Qin et al.[30,31] modified the conventional channel by inserting hydrophilic needles or plates in it and carried out CFD simulations with the VOF method to reveal the effect of the needles or plates. Their results showed that the hydrophilic needles or plates could transfer the droplet to the channel bottom and thus clear the GDL surface. Utaka and Koresawaa[32] processed sloping micro-grooves on the hydrophilic sidewalls of the flow channels and carried out experiments on PEMFC output. The sloping micro-grooves were proved to be able to reduce cell voltage fluctuation and improve cell voltage at a certain current, owing to the enhancement of water removal.

In recent years, we have also been committed to the study of water management and proposed a water removing method with hydrophobic channel walls. Based on this method, we have presented three modified flow channels which exhibited excellent water removal and transportability.[33-35] However, these modified flow channels can remove the water droplets from the GDL surface only when the droplets are transported to the channel turns or located at the channel sidewalls.

In this work, we proposed a modified flow channel including a micro-rib fixed on the channel bottom, which can remove the droplets located in the middle of the flow channel from the GDL surface, and simultaneously reduce their volumes and enhance their transport speed. This modified flow channel overcomes the disadvantages of the channels in our previous works. CFD simulations with the VOF method were carried out to reveal the effect of the micro-rib on water behaviors. To optimize the micro-rib, the height and width of the micro-rib, as well as the wettability, were investigated. Besides, PEMFC simulations were also carried out to explore the effect of the micro-rib on cell output.

 

2. Model Formulation

2.1 CFD simulations with VOF method for water behaviors in channels

2.1.1 Computational domain and assumptions

Geometry models for the conventional and modified channels were shown in Fig. 1. The conventional channel is a 20 mm long straight channel with a rectangular cross-section of 1 mm  1 mm. The modified channel includes a micro-rib fixed on the bottom of the conventional channel.

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Fig. 1 Geometry models for the conventional channel (a) and the modified channel with a micro-rib (b).

 

Generally, liquid water in the flow channel emerges from many micro-holes on the GDL surface and in a large variety of velocities. Such a complicated flow is impractical for CFD simulations and not conducive to observe water behaviors in detail. Therefore, the GDL was simplified to a homogeneous surface with only one micro-hole, which has been adopted in many previous studies.[36-38] The micro-hole is 1.0 mm away from the inlet and its diameter is 0.08 mm. Besides, diffusions of air through the GDL were ignored. The air was regarded as an incompressible ideal gas and its flow in the channel was laminar (the Reynolds number is about 300). Air and water were considered immiscible and phase change between air and water phases was neglected. The channel was assumed to be isothermal and the surface tension coefficient at the interface between the two phases was assumed a constant 0.618.

 

2.1.2 Governing equations

CFD simulations with the VOF model were conducted in available Fluent. The governing equations involved are as follows:

Continuity equation

                              (1)

 

Momentum equation

………………………(2)

where  is the averaged density,  the averaged viscosity,  the fluid velocity vector,  the pressure and  the gravitational acceleration.  is a source term representing the surface tension effect, which is expressed as[39]:

                            (3)

where  is the surface tension co