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By Valentin Izraelev and Andrei Khodak, ABI, Hopkins, MN
View the pdf of this Supplement
The blood pump housing, showing the inlet at
left and outlet at right
Cross-section of the pump in the outlet plane,
showing velocity vectors
Since the late 1990s, ABI (Advanced
Bionics, Inc.) has used Fluent software
on a regular basis in the development
of proprietary bearing- and seal-free rotary
pump technology. Blood pumps for medical
applications are a major part of ABI’s
business. The traditional theory of centrifugal
pumps, used for over 100 years, does not
apply directly to small blood pumps with
flow rates between 0.3 – 1.5 gal/min and
differential pressures of 1.0 – 6.0 psi. To complicate
the matter, small blood pumps use
variable speed drives, and have unique optimization
criteria. In addition to traditional
pump characteristics, such as flow rate,
pressure, and efficiency, blood-handling characteristics
such as hemolysis and thrombosis
are important for blood pumps. Hemolysis
is the destruction of red blood cells and is
correlated to shear stress and residence time.
Thrombosis is clot formation, and is correlated
to wall shear stress. Thus, operation
of an optimum blood pump should not lead
to too much shear stress (to have low hemolysis),
and should simultaneously maintain
a certain level of wall shear stress (to prevent
thrombosis). ABI has developed an
expertise in analyzing blood pump performance,
and has used this capability as
a virtual prototyping design tool with a number
of variables representing the pump dimensions.
A number of actual blood pump tests
have shown that the computational and
experimental results are very closely correlated.
To perform an accurate analysis, the
whole pump, and often a substantial portion
of the outlet region, needs to be
meshed. GAMBIT is used to build 3D grids
with sliding, often non-conformal interfaces
around the rotor region. This kind of flexibility
allows accurate meshing for the blades,
which have complex geometry. A typical
mesh size for the calculations is about
300,000 cells. Journal files for GAMBIT provide
an opportunity to make small
changes in the geometry and automatically
rebuild the computational grid. This
means that the mesh generation time for
each design change is significantly
reduced. This parametric approach allows
fast virtual prototyping of many variants
from the predetermined matrix of geometric
parameters.
The parametric approach is also applied
in FLUENT, where journal files are used during
the solution stage and for postprocessing.
The performance of each variant is tested
at different operating conditions. Journal files
allow automatic preparation of the case and
data file for each regime from a prescribed
set of rotation speed and flow rate boundary
conditions. For solutions in the turbulent
regime, the standard k-e turbulence
model is used. FLUENT’s enhanced wall-functions allow accurate prediction of near-wall
regions without a significant increase in the
mesh size. For unsteady calculations, the
“frozen rotor” solution, computed from the
multiple reference frames (MRF) model, is
used as an initial condition.
Postprocessing of the CFD results
includes a quantitative assessment of the
pump design performance through predictions
of pressure rise in the pump, rotor torque,
forces, and stresses. Vector plots provide
detailed pictures of the flow patterns, and
contour plots show the locations of regions
where the maximum potential blood damage
would most likely occur. Journal files are
used during postprocessing to prepare comprehensive
reports on the computational
results.
If desired, special procedures are written
using Scheme files, which can be read
by FLUENT. These procedures allow for complex
cyclical operations during solver execution
and postprocessing that expand the
capabilities of the code. Only after the optimal
virtual design has been identified is the
actual prototype built. The highly efficient
virtual prototyping system that has been developed
at ABI has significantly reduced design
time and development costs.
Contours of the wall shear stress on the rotor
wall of a Tesla-type pump; half of the pump
rotor is shown
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