INFLUENCE OF PRESSURE ON FERRO-OIL DYNAMIC VISCOSITY
In this paper authors present the results of dynamic viscosity measurements of ferro-oil in aspect of pressure changes. The investigation concerned the ferro-oil with 1.4% concentration of magnetic nano-particles in the base oil, which satisfies SAE 15W-40 standard. The dynamic viscosity measurements were performed on the Thermo Scientific Haake Mars III rheometer and by applying the measuring system called a pressure chamber. The measurements were made at ferro-oil temperatures: 30, 60 and 90 degrees of Celsius, in the range of shear rates from 15 to 200 [1/s] and at gauge pressures: 0, 10, 40, 70 and 100 bars. The results show, that the pressure has an significant impact on the ferro-oil dynamic viscosity, because the increase in pressure of ferro-oil causes a significant increase in its viscosity value. The correctness of the results obtained using a pressure chamber at zero overpressure, was verified by comparison with the results obtained with the cone-plate configuration.
Keywords: dynamic viscosity, ferro-oil, pressure influence, rheometer.
INTRODUCTION
One of the most important physico-chemical properties of lubricating oils is their viscosity. Along with other significant oil properties, as: low temperature fluidity, lubricity or density, which are separated group of the rheological properties, viscosity is the important quantity for the description of lubricants quality and also for comparing their tribological properties. Broadly defined viscosity is a measure of a internal friction of fluid and often is determined as the dynamic viscosity. In this study, authors present the results of dynamic viscosity measurements, performed for the selected ferro-oil with dependence on the pressure changes. This investigation concerned the ferro-oil, but with the absence of a external magnetic field. Changes in the viscosity values of some oils and ferro-oils, in dependence on a temperature, shear rate and concentration of magnetic particles in the ferro-magnetic oil, either under the presence of an external magnetic field and the absence of it, have been measured and described in the previous works of the authors [1, 2, 3, 4, 5, 6]. There can also be found the work of some authors, on the impact of aging on the oil properties [8].
Ferromagnetic properties of ferro-oils can be described by the quantity called “a magnetic susceptibility”. The addition of magnetic particles into base oil causes significant change of the rheological properties of ferro-oil. The presence of
magnetic particles in ferro-oil, makes it possible to change and control its dynamic viscosity, what can be very important and useful in practical applications, for example in slide bearings [7]. Moreover, the position of the ferro-oil can be adjusted by the controlling the magnetic field, what could be very useful in the absence of gravity. Then, ferro-oils have good anti-vibration properties.
The hydrodynamic lubrication of slide bearings is a theory, which describes behavior of lubricating oil in a slide bearing gap. For numerical simulations, which concern these issues, a accurate input data is required. Studying the impact of lubrication with unconventional oils, such as ferro-oils, requires knowledge about the tribological properties of the above mentioned lubricants. Taking into account the impact of such parameters, as pressure, share rate, temperature or concentration of magnetic particles, on the dynamic viscosity of oils and ferro-oil in such calculations, allows to obtain more accurate results, more closely reflect to the real values, therefore this work is focused on determining the changes of ferro-oil viscosity value, with accordance to the pressure changes. Of course there are more parameters, which could be taken into consideration. Some of such parameters are described in papers [8, 9].
MEASUREMENTS AND RESULTS
The investigation concerned the ferro-oil with 1.4% concentration of magnetic nano-particles in an base oil, which satisfies SAE 15W-40 standard. The same measurements for base oil with similar properties were described in paper [2]. This oil was also used as a base oil of ferro-fluids, investigated in papers [3, 4, 5, 6]. The dynamic viscosity measurements were performed on the Thermo Scientific Haake Mars III rheometer and by applying the measuring system called a pressure chamber, which is shown in Fig. 1, with the D100 rotor, shown in Fig. 2.
Such setup allows to determine the change in viscosity of the sample at varying pressure. Maximum allowed pressure (overpressure) in that system is 100 bars.
The dynamic viscosity measurements were performed in the constant rotation (CR) mode, so the rotation speed has been set to a constant value and the torque was measured. The measurements were made at temperature 30, 60 and 90 degrees of Celsius, in the range of shear rates from 15 to 200 [1/s] and at gauge pressures: 0, 10, 40, 70 and 100 bars. It is worth mentioning, that the rotor D100 is mounted in the pressure chamber on two sapphire bearings and is magnetically coupled with the element rigidly attached to the motor of the rheometer.
Fig. 1. The Thermo Scientific Haake Mars III rheometer with the pressure chamber
The water jacket, with water as a medium, is used for temperature control of sample oil in the pressure chamber. The Haake Mars III is a high end rheometer, but despite this, the accuracy of the results, obtained by using the described above measuring system, depends on the preparation of the device by the operator. During a calibration, before proper measurements, a researcher must take into account the effect of temperature on the magnetic interaction of the rotor and also the optimal distance between the permanent magnets should be determined. Next, it is important to perform tests for determination of the effect of non-ideal alignment of the rotor and also determination of the influence of friction forces, which occur in the rheometer bearings, which are lubricated with the test sample, but the when pressure chamber is empty, i.e. without sample. Then, it is necessary to recognize the impact of friction in the sapphire bearings of the rotor, and to calculate some corrective parameters. Such calibration should be performed for each sample. In connection with the above issues, in order to verify the dynamic viscosity values obtained for the ferro-oil at ambient pressure, additional measurements were performed using a cone-plate system, which can be used to study the dynamic viscosity of the oil at ambient pressure and with a very precise
and quick setting and measuring the sample temperature (Peltier cells). Volume of the sample in this case it is only 1 ml, while in pressure chamber 50 ml. Comparison of the results obtained using the cone-plate system with the results obtained with the pressure chamber system at pressure 0 bar (i.e. ambient pressure) is shown in Fig. 3.
Fig. 2. The rotor D100 used in the pressure chamber
Fig. 3. Comparison of the results: pressure chamber vs. cone plate system at different temperatures (overpressure p = 0 bar)
As shown in the graph above, the averaged results obtained with the use of the pressure chamber, are just slightly different from the results obtained with the cone-plate system (difference less than 5%), which may indicate a correct preparation and calibration of the pressure chamber for the measurements of dynamic viscosity of ferro-oil at different pressures.
The viscosity measurements with the pressure chamber were made at gauge pressures 0, 1, 10, 40, 70 and 100 bars. Each measurement was made for new sample of investigated ferro-oil, after the previously performed calibration process.
Graphs show results for
the average value of five measurements.
In Fig. 4 is shown the dynamic viscosity of investigated ferro-oil at temperature t = 30°C for different pressures.
Fig. 4. The viscosity vs. shear rate dependency for investigated ferro-oil at different pressures and temperature t = 30°C
During the measurements, the rise of temperature of the sample, may occur due to the viscous heating, especially at higher shear rates. The measurement of sample temperature was made not directly in the sample but in the material of the pressure chamber. This affected on the accuracy of the results and on the nature of the obtained viscosity curves. However, basing on obtained results, it can be noticed, how significant is the effect of pressure on the viscosity of ferro-oil. The overpressure 100 bar resulted in an increase of the viscosity of approx. 70% in the tested shear rates range.
Fig. 5 shows results for investigated ferro-oil at temperature t = 60°C. In this case, the obtained values and viscosity curves have uneven nature and there are some noticeable deviations from the trend line. This is the result of leakage of the sample from the pressure chamber as well as attempts to compensate for the pressure drops. In such cases, there were attempts to compensate the pressure drop by pumping it into chamber.
In this case, it can be also seen, that increase of ferro-oil pressure causes significant increase of its viscosity.
In Fig. 6 is shown the dynamic viscosity of investigated ferro-oil at different pressures and temperature t = 90°C. For this case, noticeable is relatively large decrease in viscosity in accordance to a shear rate, in relation to the characteristics obtained at lower temperatures. This is an effect of the viscous heating, which, at a temperature of 90°C becomes more noticeable, because the viscosity of the ferro-oil at this temperature is significantly lower than, for example at 30°C and 60°C.
Fig. 5. The viscosity vs. shear rate dependency for investigated ferro-oil at different pressures and temperature t = 60°C
Fig. 6. The viscosity vs. shear rate dependency for investigated ferro-oil at different pressures and temperature t = 90°C
CONCLUSIONS
In this research authors investigate the influence of pressure variations on the dynamic viscosity of the ferro-oil. The investigation concerned the ferro-oil with 1.4 % concentration of magnetic nano-particles in an base oil, which satisfies SAE 15W-40 standard. The measurements were made for the ferro-oil, but with absence of a external magnetic field. The results show, that the pressure of ferro-oil has a significant impact on its viscosity. Furthermore, comparison of the obtained results, with the results presented in the paper [2], which concerned the viscosity dependence on pressure only for base oil, allows us to conclude, that just the addition of the magnetic nano-particles causes the increase of dynamic viscosity values of this oil.
In paper [2], the authors expressed their concern about the problem “of the impact of the magnetic field generated by the rotor magnets on the sample properties”, but in this research, the authors have verified, by measuring the magnetic field around a permanent magnets of rotor, that these magnets are built in such a way, that the magnetic field does not penetrate the volume of the sample and the influence of magnetic field can be neglected.
Further studies are planned to be performed for ferro-oils but with the influence of the controlled external magnetic field, also in a wider range of shear rates. Unfortunately, tests in the pressure chamber require the use of a relatively large volume of oil sample, which by occurring leakage and other factors, such as the remaining portion of the sample volume on the walls of the chamber and other elements or losing the sample volume while reducing the pressure, becomes impossible to recover all volume of sample. Thus, the investigation of ferro-oils properties in pressure chamber are difficult due to economic reasons.
The results will be used in the studies of hydrodynamic lubrication of slide hydrodynamic bearings and for determining the constitutive relationships for ferro-oils. The results shown, e.g. in the work [7], prove that the values of oil pressure, generated in the hydrodynamic bearing gap, have relatively high values, therefore taking into account the pressure dependence of the viscosity can be crucial to achieve the required accuracy of the results of simulations on the hydrodynamic theory of lubrication of slide bearings.
REFERENCES
1. Czaban A., The Influence of Temperature and Shear Rate on the Viscosity of Selected Motor Oils, Solid State Phenomena, 2013, Vol. 199, p. 188–193.
2. Czaban A., Frycz M., Influence of Pressure on Dynamic Viscosity of Oil, Journal of KONES Powertrain and Transport, 2013, Vol. 20, No. 2, p. 49–54.
3. Czaban A., Frycz M., Horak W., Effect of the Magnetic Particles Concentration on the Ferro-Oil’s Dynamic Viscosity in Presence of an External Magnetic Field in the Aspect of Temperature Changes, Journal of KONES Powertrain and Transport, 2013, Vol. 20, No. 2, p. 55–60,
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7. Miszczak A., Analiza hydrodynamicznego smarowania ferrocieczą poprzecznych łożysk ślizgo-wych, Fundacja Rozwoju Akademii Morskiej, Gdynia 2006.
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WPŁYW CIŚNIENIA NA LEPKOŚĆ DYNAMICZNĄ FERROOLEJU
Streszczenie
W pracy autorzy prezentują wyniki pomiarów lepkości dynamicznej wybranego ferrooleju w zależ-ności od ciśnienia. Badania dotyczyły ferrooleju, w którym udział cząstek ferromagnetycznych wynosił 1,4% w oleju bazowym, który to spełnia normę SAE 15W-40. Pomiary lepkości dynamicznej zostały wykonane przy wykorzystaniu reometru Haake Mars III firmy Thermo Scientific wraz z układem pomiarowym służącym do badania lepkości dynamicznej olejów przy ciśnieniu wyższym niż ciśnienie otoczenia (tzw. komora ciśnieniowa). Pomiary przeprowadzono przy temperaturach ferrooleju: 30, 60 i 90 stopni Celsjusza, w zakresie szybkości ścinania od 15 do 200 [1/s] oraz przy ciśnieniach: 0, 10, 40, 70 i 100 bar. Wyniki pokazują, że ciśnienie ma wpływ na lepkość dynamiczną oleju, ponieważ zwiększanie ciśnienia ferrooleju powoduje znaczący wzrost wartości jego lepkości. Poprawność otrzymanych wyników przy wykorzystaniu komory ciśnieniowej przy zerowym nad-ciśnieniu została sprawdzona poprzez porównanie z wynikami otrzymanymi na układzie płytka-stożek. Słowa kluczowe: lepkość dynamiczna, ferroolej, wpływ ciśnienia na lepkość dynamiczną, reometr.
