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In the present work, the performance of oil-air two-phase flow under different lubricant oils was investigated. The simulation method was applied to study the influence of the oil viscosity on the flow pattern, velocity distribution and Re number in oil-air lubrication by FLUENT software with VOF model to acquire the working performance of oil-air lubrication for high-speed ball bearing. This method was used to obtain the optimum lubrication conditions of high-speed ball bearing. The optimum operating conditions that produce the optimum flow pattern were provided. The optimum annular flow was obtained by PAO6 oil with the low viscosity. Reynolds number influences the fluid shape and distribution of oil and air in pipe. The annular flow can be formed when Reynolds number is an appropriate value. The velocity distribution of oil-air two-phase flow at outlet was also discussed by different oil viscosities. The simulating results show that due to the effect of the oil viscosity and flow pattern the velocity decreased and expanded gradually close to the pipe wall, and the velocity increased close to the central pipe. The simulation results provide the proposal for the design and operation of oil-air two-phase flow lubrication experiments in the present work. This work provides a useful method in designing oil-air lubrication with the optimum flow pattern and the optimum operating conditions.

High speed machine tool has become the mainstream of metal cutting in machining aerospace parts, dies and moulds for finishing aluminum alloys, titanium alloys and hardened steel alloys because high speed machining is a promising technology for drastically increasing productivity and reducing production costs [_{3}N_{4} ball bearing has been widely used to support high speed ceramic rolling contact bearings or super high speed spindles of high-speed machines tool for high speed cutting because silicon nitride bearings could offer significant benefits for the rolling contact fatigue life and the low density of the material greatly reduces the dynamic loading at ball/raceway contacts in very high speed applications such as machine tool spindles [

Numerical simulation has been developed widely in an attempt to rationalize lubrication optimization, design evaluation and prediction of oil-air lubrication. It is an effective and economical method for analysis of oil-air two-phase flow. Oil-air two-phase flows are encountered in various industries. The requirement for the optimization of the operating conditions and assessment of safety operations leads to the need for the detailed information on the influences of the physical properties of oil on the flow characteristics of oil-air two-phase flow. Our previous work shows that flow pattern of oil-air two-phase flow is affected by the operating parameters such as velocity and lubrication in oil-air lubrication system [

Computational fluid dynamics (CFD) is applied to simulate the complicated flows, which has grown from a mathematical curiosity to become an essential tool in almost every branch of fluid dynamics [

1) Materials

Air and poly-alpha-olefin (PAO) oil are used as gas and fluid in oil-air lubrication system, respectively. To ensure the reliability of the mechanical parts used under extreme conditions of high speed, high load and extreme temperature, PAO oil is chosen as lubrication oil for high speed roller bearings. PAO oil is a hydrocarbon. PAO oil is a type of synthetic lubricant having excellent properties such as low volatility, high thermal stability and viscosity index, excellent hydrolytic stability and compatibility with a variety of base oils, which has been widely used in the engine oil.

Density (kg/m^{3}) | Viscosity (Pa∙s) | |
---|---|---|

Air | 1.225 | 1.8 × 10^{−5} |

PAO2 | 797 | 6.4 × 10^{−}^{3} |

PAO6 | 827 | 2. 6 × 10^{−2} |

PAO10 | 835 | 5.5 × 10^{−2} |

PAO25 | 841 | 2.0 × 10^{−}^{1} |

PAO40 | 850 | 3.4 × 10^{−1} |

PAO100 | 853 | 1.0 |

respectively.

2) Physical Model

The laws of fluid flow are the basis on establishing the fluid movement equations. There are three equations including the control of fluid flow, heat and mass transfer mathematical formulas. The governing equations of oil-air two-phase system were derived from the continuity and momentum equations. Governing equations include mass conservation equation, conservation of momentum equation and energy equation. The governing equations are listed in the following.

a) Mass conservation equation

b) Conservation of momentum equation

c) Conservation of energy equation

where _{x}, f_{y} and f_{z} is mass force, e is internal energy,

The flow pattern maps are used to predict two-phase flow regimes in horizontal tubes. The horizontal pipe has 500 mm in length and 10 mm in diameter. The end of the pipe is the entrance of air and another end is outlet for the mixture phase. The entrance of oil is located the distance of 20 mm far from the entrance of air and perpetual with the inlet of air. The inlet of oil is 4 mm in diameter. Three dimensional models were built with the grid generation program. The model is meshed when the physical model is built by the hexahedron grid, as shown in

The solver is the pressure-based solver, which is suitable for low velocity and incompressible flow. The transient state parameter is applied to obtain the flow pattern map in the time option.

VOF (volume of fluid) model is used to analyze oil-air two-phase flow in the horizontal pipe. The flow patterns of oil-air two-phase flow and the physical properties like the viscosity and density of oil and air are identified. For all flow patterns, annular flow is characterized by a thin liquid film covering the wipe wall and a central gas core loaded with smaller droplets [

The pressure is atmospheric pressure. Velocity of air and oil at the inlet of pipe is the initial velocity, respectively. Pressure at the outlet of pipe is zero. The turbulence intensity is referred as the ratio of velocity of average flow velocity. The diameter of pipe is the diameter of water power which represents the limit of turbulence flow. The oil viscosity and density of PAO oil are set in the material option in FLUENT software.

The velocity of PAO oil is 10 m /s and the velocity of air is 0.05 m /s at inlet, respectively.

The influence of the oil viscosity on flow pattern of oil-air two-phase flow is investigated by six kinds of PAO oil with different viscosities.

The annular flow of flow pattern cannot format under PAO2 oil, and the annular flow of flow pattern form with the increase of the oil viscosity. However, the oil quantity in annular flow becomes large with the increase of the oil viscosity. And the annular flow becomes bad when PAO25 oil is chosen as oil phase. The oil is put on the top of oil wall when PAO100 oil is chosen as oil phase; there is no annular flow at all in the wall. The same phenomena were observed when the velocity of air increase to 100 m/s, as shown in

The viscosity of the oil affects the lubrication condition. Oils with a low viscosity should be avoided, since at high loads the load-bearing capacity might no longer be sufficient, thereby reducing the service life of the bearings. Grease lubrication and lubrication are both commonly used in rolling bearings. However, when the temperature and operating speed are high, the base oil of the lubricating grease may evaporate and be oxidized, and lubrication performance may deteriorate rapidly. For this condition, the lubrication performance of bearings may be improved by using circular lubrication, atomization lubrication and oil spray lubrication. When operating speed of bearings is very high, bearings must have a sufficient, but not excessive amount of oil to ensure that the bearing is sufficiently lubricated and exorbitant temperature. Oil spray lubrication is an especially effective method in such situations. In oil spray lubrication, oils are injected into bearing from one side of the roller cage shatter. When bearing operates in high speed, the high strength gas flow is formed around the rolling element and the roller cage shatter through which oil is sent to bearings using ordinary lubrication methods. Therefore, oil spray lubrication can be used, and the speed of oil must reach certain value so that it can drill through the hydraulic flow layer under high speed conditions. In this study, the numerical simulation model of the

oil spraying system is established, with spraying speed, spraying angle, oil pressure, oil viscosity, structure of roller cage shatter, rotating speed as the main parameters of the model. By optimization, the best parameters for oil spraying were obtained which can meet lubrication requirement of highs-speed rolling bearing. The velocity increases in the rotational direction, and the maximum value appears at the top of roller. The flow field was influenced mostly by the velocity boundary due to hydrodynamic action of the air flow. The pressure long the diametrical direction decreases with the rotating speed of bearings and the distribution regularity of pressure remains the same. It is concluded from the simulation results that PAO6 oil or other oils with the viscosity close to PAO6 oil are proposed to use the lubricants in oil-air lubrication.

The influence of Reynolds number on flow pattern of oil-air two-phase flow is investigated under different air velocities conditions. In hydromechanics theory, Reynolds number is usually used to characterize the analogy number (Re) about the viscosity effect, as shown in Equation (6).

where,

The influence of the oil viscosity and Reynolds number on flow pattern of air-oil two-phase flow for high-speed rolling bearing is investigated with FLUENT software and the simulation results are presented. The viscosity of PAO oil influences flow pattern of oil-air two-phase flow. The optimum annular flow of flow pattern is obtained by PAO6 oil with the low viscosity. It is concluded that PAO6 oil or other oils with the viscosity close to PAO6 oil are proposed to use

the lubricants in oil-air lubrication. Reynolds number influences the fluid shape and the oil distribution in two-phase flow pipe. The annular flow of flow pattern is formed when Reynolds is appropriate. The velocity of oil and air distributes non-uniformity in oil pipe with different oils and is much larger than that of oil and air at inlet. The annular flow pattern results in a concentric circular velocity distribution at outlet, which is beneficial to maintain the annular flow.

The present work is financially supported by the grant from National Natural Science Foundation of China (No. 51305331, 51675409).

No potential conflict of interest was reported by the authors.

Zeng, Q.F., Zhang, J.H., Hong, J. and Su, W.J. (2017) Numerical Investigation on Flow Pattern of Air-Oil with Different Viscosities Lubrication. Engi- neering, 9, 1-13. http://dx.doi.org/10.4236/eng.2017.91001