Tech Briefs Magazine - August 2023 - 23

through the porous media, and that
the desired chemical reactions will occur
proportionally to the reactant concentration.
We run these simulations
to arrive at an optimal distribution of
fluid flow orientation through the
pores. This process gives us valuable
results with a huge reduction in computational
complexity. "
Zhou describes this part of the overall
design process as homogenization. Having
now established a pattern of ideal
trajectories of fluid through the plate's
pores, the next step is dehomogenization.
This step involves the equation-driven
definition of microchannel
forms that will force fluid to follow these
optimal paths.
Generated Designs That Maximize
Flow, Reaction, or Both
The dehomogenization step is needed,
Zhou said, because " we cannot fabricate
an ideal porous material with each
pore individually designed. We need to
install walls and channels to direct fluid
through the pores in ways that approximate
the ideal. To generate this design,
we use COMSOL Multiphysics® to solve
a customized partial differential equation
(PDE) for pattern generation. The
software also gives us plotting functions
we can use to visualize the results. "
Two of the formal options created by
TRINA's dehomogenization equations
are shown in Figures 3 and 4. Their
guiding performance objectives are to
reduce resistance to reactant flow and
to enhance reactant supply and reaction
uniformity across the entire plate.
These objectives are represented by
governing variables in the model's PDE.
By assigning different weighting factors
to these two objectives, Zhou and his
team can induce the model to generate
different design options. They can then
evaluate the relative merits of each option
and make adjustments to produce
further iterations.
Of the design shown in Figure 3, Zhou
said, " We call this the 'flow design' because
it leads to the smallest pressure
drop across the entire flow field surface.
The model generated paths that are relatively
parallel and straight, without
much side branching. "
While this design effectively moves fluid
across the plate, it does less well at distributing
reactant evenly through the
porous material layer. Simulation shows
lower reactant concentration on the outlet
side of the design, which can limit
Tech Briefs, August 2023
Figure 5. A machined metal prototype flow field plate based on one of TRINA's generated designs.
reaction uniformity and the resulting
power output from the fuel cell.
If the weighting factors in the governing
equation were adjusted to prioritize
reaction uniformity, the model would
then generate a design like the one
shown in Figure 4, which Zhou calls the
" reaction design " . High reactant concentrations
(shown in red and orange in the
lower-right image) now predominate,
indicating that a larger share of the available
reactant is being put to work.
" Most commercial microreactors
would use a design somewhat similar to
the 'flow design', " said Zhou. But naturally
occurring systems that distribute
fluid reactants - such as leaves, lungs,
and blood vessels - more closely resemble
the forms of Figure 4.
" Engineers might prefer to use
straight channels with no side branching,
but nature chooses the 'reaction design', "
Zhou said. The TRINA team's research
paper notes that while some have
previously experimented with natural-looking,
fractal, or hierarchical forms
selected a priori for flow field channels,
" this is the first time that such large-scale
branching flow fields have been discovered
using an inverse design approach
without assuming prescribed layouts. "
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Rather Than Trying to Predict
the Future, Create It
Along with the " flow versus reaction "
comparison illustrated above, TRINA
produced two further designs (not
shown) that combined attributes
of
those in Figures 3 and 4. Every one of
TRINA's four iterations outperformed
baseline conventional designs across
key reaction-fluid performance metrics.
An additional design that was fabricated
and experimentally tested by the
TRINA team is shown in Figure 5.
So, what is the ideal design for a
plate? There is no such
flow field
thing, just as there is not a single ideal
technology for replacing gasoline-powered
automobiles. " From our point of
view, we succeed by providing multiple
good options for our engineers to consider, "
Zhou said.
Zhou also shares some advice that
guides him and his colleagues: " Our
chief scientist has said: 'We must stop
trying to predict the future, and just
work on trying to create it.' "
This article was written by Alan
Petrillo, COMSOL Inc. (Burlington,
MA). For more information visit www.
comsol.com.
23

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Tech Briefs Magazine - August 2023

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