Many electrical appliances around the world are installed outdoors
where they are continuously exposed to the elements. In designing
these appliances, a rain test is performed in a shower to evaluate
the effectiveness of the waterproofing and to examine the possibility
of water penetration and damage to important internal components.
However, while these tests can determine whether water has
penetrated the product, it is not easy to determine where it entered.
In order to develop these products more efficiently, numerical
simulation can be used to predict the intrusion of rainwater and its
path into the product so that appropriate design changes can be
made and waterproofing measures can be taken before prototyping to
improve product performance.
Rainfall or the flow of agglomerated raindrops on a product represent
a complex free-surface phenomenon. Particleworks, a particle-based
computational fluid dynamics (CFD) software that can efficiently
process water droplet behavior and free-surfaces, is well suited to
simulating such problems.
In this article, we will introduce a simulation of precipitation on the
outdoor unit of an air conditioner as a concrete example of dealing
with such a problem. Initially, the problem was posed as a challenge
by the manufacturer, Blue Star Limited in India, where it often rains
heavily. In recent years, torrential rains, which are considered to be
an abnormal weather condition resulting from climate change, have
occurred every year in many parts of the world. As a result, appliances
installed outdoors need to be more waterproof than ever before.
These waterproofing measures extend the life of the product by
preventing critical electronic components from being exposed to
water, compromising product functionality and also prevent corrosion
of internal components if they are exposed to water. For outdoor
units, this includes reducing the ingress of water through the large
front grille openings used to control airflow as much as possible.
Numerical simulations play an important role in detailing the behavior
of these raindrops, identifying the path of water penetration into the
product, and using this information for waterproofing measures.
Fig. 1 shows the actual precipitation test. Assuming torrential rain, the
amount of rainfall is set to 80mm per hour. The turntable on which the
product is placed rotates slowly, allowing raindrops to hit the product
from all directions, while two blowers behind the test chamber
constantly blow strongly. Fig. 2 shows the model geometry prepared
to evaluate the product by simulation under these conditions.
Fig. 2 - Geometry of the simulation model
Overall, the dimensions of the product are approximately 860mm
wide, 540mm high, and 280mm deep. Although the actual product
is composed of many components, the simulation models only the
major components that affect raindrop behavior: enclosure, fans,
turntables, and electronic components to be assessed for water
damage.
Particleworks does not require meshing and the CAD data itself can
be used directly for simulation, so there is no need to spend a lot of
time on pre-processing. Importantly, the product has a small gap
above the handle through which raindrops may enter. The width of
the opening is only 0.95mm, but since it is close to the electronic
components, the possibility of raindrops entering from here cannot
be ignored.
The water sprinkling from the shower was set as an analysis condition.
In the test, three rows of showerheads were installed at a height
of about 2 meters from the product. However, only the water from
two rows of showerheads had an effect, so eight showerheads were
modeled for the two rows that would be needed for the simulation.
In Particleworks, we created an inlet above the showerheads and set
a flow rate of 80mm per hour, similar to the test. Considering the
precipitation and the area of the sprinkler holes, we defined the flow
velocity as 2,000mm/s. As boundary conditions, the fan speed inside
the outdoor unit was set to 3,600rpm, and the turntable speed was set
to 2.3rpm, estimated from the test video.
We also had to consider the effect of airflow. In order to reproduce
the actual rainfall test, we had to consider the effects of the airflow
generated by the fan and its rotation on the trajectory of the raindrops
inside the outdoor unit. Such an airflow field can be obtained by
modeling the air around the outdoor unit with particles, but this is not
efficient in practice due to the large number of particles.
In this study, the results of the airflow field calculated by general
CFD software were imported into Particleworks as a set of spatial
coordinates and velocity vectors in CSV format, and the airflow field
was examined as shown in Fig. 3. This airflow field can
be used to calculate the state of the raindrops under
the influence of wind as a so-called one-way coupling
simulation.
The water falling on the entire outdoor unit of the air
conditioner was modeled with 2mm diameter particles
for practical reasons of calculation time. However, this
model geometry has a 0.95mm gap above the handle,
so a smaller particle size had to be set to model the
particles passing through this gap.
Particleworks provides a feature called “zooming” that allows the
spatial resolution, or particle size, of certain areas within the analysis
space to be fine-tuned. The zooming function is also intended to
reduce the computational load. As shown in Fig. 4, in this simulation,
a single particle with a diameter of 2mm that reaches the vicinity of the gap is replaced with approximately 500 particles with a diameter
of 0.25mm, and a high-resolution calculation without partial increase
in the number of particles in the entire analysis space is performed.
Fig. 5 shows the simulation results. Water was sprinkled from the
eight showerheads and fell onto the outdoor unit. The test confirmed
that the surface of the electronic component box was covered with
water. The simulation also shows that water droplets adhere to the
box.
Fig. 5 - Comparison between the test and simulation results
As shown in Fig. 6, the raindrops that penetrated the outdoor unit
reached the top of the electronic component box and flowed through.
Using the post-processing function, it is possible to visualize the
infiltration path of the raindrops more
clearly by plotting the streamlines of
the raindrop particles that reached
the top of the electronic component
box (Fig. 7).
Here, we plotted the streamlines
of several particles and found that
there are two major infiltration paths.
One is the path shown by the green
line, where particles entering the
enclosure through the front grille
hit the rotating fan blades and are
blown away towards the top of the
electronics box. The other is the path indicated by the pink line, where
the fan’s airflow also blows particles entering through the grille up
towards the electronics.
Next, we focused on the behavior of the raindrops around the small
gap. As mentioned earlier, we used the zooming function here to
create particles with a 0.25mm diameter. Looking at the aperture from
the inside, we could see that raindrops were entering through the gap,
as shown in Fig. 8.
Therefore, the ability to visualize the ingress path of the raindrops,
which is difficult to observe in the actual test, is a great advantage of
simulation.
As mentioned earlier, it is not easy to identify the path of water
penetration into the product simply by testing the actual product and
then applying waterproofing and water damage countermeasures in
the early stages of design.
Fluid simulation with Particleworks can be applied to the design and
development of various home appliances to apply waterproofing and
water damage countermeasures more accurately and efficiently.
The simulation presented in this paper uses the 3D CAD data of the
outdoor unit of an air conditioner, actual test results, and photographs
provided by Blue Star (India). This study was also presented at the
International CAE Conference held in November 2020.
Newsletter EnginSoft Year 18 n°1
By Indraneel Samanta | R&D, BlueStar Ltd
Sunao Tokura and Akiko Kondoh | Prometech Software, Inc.
