4.1 Visualization of the air-oil flow field

The visualized flow field with the corresponding simulations inside the bearing at three different speeds is shown in Figure 4. The cooling oil is injected in a rate of 0.15 l/min from a 0.5 mm nozzle that leads to a jet velocity around 12 m/s. The bearing contrarotates at speeds of 500, 2000, and 4000 r/min, respectively. It is seen that all the rolling elements are evenly covered with the cooling oil, indicating

Figure 4. Cooling oil distribution at different speeds: (a) measured results and (b) simulated results.

a well-distributed oil film at the speed of 500 r/min. When the bearing speed rotates at 2000 r/min, some of the rolling elements become apparent, indicating a decreased film thickness at a higher speed. Moreover, there are no noticeable bubbles observed inside the cooling oil. As the rotating speed reaches 4000 r/min, rolling elements are seen to be uncovered at certain spots, which indicate an insufficient oil supplement at this condition. Microbubbles, which are evident in the air entrainment, are also observed in this condition.

The contours of oil volume fraction in the simulation results well resemble the visualized flow field at each speed, which again confirms that the oil volume fraction inside the bearing decreases with the increase of bearing rotating speed. Thus, the heat transfer characteristics should be treated as a two-phase flow, especially at higher speeds, since the heat capacities of the oil and air are different.

Figure 5 represents the visualized and simulated flow field inside the bearing with respect to different flow rates. The bearing again rotates counterclockwise, and the speed is set to be 4000 r/min constantly. With nozzle diameter of 1.5 mm, the test flow rates are 0.3, 0.5, and 0.9 l/min, respectively. The general observation is that, in all tested flow rates conditions, the thickness of the film increases along the outward radial normal direction, which is evidenced by the phenomena that the cooling oil distributes more preferably around the outer ring due to the centrifugal effect.

A further general observation is that the oil film thickness near the outer ring gradually reduces along the direction of rotation under all conditions. For example,

Figure 5. Cooling oil distribution at different flow rates: (a) measured results and (b) simulated results.

### Flow and Heat Transfer in Jet Cooling Rolling Bearing DOI: http://dx.doi.org/10.5772/intechopen.84702

in the 0.3 l/min case, the oil volume fraction becomes lower along the direction of rotation, and the lowest oil volume fraction appears at the upstream of the jet position, leading to a lack of oil. For the 0.5 l/min case, the oil lack area shrinks at the same observation positon, while there is no sign of insufficient oil supplement under the 0.9 l/min condition. This observation indicates that large flow rate is beneficial in homogenization of the cooling oil distribution inside the bearing. Nevertheless, please be noted that the power efficacy could be damaged if an unduly large flow rate of oil is used since the churning loss can be quite high.

Figure 6 shows oil volume fraction simulations inside the same bearing at highspeed conditions. The cooling oil distributions at different speeds show similar trends, as shown in Figure 6(a). The oil volume fraction reaches its peak value at a certain downstream vicinity of the jet position and gradually reduces along the bearing rotation direction from then on. However, the peak oil volume fraction emerges at random position. The area of the peak oil volume fraction also changes under different speeds, as shown in Figure 6(b). It can be seen that the highest oil

Figure 6. Cooling oil flow field at higher speeds: (a) simulated oil flow field and (b) oil flow at the jet position.

volume fraction position moves along the rotation direction. This is because that the circumferential angular velocity of the oil increases rapidly at a higher speed. The oil is driven by the roller and cage. Further, part of the oil passes through the bearing directly at a lower speed. This reduces the utilization efficiency of the cooling oil.

### 4.2 Parametric studies on the air-oil flow distribution

Figure 7 shows the calculated air-oil distribution with different nozzle numbers. The speed of the inner ring is 10,000 r/min, and the oil flow rate is 3.0 l/min. The oil volume fraction contours from three to eight nozzles are given in Figure 7(a). It is seen that the oil volume fraction rises with the increase of the nozzle number. Figure 7(b) presents the oil volume fraction distribution around the circumference. The parametric results of the oil volume fraction indicate the effect of the different nozzle numbers. It seems that the uniformity of the oil volume fraction inside the bearing rises with the increase of the nozzle number. The oil volume fraction also

Air-oil distribution with different numbers of nozzles: (a) oil volume fraction contours in the center cross section; (b) oil volume fraction distributions; and (c) average oil volume fraction around the circumference vs. the number of nozzle.

Flow and Heat Transfer in Jet Cooling Rolling Bearing DOI: http://dx.doi.org/10.5772/intechopen.84702

increases with the increase of the nozzle number. However, when the nozzle number is larger than 6, the oil volume fraction improvement becomes unapparent, as shown in Figure 7(c). Further, when the nozzle number exceeds 4, the variation of the oil volume fraction and the uniformity of the oil volume fraction inside the bearing are less than 10%, as shown in Table 5. The multiple-nozzle jet requires a more complex mechanism than the single-nozzle jet.

Figure 8 shows the correlations between the air-oil distribution and jet velocity. The rolling bearing with a single-nozzle jet configuration is simulated; the oil volume fraction is obtained under a revolution speed of 10,000 r/min, and the oil flow rate is 3.0 l/min. The oil volume fraction is shown staying around both the inner


#### Table 5.

Uniformity of the oil volume fraction with different nozzle numbers.

#### Figure 8.

Air-oil distribution with different jet velocities: (a) oil volume fraction contours in the center cross section; (b) oil volume fraction distributions; and (c) average oil volume fraction around the circumference vs. the jet velocity.

ring and the outer ring which becomes much lower with the increase of the jet velocity, as shown in Figure 8(a). However, the oil volume fraction becomes more uniform with higher jet velocity. The oil volume fraction distribution around the circumference decreases with the higher jet velocity, as shown in Figure 8(b). However, the jet velocity has little effect on the tendency of the oil volume fraction distributions around the circumference. Further, there is a jet velocity at which the average oil volume fraction achieves the largest value. The calculated result of the jet velocity is between 15 and 20 m/s in the given operation conditions, as shown in Figure 8(c). The detailed relationship between the jet velocity and the oil volume fraction still needs more effort to investigate.

Figure 9 shows the simulated air-oil distribution with different oil flow rates under a constant speed of 10,000 r/min. As indicated in Figure 9(a), the oil volume

Figure 9.

Air-oil distribution with different oil flow rates: (a) oil volume fraction contours in the center cross section; (b) oil volume fraction distributions and (c) average oil volume fraction around the circumference vs. the operation speed.

Flow and Heat Transfer in Jet Cooling Rolling Bearing DOI: http://dx.doi.org/10.5772/intechopen.84702

fraction inside the bearing is increased as the oil flow rate increases. Figure 9(b) indicates that the oil volume fraction distribution under different oil flow rate condition shows similar trends; however, the amplitude of the oil volume fraction rises with the increase of the oil flow rate. Moreover, the oil volume fraction distributed more evenly at locations far from the jet position. The average oil volume fraction increases with the decrease of the speed, and the increase tendency becomes faster at higher speed, as shown in Figure 9(c). Large flow rate is beneficial in homogenization of the cooling oil distribution inside the bearing and therefore the heat dissipation of the bearing. Nevertheless, the power efficacy could also be damaged due to the resulting unduly churning loss.
