EV Battery Innovation Special Report - November 2023 - 13
leak tightness during leak testing
with test gas. In this paper, the
smallest acceptable cross-section or
diameter of a leakage channel for
the coolant glycol is derived and the
leakage rate value to be assigned
for the test gas leakage test is given.
Compared to the requirements
regarding IP67, three essential
differences must be considered in the
application case of a coolant circuit.
In the coolant circuit, overpressure
of up to 5 bar prevails during operating
conditions, whereas the requirements
for IP67 generally consider an
effective force at the leakage channel
corresponding to a pressure difference
of 1100 mbar against 1000 mbar.
With increasing pressure difference
at the leakage channel, the leakage
rate increases correspondingly with
the same leakage channel geometry
and significantly more medium
leaks out than under the IP67 test
conditions and thus requirements
for test criteria for testing a coolant
circuit must be defined more strictly.
Furthermore, the temperature in
the coolant circuit is significantly
increased during operation, which
in turn affects the viscosity of the
medium. As the temperature increases
the viscosity decreases, which in
turn increases the leakage rate.
The difference in temperature from
room temperature to the operating
temperature in the coolant circuit
changes the viscosity by up to an order
of magnitude, which correspondingly
increases the leakage rate.
Third, the property of surface tension
or wetting angle of the liquid in a
leakage channel and its wall affects the
channel geometry of both the leakage
flow, which may be prevented due
to blockage of the leakage channel.
Thus, when setting rejection limits for
leak testing, the property of the liquid
medium used must be considered.
Theory of Leak-Channel
Behavior
The blocking of a leak channel with
a liquid, e.g., a water-glycol mixture,
depends mainly on the surface tension
EV BATTERY INNOVATION SPECIAL REPORT
(σ), the contact angle (θ) between
the solid-state material and the fluid,
and the maximum overpressure (p).
The leakage channel radius at
which a liquid can no longer escape
from a leakage channel is described
in the appendix. Using equation
(1), the leakage channel radius at
which the leak channel is blocked
by the liquid due to capillary forces
and prevents the liquid escaping
the tube may be calculated.
Holder
r = 2*σ*sinӨ
p
Where:
p = pressure inside the drop of liquid
σ = surface tension of the liquid
θ = contact angle Theta
r = radius of the leak channel
In the case of a cooling system,
the maximum overpressure (p)
varies typically between 2.5 to 5 bar
overpressure, depending on the cooling
system. The surface tension (σ) of pure
water and ethylene glycol is given with
72.7·10-3 N/m and 48.0·10-3 N/m.
In our experiments, glass
capillaries are used because only
glass capillaries are available in such
a range of small inner diameters. In
our previous SAE-paper a contact
angle for glass capillaries of 25° for
pure distilled water was used.
However, this paper describes
the wetting properties of different
liquids: pure distilled water, a waterethylene-glycol
mixture and pure
ethylene glycol. Thus, we use a
slightly different contact angle for
glass, substituting the contact angle
of quartz. Instead of a contact angle
for glass of 25° we used the contact
angle for quartz of 29° since glass
has a quartz content of 80 percent.
Using this equation, the
leakage channel radius at which
the glass (quartz) leak channel
theoretically is blocked by using
pure distilled water and pure
ethylene glycol can be calculated.
Figure 1 shows the results of the
blocked leak channel diameter at
different overpressures of a cooling
system using derived surface tension
Glass
capillary
Liquid
droplet
Figure 3: A glass capillary with liquid droplet.
(Image: Inficon)
and contact angles for pure distilled
water and pure ethylene glycol.
The contact angle (θ) between the
solid material, the liquid overpressure
(p) and the surface tension (σ) of the
liquid affects the blocking behavior
of a leakage channel. Thus, the hole's
diameter from which a leakage channel
blocks with a liquid essentially depends
on the material used, the surface
tension of the cooling liquid, and the
overpressure of the cooling system.
If the radius of the leak channel
is bigger than the calculated radius,
fluid will escape and a leak channel
has been formed. If the radius of the
leak channel is smaller or equal to the
calculated radius, the leak channel
is blocked by the fluid due to the
capillary force and no fluid will escape.
Thus, the cooling system is leak tight
if the radius of leak channel is smaller
or equal to this calculated radius.
Experimental Setup
To simulate a leaky cooling system,
a special test setup was developed
in which glass capillaries with a
length of 30 mm and different inner
NOVEMBER 2023 13
EV Battery Innovation Special Report - November 2023
Table of Contents for the Digital Edition of EV Battery Innovation Special Report - November 2023
EV Battery Innovation Special Report - November 2023 - Cov1
EV Battery Innovation Special Report - November 2023 - Cov2
EV Battery Innovation Special Report - November 2023 - 1
EV Battery Innovation Special Report - November 2023 - 2
EV Battery Innovation Special Report - November 2023 - 3
EV Battery Innovation Special Report - November 2023 - 4
EV Battery Innovation Special Report - November 2023 - 5
EV Battery Innovation Special Report - November 2023 - 6
EV Battery Innovation Special Report - November 2023 - 7
EV Battery Innovation Special Report - November 2023 - 8
EV Battery Innovation Special Report - November 2023 - 9
EV Battery Innovation Special Report - November 2023 - 10
EV Battery Innovation Special Report - November 2023 - 11
EV Battery Innovation Special Report - November 2023 - 12
EV Battery Innovation Special Report - November 2023 - 13
EV Battery Innovation Special Report - November 2023 - 14
EV Battery Innovation Special Report - November 2023 - 15
EV Battery Innovation Special Report - November 2023 - 16
EV Battery Innovation Special Report - November 2023 - 17
EV Battery Innovation Special Report - November 2023 - 18
EV Battery Innovation Special Report - November 2023 - 19
EV Battery Innovation Special Report - November 2023 - 20
EV Battery Innovation Special Report - November 2023 - 21
EV Battery Innovation Special Report - November 2023 - 22
EV Battery Innovation Special Report - November 2023 - 23
EV Battery Innovation Special Report - November 2023 - 24
EV Battery Innovation Special Report - November 2023 - 25
EV Battery Innovation Special Report - November 2023 - 26
EV Battery Innovation Special Report - November 2023 - 27
EV Battery Innovation Special Report - November 2023 - 28
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