Determination of the effective thermal conductivity of gas diffusion layers in polymer electrolyte membrane fuel cells: a comprehensive fractal approach

Abstract

SUMMARYAccurate determination of the effective thermal conductivity of gas diffusion layers used in polymer electrolyte membrane fuel cells is very critical for efficient thermal and water management. In this paper, effective thermal conductivity of a diffusion medium material with and without polytetrafluoroethylene (PTFE) was estimated using a fractal approach. Fractal dimensions were obtained using the measured pore size distributions of the samples. The predicted through-plane thermal conductivities were in good agreement with the experimentally measured data. It was demonstrated that such a fractal technique can be employed, with slight modifications, to estimate in-plane thermal conductivity. Besides, application of a random multifractal simulation to obtain the required tortuous fractal dimension is introduced, which can be helpful in the absence of scanning electron microscopy images. It was shown that the tortuous fractal dimension obtained is in good agreement with the reported value in the literature. Finally, variation of the tortuous fractal dimensions with PTFE contents was also determined, which is of practical importance when estimating thermal conductivity of diffusion media with different amount of PTFE contents.

A Comprehensive Fractal Approach in Determination of the Effective Thermal Conductivity of Gas Diffusion Layers in Polymer Electrolyte Membrane Fuel Cells

“…For numerical simulation method, the difficult to analyst detail of GDL geometry model of porosity structure. (Nikooee et al, 2011) Fractal approach can be used to estimate the thermal conductivity of diffusion media.…”

“…In a porous medium having fractal characteristics, surface area, pore volume and the length of capillary pathways follow a power law (Nikooee et al, 2011) (Shi et al, 2012. Figure 2.4 shows the microstructure and material parameters for the fractal model.…”

“…Based on the method of fractal analysis the pore fractal dimension can vary, for instance, if the image of the GDL cross section is used, the pore fractal dimension would vary between 1 and 2 and if the pore size distribution (example as obtained by porosimetry methods) is employed the dimension would be between 2 and 3 (Nikooee et al, 2011) (Zheng et al, 2012.…”

“…The series-parallel model (Figure 2.5), Nikooee et al,(2011) claimed that Shi et al, (2012), was proposed a fractal model for estimation of thermal conductivity (Nikooee et al, 2011) (Shi et al, 2012. The effective thermal conductivity of the medium can be obtained as equation (4) :…”

“…Based on Nikooee et al, (2011) this simplifying assumption and for practical applications is suitable used for two parameters can be related using a more general function, f(ξ)as equation (9) below:…”