EV Battery Innovation Special Report - November 2023 - 15
The number of drops as a function
of the overpressure shows a good
linear correlation as expected from
the Hagen Poiseuille law for liquids.
In a second experiment, the dripping
behavior of the various capillaries
with a solution of distilled water
and ethylene glycol in a mixing ratio
of one-to-one at various operating
pressures at 20° C was investigated.
It can be seen that the number
of drops has clearly decreased in
contrast to the previous measurement
with pure distilled water.
Here, within the measuring time
of 60 minutes, not a drop fell from
the 10μm capillary for every pressure
difference. In the case of the 5μm
capillary, no drop formation could
be determined visually. Transferring
the measurement results into a
double logarithm scaled diagram
(Figures 3a and b), the dependence
of the drop rate to the fourth power
of the radius can also be seen in a
good approximation, just like the
influence of the pressure in the system
which is described in the theory.
Water has a particularly low
dynamic viscosity of 1.002 mPa/s
and pure ethylene glycol has a high
dynamic viscosity of 19.83 mPa/s
at a temperature of 20° C. A waterethylene
glycol solution of 1:1 (50
percent volume distilled water and
50 percent ethylene glycol) has a
dynamic viscosity of 4.1 mPa/s.
Summary and Conclusions
The measurements show very
clearly that the enumerated liquid
droplets or liquid leakage rate is
linearly dependent on the pressure
difference, with the pressure always
being at atmospheric pressure. This
linear relationship is very clear in
Figure 4. In addition, it is easy to
see in the graph that the magnitude
of the leak rate decreases as the
viscosity of the liquid increases.
The leakage rate for glycol shows a
factor of 10 times lower and for the
50/50 water glycol mixture a factor
of 2.3 lower than for pure water, all
other parameters being equal.
EV BATTERY INNOVATION SPECIAL REPORT
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100
80
60
40
20
1
Water drops
Water-Glycol drops
Glycol drops
2
3 45 6
Overpressure cooling system [bar]
Data fit to water drops
Data fit to water-glycol drops
Data fit to glycol drops
Figure 4: Number of drops as a function of the pressure difference of a Ø25 μm diameter glass
capillary with 30 mm length. (Image: Inficon)
As the operating pressure of the
coolant in the cooling circuit increases,
the amount of liquid leakage
increases linearly. Temperature
influences leakage rate because the
viscosity of the coolant decreases
with increasing temperature. For
example, the leakage rate for glycol
as a coolant increases by a factor of
five when the operating temperature
rises from 20 °C to 80 °C. This is true
over long periods of time. In terms
of longer periods of time, even a few
drops of loss over the time scale of
minutes or hours mean significant
loss of coolant during a year and
should be avoided accordingly.
However, in the case of the cooling
circuit of a combustion engine under
operating conditions, especially
at high operating temperatures,
losing a few drops of coolant
does not have a harmful effect
on the immediate environment
because of the evaporation rate.
Unfortunately, the same cannot be
said of battery cells, modules, and
packs. The harmful effect of even a
small amount of liquid, droplets or as
water-bearing vapor, is much more
dangerous to any traction battery
module or pack. There, the liquid
coolant or water vapor can destroy
battery cells or generate a short
circuit. The leak test of cooling circuits
of traction battery packs and for
fuel cells must be done with higher
reliability compared to tests of cooling
systems of combustion engines.
The corresponding limiting leakage
rates for such leakage channels are
presented in the paper; the leakage
rates specified here for gas pretesting
are an order of magnitude
significantly below the detection
limit of classical test methods
such as pressure decay or mass
flow leak testing. However, when
using modern test gas methods
with forming gas or helium as
the test gas, reliable detection of
critical leakage rates is possible.
This article was written by Marc
Blaufuß, Application Engineer, Leak
Detection Tools, and Daniel Wetzig,
Research Manager, both at Inficon
GmbH (Köln, Germany). This article
is a condensed and edited version of
SAE Technical Paper 2022-01-0716. For
more information, visit www.sae.org/
publications/technical-papers.
NOVEMBER 2023 15
Number of drops in 60 minutes
https://sae.org/publications/technical-papers
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
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