Transport phenomena in high porosity open-cell fibrous structures have been the
focus of many recent industrial and academic investigations. Unique features of these
structures such as relatively low cost, ultra-low density, high surface area to volume
ratio, and the ability to mix the passing fluid make them excellent candidates for
a variety of thermofluid applications including fuel cells, compact heat exchangers
and cooling of microelectronics. This thesis contributes to improved understanding
of thermal transport phenomena in fuel cell gas diffusion layers (GDLs) and metal
foams and describes new experimental techniques and analytic models to characterize
and predict effective transport properties.
Heat transfer through the GDL is a key process in the design and operation of
a proton exchange membrane (PEM) fuel cell. The analysis of this process requires
determination of the effective thermal conductivity as well as the thermal contact
resistance (TCR) associated with the interface between the GDL and adjacent surfaces/
layers. The effective thermal conductivity significantly differs in through-plane
and in-plane directions due to anisotropy of the GDL micro-structure. Also, the high
porosity of GDLs makes the contribution of TCR against the heat flow through the
medium more pronounced.
A test bed was designed and built to measure the thermal contact resistance
and effective thermal conductivity in both through-plane and in-plane directions under
vacuum and ambient conditions. The developed experimental program allows
the separation of effective thermal conductivity and thermal contact resistance. For
GDLs, measurements are performed under a wide range of compressive loads using
Toray carbon paper samples. To study the effect of cyclic compression, which may
happen during the operation of a fuel cell stack, measurements are performed on the
thermal and structural properties of GDL at different loading-unloading cycles.
The static compression measurements are complemented by a compact analytical
model that achieves good agreement with experimental data. The outcomes of the
cyclic compression measurements show a significant hysteresis in the loading and unloading
cycle data for total thermal resistance, TCR, effective thermal conductivity,
thickness, and porosity. It is found that after 5 loading/unloading cycles, the geometrical,
mechanical, and thermal parameters reach a“steady-state”condition and
remain unchanged. A key finding of this study is that the TCR is the dominant
component of the GDL total thermal resistance with a significant hysteresis resulting
in up to a 34 % difference between the loading and unloading cycle data. Neglecting
this phenomenon may result in significant errors in evaluating heat transfer rates and
temperature distributions.
In-plane thermal experiments were performed using Toray carbon paper samples
with different polytetrafluoroethylene (PTFE) content at the mean temperature of
65−70◦C. The measurements are complemented by a compact analytical model that
achieves good agreement with experimental data. Results show that the in-plane
effective thermal conductivity remains approximately constant, k ≈ 17.5W/mK, over
a wide range of PTFE content, and it is approximately 12 times higher than the
through-plane conductivity.
Using the test bed designed for the through-plane thermal conductivity measurement,
the effective thermal conductivity and thermal contact resistance of ERG
Duocel aluminum foam samples were measured under varying compressive loads for
a variety of porosities and pore densities. Also, an experimental program associated
with an image analysis technique is developed to find the size and distribution of
contact spots at different compressive loads. Results show that the porosity and the
effective thermal conductivity remain unchanged with the variation of pressure in the
range of 0 to 2 MPa; but TCR decreases significantly with pressure due to an increase
in contact area. Moreover, the ratio of contact area to cross-sectional area is 0-0.013,
depending upon the compressive force, porosity, and pore density.
This study clarifies the impact of compression on the thermal and structural properties
of GDLs and metal foams and provides new insights on the importance of TCR
which is a critical interfacial transport phenomenon. / Graduate
| Identifer | oai:union.ndltd.org:uvic.ca/oai:dspace.library.uvic.ca:1828/3628 |
| Date | 19 October 2011 |
| Creators | Sadeghi, Ehsan |
| Contributors | Djilali, Nedjib, Bahrami, Majid |
| Source Sets | University of Victoria |
| Language | English, English |
| Detected Language | English |
| Type | Thesis |
| Rights | Available to the World Wide Web |
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