Two-phase flow in porous metal foam flow fields of PEM fuel cells

TL;DR

This work addresses the pore-scale gas–liquid two-phase flow in porous metal foam (PMF) flow fields for PEMFCs, a key factor in water management and performance. It combines optical visualization, high-resolution pore-structure characterization, and real-time pressure-drop measurements on PMFs with two pore sizes (20 PPI and 40 PPI) and two wettabilities (hydrophilic and hydrophobic). The study identifies five flow patterns—film, plug, ligament, slug, and droplet—and shows how pore size and surface wettability shift pattern boundaries, while introducing the two-phase friction multiplier $\phi_g^{2}$ and spectral pressure-drop analysis as tools to diagnose transitions. The findings offer actionable design guidance toward improved water management (e.g., hydrophobic treatments and optimized pore size) and introduce PSD-based online monitoring as a practical diagnostic for PMF-based PEMFCs.

Abstract

Porous metal foam (PMF) flow field is a potential option for proton exchange membrane fuel cells (PEMFCs) due to its excellent capabilities in gas distribution and water drainage. However, the gas-liquid two-phase flow in the PMF flow field on the pore scale is still unclear. In this study, we investigate the gas-liquid two-phase flow in the PMF flow field. Film, plug, and ligament flows are found in the hydrophilic PMF flow field, while slug and droplet flows are found in the hydrophobic PMF flow field. The results suggest that optimizing the pore size, increasing the metal foam surface hydrophobicity, and optimizing the operating condition are helpful for the water management of the PMF flow field. The frequency analysis of the pressure drop also shows that the dominant frequency can be used as an indicator to analyze the transition between different flow patterns.

Figures (8)

• Figure 1: (a) Schematic diagram of the transparent assembly of the PMF flow field structure. (b) Schematic diagram of the test loop.
• Figure 2: Metal foam structure used in the experiment: (a) Metal foam structure obtained by X-ray tomography. (b-c) Images of pore morphology for foams of 20 and 40 PPI. (d-e) Pore size distribution for foams of 20 and 40 PPI. (f-h) Advancing and receding contact angles of liquid water on copper surfaces with different wettabilities. The substrates beneath the droplets are smooth copper surfaces, and the contact angles in the images are the intrinsic contact angles.
• Figure 3: Flow patterns in the PMF flow field. (a) Film flow; (b) Plug flow; (c) Ligament flow; (d) Slug flow; (e) Droplet flow. The left column is experimental images, and the right column is schematic illustrations. In the illustrations, the brown, blue, and white colors indicate ligaments of the metal foam, liquid water, and gas, respectively. Images in Figs. \ref{['fig:03']}(a-c) were taken in the hydrophilic PMF flow field with blue dye to increase the contrast, and images in Figs. \ref{['fig:03']}(d-e) were taken in the hydrophobic PMF flow field without dye. The superficial gas velocity ($U_{sg}$) and the superficial liquid velocity ($U_{sl}$) are respectively: (a) $U_{sg}$ = 0.56 m s$^{-1}$, $U_{sl}$ = 0.0043 m s$^{-1}$; (b) $U_{sg}$ = 2.22 m s$^{-1}$, $U_{sl}$ = 0.0043 m s$^{-1}$; (c) $U_{sg}$ = 4.44 m s$^{-1}$, $U_{sl}$ = 0.0021 m s$^{-1}$; (d) $U_{sg}$ = 0.56 m s$^{-1}$, $U_{sl}$ = 0.0043 m s$^{-1}$; (e) $U_{sg}$ = 4.44 m s$^{-1}$, $U_{sl}$ = 0.0021 m s$^{-1}$.
• Figure 4: Flow pattern map plotted against the superficial gas velocity ($U_{sg}$) and the superficial liquid velocity ($U_{sl}$) in the PMF flow field with different pore sizes: (a) Foam 20 PPI, (b) Foam 40 PPI. The dashed lines are guidance for the eyes.
• Figure 5: Flow pattern map plotted against the superficial gas velocity ($U_{sg}$) and the superficial liquid velocity ($U_{sl}$) in the PMF flow field with different surface wettabilities: (a) Hydrophilic, (b) Hydrophobic. The experiment was performed using Foam 20 PPI with surface modification as discussed in Figs. \ref{['fig:02']}(f-h). The dashed lines are guidance for the eyes.