Study On The Generation Of Bubbly Oil And Its Viscosity Characteristic At High Shear Rate

Increasing the maximum working speed of bearings is the premise to ensure machine tools could be improved by a move towards high speed,high precision and low power consumption.However,due to the high viscosity of lubricating oil,the oil film will produce a lot of heat after shearing.Therefore,the operating speed range of oil-lubricated journal bearings is narrow.Bubbly oil lubrication is a new type of lubrication with gas-liquid two-phase flow containing micro bubbles as lubricating medium.Compared with pure oil,the viscosity of bubbly oil is lower,and its viscosity can be changed by adjusting the air volume fraction.The use of bubbly oil lubrication is expected to broaden the operating speed range of journal bearings.Therefore,the research on the viscosity characteristic of bubbly oil at high shear rate has scientific significance and engineering value.In this paper,the generation of bubbly oil,the observation of microbubbles as well as the measurement of air volume fraction and viscosity are studied.The main work contents are as follows:(1)Based on dispersed air method and pressure dissolved air method,a bubbly oil generation system is developed to realize the generation of bubbly oil with high air volume fraction as well as the adjustment of air volume fraction.(2)Based on the sampling method and camera method,a microbubble observation device is built to capture images of microbubbles in bubbly oil with different air volume fraction.(3)Based on digital image processing technology,the invalid information such as noise in bubble image is eliminated,the overlapping bubbles in images are segmented,geometric characteristics of bubbles are extracted from images,the distribution law of bubble size in bubbly oil with different air volume fraction is obtained,and air volume fraction is measured.(4)Based on the performance test bench for journal bearing,a high-speed rotary viscosity test bench is developed.The viscosity measurement of bubbly oil at high shear rate is carried out.The viscosity model of bubbly oil is established according to the measured value.Based on law of Newton inner friction,the theoretical model of equivalent viscosity of bubbly oil is modified by considering the effects of low viscosity of air,the effect of surface tension of microbubbles,bubble size distribution and the glue temperature characteristic of lubricating oil.The viscosity model before and after modification is compared with the measured value.The results show that: firstly,the bubbly oil generation system developed in this paper can adjust the air volume fraction by changing the oil tank pressure and cycle time.The air volume fraction increases with the increase of cycle time,but there is a limit value.The limit value of air volume fraction increases with the increase of oil tank pressure.Secondly,the bubble radius distribution in bubbly oil meets lognormal distribution,and the average bubble radius increases with the increase of air volume fraction.Thirdly,at high shear rate,the bubble surface tension has little effect on the viscosity of bubbly oil,microbubbles can significantly reduce the viscosity of bubbly oil,and the viscosity decreases with the increase of air volume fraction.Last but not least,at high shear rate,bubbly oil has shear thinning phenomenon,that is,the viscosity of bubbly oil decreases with the increase of shear rate.The research of this paper improves the generation mode of bubbly oil,reveals the influence of air volume fraction,shear rate and temperature on the viscosity of bubbly oil,and lays a foundation for the popularization and application of bubbly oil lubrication.