Effects of Bubble on the Viscosity Properties of Lubricant

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Proceedings of WTC2005 World Tribology Congress III September 12-16, 2005,Proceedings Washington, of WTC2005 D.C., USA World Tribology Congress III September 12-16, 2005, Washington, D.C., USA

WTC2005-64114WTC2005-64114

EFFECTS OF BUBBLE ON THE VISCOSITY PROPERTIES OF LUBRICANT

Ng, W. S. Levesley, M. C. Priest, M.

School of Mechanical Engineering School of Mechanical Engineering School of Mechanical Engineering The University of Leeds The University of Leeds The University of Leeds Leeds, LS2 9JT, UK Leeds, LS2 9JT, UK Leeds, LS2 9JT, UK

ABSTRACT Einstein [1] reported the increase of viscosity with gas volume Air ingestion and entrapment is prevalent in the lubricant fraction in a dilute suspension of solid particles: circulating around internal combustion engine and can lead to η = (1+ 2.5φ)η (1) formation of two-phase flow system referred to as bubbly oil l which affecting the oil viscosity and overall damping Taylor [2] derived a theoretical viscosity model which shared capability. Experimental work is conducted to study the the same conclusion with Einstein. effective viscosity of bubbly oil in a simple shear rheometer η = (1+ φ)ηl (2) with the aim of gaining a general understanding of the two- For Hayward [3], he experimentally derived the viscosity phase mixture flow and developing lubrication models to be model of bubbly oil as used in subsequent analysis. A laboratory simulator is used to (3) produce bubbly oil by controlling the system pressure, η = (1+ 0.15φ)ηl temperature, and gas content entrained from atmosphere and an All researchers shared the same conclusion that the bubbly oil enclosed rheometer is employed to measure the mixture viscosity increases with gas volume fraction or bubble content viscosity. A new detailed design is incorporated into the percentage (vol%). The only difference among those models is commercial rheometer with the purpose of maintaining the the coefficient of the gas volume fraction. distribution of bubbles within the lubricant. The transparent test chamber is initially pressurized with air to control the bubbly The aim of this research is to study the effective viscosity oil level and the bubbly oil viscosity is tested over a wide range of bubbly oil with the knowledge of bubble content using a of shear rates. Under shearing motion, the viscosity of bubbly commercial aerator and sealed cone-and-plate rheometer. oil varies with time and the results obtained are compared to Bubbly oil viscosity is predicted to be shear and time that of single-phase oil. dependence since bubbles are deformable under the rheometer shearing motion. Keywords: bubbly oil, viscosity, shear rate, gas volume fraction NOMENCLATURE INTRODUCTION η Bubbly oil effective viscosity, Pa.s Air entrainment phenomenon in journal bearings or ηl Lubricant viscosity, Pa.s squeeze film dampers has been reported for years and its effect φ Gas volume fraction on load carrying capacity is of significant concern. The effect is attributed to the formation of gas bubbles in lubricant. Bubbly METHOD oil is defined as two-phase flow where the mixture consists of The experimental setup consists of two main parts, an continuous liquid phase and discrete bubbles. The flow aerator and a rheometer. The aeration apparatus is behaviour of bubbly oil attracts the attention of variety of fields manufactured by IFT (Ingenieurgesellschraft fur of researchers because of its complexity exhibited. This is Triebwerkstechnik) mbH, Germany, and is designed for mainly due to the existence of bubbles has modified the supplying aerated oil. Oil is drawn from a heated bath by an properties, especially the viscosity as the main concern in electrically driven oil pump. The oil supply unit is equipped engine lubrication, of a single-phase unaerated lubricant. Much with an oil aeration unit to adjust the content of air in the oil at work in investigating bubbly oil viscosity has been done but the low pressure inlet to the pump. The pump discharges into a most of the models developed are not applicable in journal high pressure circuit (about 20 to 30 bars) to ensure complete bearing performance analysis. air dissolution into the oil and then through a pressure regulator

1 Copyright © 2005 by ASME to a lower pressure circuit to release air as bubbles in a controlled manner and achieve the pressure or flow necessary to feed the rheometer. In the progress of delivering bubbly oil 70.00 5 vol% into the rheometer, the gas volume fraction is measured by 10 vol% comparing the volume difference of (un)aerated oil collected in 60.00 15 vol% a cylinder. 20 vol% As shown in Fig. 1, in the rheometer, a metal platform base 50.00 Viscosity, mPa.s (4) is duplicated and the aerated oil is delivered in (Qin) and out 40.00 (Qout) from bottom tappings located in the lower base plate. The bubbly liquid is delivered into the test station under the rotating bore, which is cone-and-plate. In addition, a metal enclosure 30.00 plate (3) is used to close the test station and to house two 20.00 tappings; one for a pressure regulator and the other for a tube. Effective Average The pressure regulator and tube are needed to pressurize the 0 50 100 150 200 250 300 inner space of the enclosure to control the bubbly oil level and Shear Rate, 1/s to suck out the excessive bubbly oil respectively. A transparent Figure 2. Average effective viscosity for 5, 10, 15, and 50 vol% perspex enclosure wall (5) is installed to allow observation of aeration at 30°C under the shear rate range from 50 to 250s-1. potential phenomena such as bubble coalescence and large- scale flow patterns. To further improve the sealing effect, a low The first stage of experiment was carried out to obtain the motion resistance Teflon/Delrin ring (2) has been employed. single phase unaerated lubricant viscosity at 30°C as a

reference for bubbly oil. Viscometry measurement showed that

5, 10, 15, and 20 vol% aeration has increased the unaerated lubricant viscosity. Generally, the bubbly oil showed a shear- thinning characteristic where its viscosity decreases with shear rate. At low shear rate of 50s-1, the bubbles have significantly increased the lubricant viscosity; at high shear rate of 100s-1 onwards, the bubbly oil viscosity remains almost constant but is higher than the unaerated lubricant viscosity. In addition, the bubbly oil viscosity also increased with vol% at low shear rate of 50s-1 due to the bubbles shape remaining unchanged and increasing the resistance of flow. At higher shear rates of 100s-1 onwards, however, the applied

shear rate has brought the bubbles into interaction and breakup. Figure 1. Sealed rheometer This has been observed that the lubricant volume reduced when the measurement was completed. However the viscosity of Effective viscosities as a function of rotation rate are bubbly oil is still higher than that of unaerated oil. This could determined by comparing the torque required to shear the gap- be explained where small bubbles were observed at the centre filling bubbly oil to the torque required to shear pure lubricant of cone-and-plate when the cone-and-plate is lifted for next at the same shear rate and temperature. Single shear rate at measurement. particular temperature is applied for approximately 200s to investigate the time-dependent viscosity; average effective CONCLUSIONS viscosity at different shear rates is obtained by averaging the A simple experiment has been conducted to evaluate the time-dependent viscosity to view the shear rate effect on bubbly effect of gas content on the viscosity of lubricant. In general, oil viscosity. the existence of bubbles has elevated the single phase lubricant. However, the original structure of bubbles contained in Table 1. Test Parameters lubricant has been destructed due to the small gap setting Parameter Data (150µm) under cone-and-plate rheometer and the high applied Lubricant HVI 60 (single grade shear rate. From the data collected and observation, the bubbles hydraulic oil) appear to coalesce to form large bubbles under the cone-and- Lubricant viscosity 37.1 mPa.s at 30°C plate at the beginning of the viscosity measurement and Lubricant aeration at 1.013 bar 5, 10, 15, and 20 vol% breakup or expel under high operating shear rates. Cone-and-plate gas setting 150 µm Operating shear rate 50-250 s-1 REFERENCES [1] Einstein, A., 1906, “Eine neue Bestimmung der RESULTS Molekuldimensionen,” Annln Phys., 19, 289-306.