Tech Briefs Magazine - December 2022 - BET9

Holder
using derived surface tension 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
Glass
capillary
Liquid
droplet
A glass capillary with liquid droplet. (Image:
Inficon)
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 SAEpaper
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 water-ethylene-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
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 diameters were
adapted. With this test setup, it is possible
1000
sure regulator, a pressure difference to the
atmosphere of 1 bar, 2 bar, 3.5 bar, and 5
bar were set, which were read on the pressure
gauge in the middle of the adapter
construction. The different coolant compositions
are distilled water, a 1:1 solution
of distilled water and ethylene glycol (EG),
and 99 percent ethylene glycol.
Over time, different numbers of drops
develop at the glass capillary ends during
the experiment. These drops drip off once
reaching a certain size. The number of
dripping
drops
provides
information
about the leakage at the various capillary
diameters at the various pressures.
Experiments
We conducted experiments to determine
if theory fit practice when pressurized
fluid is expressed through a variety
to count the number of liquid drops from
different leak channel diameters.
The experimental setup shown in
Figure 2 consists of a liquid reservoir, a
water pump, several stainless-steel pipes
and adapters, an inspection glass with a
paddle wheel inside, a pressure gauge,
five glass capillary slots, a pressure regulator,
and several plastic tubes for connecting
the individual components.
By reducing the flow rate at the pres100
10
y
= 2.8E-05x4.6E+00
y = 5.4E-05x4.3E+00
y = 1.6E-05x4.6E+00
1
10
Capillary diameter [µm]
Overpressure 5.0 bar
Overpressure 3.5 bar
Overpressure 2.0 bar
Data fit to overpressure 5.0 bar
Data fit to overpressure 3.5 bar
Data fit to overpressure 2.0 bar
Figure 3a: Number of drops in 60 minutes from glass capillaries with 30 mm length and
different capillary diameters @ 2.0 bar, 3.5 bar, and 5.0 bar overpressure for pure distilled
water. (Image: Inficon)
Battery & Electrification Technology, December 2022
9
100
Number of drops in 60 minutes

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