58
pressurized with gaseous nitrogen to the desired pressure. The lubricant was then allowed to
obtain equilibrium with the gaseous nitrogen before the gas flowing through
the experiment was
switched over to the test gas. Though this initially proved fruitful, ASTM standard D2779-
92(2020) was consulted and it was noted that nitrogen solubility was on the same order of
magnitude as methane solubility for the temperature range under consideration as shown in
Figure 28.
Figure 28: Chart of Ostwald Coefficients at varying temperatures. Adapted from ASTM D2779-92(2020). Red box
shows temperature range investigated
This meant that rather than mixing the test gas with
a lubricant at high pressure, the test gas
was being mixed with a solution of lubricant and gaseous nitrogen. It was determined that this
procedure was not desirable as the test gas took many hours to equilibrate
with the lubricant-
59
gaseous nitrogen mixture while taking less than one-half hour to equilibrate the test gas with the
lubricant when no nitrogen precursor was used. Thus, the nitrogen in the experiment was
affecting how the dilution process occurred and made conditions dissimilar to what would be
seen in the field. The nitrogen precursor was abandoned after for further testing.
As it was still of interest to determine the relationship
between viscosity and pressure, the
primary lubricant studied in this work (Mobil Pegasus 805 Ultra) was pressurized with nitrogen
without the gear pump circulating the lubricant. This prevented the nitrogen
from fully mixing
with the lubricant and allowed for a determination of the impact of the pressure on
the lubricant’s
viscosity. The lubricant was then depressurized to ensure that there were no residual effects
from the nitrogen left in the lubricant. The lubricant’s viscosity was measured as the pressure
increased and decreased. Small variations in temperature were accounted for to discern the
pressure effects on viscosity. These tests were completed at temperatures of 50, 100, and
150°C and pressures up to 86.2 bara. Coefficients were determined at each
temperature for the
Barus equation which is given by:
𝜇 = 𝜇
0
𝑒
𝛼𝑃
Equation
42
Where
𝜇
0
is the viscosity at atmospheric pressure and
𝛼
represents the pressure-viscosity
coefficient (Barus, 1893). Use of the Barus equation allowed for the pressure dependency to be
effectively removed allowing for comparisons of dilution data collected at different pressures
with a comparison of the measured and fitted viscosity data shown in Figure 29, Figure 30, and
Figure 31.
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Figure 31: Estimated and measured viscosity-pressure dependence at 150°C for Mobil Pegasus 805 Ultra
Table 8: Coefficients determined for the Barus equation at 50°C, 100°C, and 150°C
Temperature
[°C]
Pressure-Viscosity Coefficient (
α
)
[bara
-1
]
50
0.0024
100
0.0019
150
0.0017
The coefficients shown in Table 8 were used to fit the data shown in Figure 29, Figure 30, and
Figure 31. The pressure-viscosity coefficients have the same order of magnitude as those
measured for other lubricants (van Leeuwen, 2009) and were used to
account for pressure
fluctuations in the primary lubricant studied in this work (Mobil Pegasus 805 Ultra). The study of
the dependence of viscosity on pressure is typically of interest for extreme pressures seen in
62
bearing contacts. The coefficients shown in Table 8 indicate that the pressures in this
experiment can increase the viscosity by up to 23%.
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