Aerodynamic Design of an Axial Low-Pressure Fan


In this video we’ll be demonstrating the
aerodynamic design of an axial low-pressure fan including a CFD
simulation. First you create a new project choosing the ventilator design
module. Insert the design point of the fan. The design point is defined by
volume flow rate, total pressure rise, and rotational speed. In this case the fluid
is air at 20 degrees Celsius. You can choose any fluid properties by choice or
by applying a fluids database. We can switch the rotational direction or add
an inflow swirl if required. On the right side you’ll see the specific speed and
the general machine type. Now we add a new component, selecting the axial
impeller option. The main dimensions window opens. In this example we use an
unshrouded standard impeller. We have two general options for blade design: airfoil
or meanline mode. Here we have chosen the airfoil method. In CFturbo there is
also an option for multistage design including contra-rotating impellers. This
time we won’t use it. It will be in a separate demo video. To calculate the
impeller main dimensions, CFturbo uses numerous empirical correlations in the
parameters section. In most cases, non-dimensional approximation functions are taken from textbooks, research papers, and our own data. Here we use the work
coefficient to calculate the impeller diameter, and a diameter ratio function
to get the hub diameter. Additionally, several empirical efficiency
correlations are implemented too, to get reasonable initial values of the
impeller main dimensions. Now the impeller main dimensions have been
computed. On the right side of the window you see important physical values, a
sketch of the meridional section, the Cordier diagram and initial velocity
triangle on inlet and outlet of the impeller. In CFturbo all
geometrical parameters can be adjusted manually if required. Here we modify hub
diameter and impeller diameter slightly. This will only have a minor influence on
subsequent design steps. The meridional view allows the design of the meridional contours of the impeller. On the right side of the window there are
diagrams for area progression, static moment, and curvature, which can assist
you in shaping the meridional contour of the impeller. All Bezier points can be
modified numerically by changing numbers in the pop-up window. The user can adjust axial length of the impeller as well as shape and position of leading and
trailing edge. Now we add hub solids. For example, we can split the curves or transform them into Bezier splines. For demonstration
purposes, we’re choosing a very simple model of the hub contour. This tool is
very flexible for modeling hub and shroud geometry if applicable. In the blade properties, at first, we set
the number of blades, which is an input value with strong effects on the blade
profile geometry. Then a method must be selected to calculate the radial
equilibrium from hub to shroud. The meridional and circumferential velocity
components can be adjusted to balance pressure and centrifugal forces. In the
profile selection window, the user selects blade profile. It could be done
from a NACA profile catalog or by point based data. User-defined blade profiles
can be easily implemented. For this demo we have selected a NACA 6509 profile. On the right side there are several
diagrams with aerodynamic values and criteria to evaluate the profiles, for
example solidity or De-Haller criteria. If required, the profile properties like
stagger angle or chord length can be adjusted manually. in the next window there is the actual
status of the blade profiles, showing more detailed information of the current
design. For example, we can evaluate blade thickness, blade angles on leading and on
trailing edge, blade passage area, a blade to blade view, and so on. For NACA
profiles there is a possibility to set a finite thickness on the trailing edge of
the blade. Blade sweeping is a possibility to
deform the blade profiles in meridional and circumferential position. It
is mainly done to get better acoustics. Axial fans can become quieter this way.
The empirical, so-called acoustic benefit is shown on the upper right side of the
window. However, usually this goes along with some losses and efficiency and
performance. It is a trade-off. Now we can see the impeller in its
actual state in our 3D viewer. On the left side of the window you have the
model tree with all components and subcomponents. This allows for easy
navigation, renaming, and graphical adjustment. The user can modify color and translucency for each component or sub-component. To prepare the CFD setup, first we add an inlet extension including a simplified hub nose. For this purpose we
apply the stator module of CFturbo, which can be used to make vaned and
unvaned turbomachinery components like pumps or guide vanes. The inlet pipe
should have a certain length in order to prevent an impact from the boundary
conditions to the fan stage itself when running a flow simulation. In our example
we want to create a simplified hub nose which will be placed directly upstream
of the impeller. We split the hub curve, change the curve type to Bezier, and
create a rounded shape of the hub nose. Every single Bezier point can be
numerically defined as shown for the Bezier point on the z-axis. As done
before for the impeller we have added a solid shape for the material domain of
the hub nose, and additionally for the outer diameter. Now the model will be
shown in the 3D viewer again. Both components can be shown or hidden. Solid models of the whole impeller including blades and blade fillets, or of the
stator components, can be exported into any CAD format or meshing tool for CFD/CHT/FEA analysis. Direct export to rapid prototyping via STL is also
possible. In the same way as on the inlet side, we should add a pipe or a diffuser
downstream of the impeller. Here we add an axial stator. Again, the stator should
have an appropriate length. A recommendation for an outlet pipe length
should be 5 times pipe diameter or more. As before hub and shroud solid will be
generated. Our model here uses the inlet pipe and the outlet pipe just for
numerical reasons to run the 3D CFD simulations correctly. Of course a user
can create more complex geometries like guide vanes or bends, upstream or
downstream of the fan impeller. Even a virtual test rig could be prepared to
compare 3D CFD data and experimental results later. just like in the previous
design steps the model can be shown and updated in our 3D viewer. The final
design step would be model finishing in CFturbo, including fillet design. Model
finishing means a geometrical operation to create solid models for the fluid
domain and for the material domain in excellent quality. This finishing is a
precondition for many geometry export formats. It will be used often but not
for all CAD or CFD systems. For some of the export formats the model finishing
will be done automatically. Let’s save the model before we export
the geometry and prepare the simulation. We open the export window. CFturbo has
export formats to all major CFD codes and CAD systems as well as neutral
formats like STEP, STL, or ParaSolid. This fan model will be exported to SimericsMP. We have to choose settings for meshing and simulation. For this demo we intentionally generate a very coarse mesh. We’ve chosen mesh settings which
will create a mesh of about 500 thousand nodes only. It will become a hexahedral
binary tree mesh. For single-stage turbomachinery CFD simulations we
recommend two to ten million nodes depending on the number of blades and
complexity of the secondary flow path. In the solver settings we must define the
number of iterations or time steps and we have to choose between steady state
or transient simulation and select a differencing scheme. To export the model to SimericsMP, STL
files are generated. Then the computational model will be exported,
including boundary conditions. The user has the option to just to export mesh
and solver settings. Or, as it is shown here, they can start the meshing process
and the CFD software directly. By choosing this option, SimericsMP will be
launched and the meshing process will be done. SimericsMP is an affordable, fast
and robust general-purpose 3D Navier-Stokes solver. CFD simulations
with SimericsMP provide realistic results that compare accurately with
multiple field tests on various types of rotating machinery like pumps, blowers,
compressors, and turbines. Now the model is ready for simulation. All boundary
conditions are set for the design point. The project file will be saved before
the simulation starts. Besides residuals we can observe the
flow field in real time. We can start dynamic streamlines or particles, show
isosurfaces or important physical values of the flow field on all surfaces, or on
sections. Compared to others, the SimericsMP CFD solver is extremely
fast. Steady state and transient flow simulations can be run easily on a
laptop. This steady state simulation took just three minutes on a laptop with an
Intel i7 processor for models with larger mesh sizes, such as for
multi-stage machines, there are options for distributed parallel computing
available to fulfill higher computational requirements. Additionally
we’re able to monitor physical properties and integral values like total
pressure of impeller stage, hydraulic efficiency of impeller or stage, shaft
power, and torque. All results can be exported to Excel or other formats for
further post-processing. In addition to starting the simulation manually as is
shown herem batch mode runs can be prepared to run performance curves or maps. We see here a converged solution for the steady state simulation. We
always recommend doing transient flow simulation to get a higher level of
accuracy in your predictions. We always recommend 3D CFD simulations to evaluate turbomachinery components designed in CFturbo. In recent years it has become
more and more affordable to combine turbomachinery design software and CFD
codes with optimization algorithms due to lower computational cost. In case design
adjustments are necessary, after evaluating the simulation results you
can go back to CFturbo and modify the fan interactively. Here’s a 3D printed prototype of the
impeller we just created! It was exported directly to a 3D printer from CFturbo.
There’s no need to use an external CAD system. This high quality impeller
prototype made of nylon is fully functional. It’s ready for testing. As
you’ve seen, CFturbo is a user-friendly straightforward solution for axial fan
design and 3d CFD simulation. To register for a free trial and to learn
more about our engineering services, visit CFturbo.com.

Leave a Reply

Your email address will not be published. Required fields are marked *