When it comes to cars, people tend to focus on driving performance
and safety. However, one should not forget about the air conditioning
system, which plays a crucial role in enabling a comfortable ride over
long periods, even in extremely hot or cold weather conditions or in
heavy rain. There was a time when air conditioning in cars, which
we now take for granted, was a luxury
only found in expensive cars. After
years of development and innovation,
all vehicles today are equipped with
air conditioning systems. However,
the design of such systems is not
simple since they operate at full
capacity in all weather conditions.
As with the vehicle body and various
other automotive parts, it is necessary
to evaluate the system’s performance
through experiments that assume
driving under real weather conditions
and through simulations using CAE.
This article introduces a case study on the waterproofing of an automotive air conditioning system for rainy
conditions using Particleworks, a particle method CFD software.
Air conditioning in cars inevitably requires ventilation inlets to
introduce external air. A waterproof design is necessary since
rainwater is likely to enter with the exterior
air. To evaluate the impermeability of an air
conditioning system, a simulation tool that
considers water droplets and free surfaces is
required. Using these tools usually requires a
high degree of skill. However, Particleworks,
which uses the particle method, is a simple
tool that non-professionals can use to study
waterproofing.
In many modern cars, the air conditioning
system, commonly referred to as an HVAC,
provides integrated air conditioning control
with heaters (heating), ventilation, and air
conditioning (air conditioning and cooling).
The heater heats the interior air of the vehicle with a radiator using
hot coolant from the engine, while the ventilation function controls
between recirculating internal air and introducing external air. In
addition, air conditioning sends cool air into the car through a
refrigeration cycle that uses a compressor and a condenser. This
simulation using Particleworks, focused on the ventilation portion of
the system.
Internal air recirculation, which is one of the ventilation functions,
only enables the recirculation of air inside the car without any
incoming air from outside. It is used to improve the cooling capacity
in scorching weather. It operates when the user deliberately blocks
polluted external air from entering, such as when driving behind a
large vehicle. However, there are some disadvantages, including
the tendency for windows to fog up due
to increased humidity in the cabin. There
is also the possibility that the driver’s
concentration may fall due to the lack
of oxygen because of decreased air
freshness.
On the other hand, external air induction
is a function that delivers fresh air from
outside the car into the cabin. By drawing
external air from the front of the vehicle
and expelling it from vents in the rear of
the vehicle, the air inside the car cabin
is refreshed. Most modern cars are equipped with automatic air
conditioners. In setting up their control, it is common to essentially
set the air conditioner to introduce external air, except in scorching
weather.
So, what do we need to pay attention to when introducing external air?
Since the HVAC system is located in the front of the vehicle, external
air is introduced from the front of the vehicle. However, the front of the
vehicle is also home to the engine compartment that generates heat
due to the combustion from the engine itself, and is also the site of
the heat dissipation from heat exchangers such as the air conditioning
condenser and radiator. This means that it is essential to introduce
fresh air that avoids these high temperatures. Therefore, as shown
in Fig. 1, it is common to introduce external air from the upper cowl
vents between the windshield and the hood. However, the problem
arises that water enters through these vents during rain. It is therefore
necessary to design a waterproofing system that separates the air
from the liquid, allowing only the air to enter the HVAC system and
preventing water from entering the passenger compartment.
In the Particleworks simulation, we first analyzed the rain patterns for
the full vehicle when parked to reproduce the overall rain situation.
Here, assuming that the car was parked on a flat surface, we were able
to confirm that the rainwater falling on the vehicle body flowed to the
top of the cowl across the windshield. Next, to evaluate the results in
more severe conditions, a rainfall analysis was performed with the
vehicle body inclined uphill at a 15-degree angle. This confirmed
that a large amount of rainwater flowed back to the car cabin from
the hood area, and mainly from the windshield when stopping the
car (Fig. 2). In other words, it was apparent that it was necessary to
design the top section of the cowl to allow fresh air to flow in and to
enable waterproofing of the cabin.
To evaluate the water flow into the top of the cowl in greater detail,
a simplified model as shown in Fig. 3 was used. The area enclosed
by the black dotted frame and surrounded by the windshield, hood,
and fenders, and including the exterior air inlet, is the subject of this
analysis. This model is well separated from the engine compartment
when viewed from the side and has holes on both ends for drainage. The
red frame seen on the inside of the model is the air intake for the HVAC
unit and we will study its design to prevent rainwater from entering here.
Fig. 4 shows the results of the water flow analysis at the top of the cowl
top with a 15-degree inclination. In this case, based on the results of
the whole car rain analysis, the mass flow rate conditions were set to
reproduce the flow from the windshield and hood areas, and the other
conditions were analyzed using the Particleworks default settings. We
were checking to see if water was entering the exterior air inlet. At this
15-degree inclination, it was confirmed that the water
flow from the hood was the inflow path to the top of the
cowl. We also found that the water that had accumulated
due to the slope was overflowing into the vicinity of the
external air inlet.
Having confirmed the above, it was found that there
are two possible routes for water to enter the external
air inlet: one being the path where raindrops that have
entered and dripped down enter as droplets, the other
being the water that has accumulated near the opening overflows
and enters when the car inclines. Various countermeasures can
be considered here, and the design plan will be discussed while
factoring in the cost.
In this study, we considered two countermeasures for the original cowl
top and conducted simulations. Countermeasure 1 adds a simple
wall to the bottom edge. Although it is basically necessary to prevent
all splashes, and a simple wall would not be sufficient, we decided to
only add a wall in countermeasure 1 to see what improvement could
be achieved at the lowest cost. In countermeasure 2, we examined
the shape of a cover to simultaneously prevent splashing and water
overflow. We modeled a part that inverted a typical range hood or
duct cover (see Fig. 5).
Fig. 6 shows a comparison of the initial shapes, countermeasure
1 and 2 and the simulation results, and Fig. 7 shows a graphical
comparison of the number of particles (the
amount of water) entering into the air inlet
with the initial shape, with countermeasure
1, and with countermeasure 2. We found that
countermeasure 1 prevented the water from
overflowing, but was not sufficient to prevent
droplets. In contrast, countermeasure 2,
a design proposal that adds a duct cover,
reduced water infiltration by about 90%
compared to the initial shape.
Using Particleworks to study the waterproofing
design of the external air intake of a car air
conditioning system, it is possible to visualize the event itself by
simulation, to clarify the problems and causes generated by the
initial shape, and study the countermeasures and their effectiveness.
As you can see from the simulation results, we were successfully
able to use Particleworks to improve the waterproofing performance
by preventing drops from entering the cabin.
The advantage of using Particleworks in this project was that it was
easy to model and simulate without any meshes. In particular, we
were able to quickly compare multiple design proposals, reflecting
simple countermeasure shapes.
Particleworks is expected to be used more widely in the future as a
tool that can quickly and stably simulate such various fluid behaviors.
Fig.7 - Comparison of water inflow for each countermeasure design idea
