Transcript Modeling and Simulation of Slow and Frictional Multiphase Flow

TOWARDS A VIRTUAL REALITY
PROTOTYPE
FOR FUEL CELLS
Steven Beale Ron Jerome
National Research Council
Anne Ginolin
Institut Catholique d’Arts et Métiers
Martin Perry, Dave Ghosh
Global Thermoelectric Inc.
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Introduction

Fuel Cells convert chemical energy (hydrogen and oxygen)
to electrical energy

Potential replacement for IC engines

National Fuel Cells Initiative: Kyoto summit

Three main parts: Anode, cathode, electrolyte

Solid Oxide Fuel Cells (SOFC’s) can use methane/natural
gas in place of hydrogen

Built in stacks of 10-50 cells: Connected in parallel
hydraulically; electrically in series
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Introduction

SOFC’s operate at up to 1000 ºC

If supply of fuel and air non-uniform, reaction rates and
hence temperatures will vary

Temperature uniformity important: If too cold, cell reaction
shuts down; Too hot, mechanical failure

Current work to model fluid mechanics (mechanical
design). Goal: Uniform delivery of air and fuel to the
membrane-electrode assembly

Chemistry not considered at present time
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Introduction

Single cell model

Stack model: 10-50 cells. Two approaches
–Direct Numerical Simulation (DNS).
–Distributed Resistance Analogy (DRA)

For the DNS require large amounts of storage and memory

Certain details lost with the DRA

Several different DRA implementations are possible
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DRA approach


p  FU


U  ru


u  Interstitial velocity
U  Superficial velocity
 ri  

   ri ui   0
t


 ri ui  
 

   ri ui ; ui    ri pi  Fri2ui
t
F is a ‘distributed resistance’ obtained from theory,
experiments, or detailed numerical simulations. ri are
volume fractions of air and fuel.
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Determination of resistance term

Many internal flows are correlated in the form;
f  a Re
where the Reynolds number is written in terms of a
“hydraulic diameter”
Re  Dhu 
The distributed resistance is just,
F
2a 
r Dh2
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Example: Plane duct
For a plane duct:
rH P
f 
w
12

2
1
Hu
2 u
12P
F
H3
12Lu 12LQ
p 

2
H
nBH 3
For more complex geometries must use numerical
integration empirical correlations
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PHOENICS settings

Used PHOENICS VR to construct SOFC stack model

Diffusion terms turned off using Group 12 patches
GP12DFE etc. for DRA

Source term with PATCH type PHASEM in momentum
equations, and Coefficient C = F/r

2-D flow imposed in core (w = 0)
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Results
Phase 1 ends
Phase 1 CFD
modelling begins
Phase 2 CFD
modelling begins
Source:
http://www.globeinvestor.com/
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Results

Results of flow calculations for 24 designs were displayed
as 3-D VRML files using a secure web site to the client in
Calgary across internet.

This allowed us to work together “at a distance”

Images were also displayed locally to client in Ottawa
using NRC Virtual Reality (VR) wall

SOFC stack completely redesigned as a result of this work
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Results
Fuel cell stack
Inlet manifold
Exit manifold
FLOW OUT
FLOW IN
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NRC Virtual Reality Wall
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Pressure in SOFC manifolds and
stack
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Velocity vectors in manifolds and
stack
NB: Vector scale different in
stack from in manifolds
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Discussion

Geometry is quite simple, but flow in inlet manifold
complex; Pressure maximum at front of step. Due to
horizontal inlet

Flow within the core of this SOFC stack is uniform, i.e.,
design is good. Little variation in vectors, in spite of inlet
design

Core flow is a low Reynolds number (creeping) flow, driven
primarily by the pressure gradient

Pressure drops consistent with values based on theory

Flow in outlet manifold is less complex than inlet: Size and
form less critical.
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Discussion

Gradient across stack is uniform horizontal.

In manifolds gradient is relatively small and decreases with
height - due to injection/suction

Manifold losses are in many cases quite significant, with
substantial variations observed, depend on particular
configuration under consideration.

For uniform flow the ratio of Pstack/Pmanifolds should be
large.

Parametric studies identified which parameters important allowed for the stack design to be optimised.
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Comparison of DRA and DNS
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Comparison of DRA and DNS
approaches

DRA and DNS results are similar with minor systematic
deviations.

Details of velocity profile lost with the DRA.

If core resistance is small, inertial effects become
significant: Pressure and velocity less uniform.

For tall stacks need large pressure gradient within core, so
inertial effects due injection/suction of working fluid from
manifolds does not lead to starvation at top of core.

Various DRA approaches are possible. Minor differences
occur due to convection terms.
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Conclusions

DRA model may be used as an engineering tool to design
SOFC's with a measure of confidence: Certain details of
flow field are lost. However combines computational
speed with accuracy

Certain SOFC models superior.

Back pressure across stack should be large to maintain
uniformity of pressure and velocity across core.

Geometric features by which this may be achieved were
identified using parametric studies

SOFC design re-configured as a result of CFD.
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Future (current) work

SOFC’s with more complex passages.

Flow of two working fluids, combined with inter-fluid heat
transfer and Ohmic heating.

Initial anlysis suggests the conventional DRA for heat
transfer may need to be modified due to low Reynolds
number effects.

Concurrent display and manipulation of graphics data,
locally on VR walls, and across the country via CA*net 3.

Experimental facilities to gather empirical data and conduct
flow visualisation studies for model validation.
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