3.5 Coupling procedure
4.1.2 Velocity field outside the motor housing
As it was mentioned in Section2.1, the PIV technique was planned to be used in the study but the quality of the records was not sufficient. For this reason, there was no possibility to estimate the velocity field using this technique. However, during the PIV data collection, the flow visualisation was possible by the laser plane crossing the air above the motor with the accompanying smoke seeding. Therefore, in Fig. 4.7, four images are shown to illustrate the flow behaviour after achieving a thermal steady state, i.e. constant temperature values recorded within the motor. In the presented frames from (a) to (d) taken in one-second intervals, the various eddies are visible. They represent the transient character of the flow outside the motor caused in this region by natural convection flow. The images confirm that the high air velocity region is expected, e.g., above the rear bracket. However, without results from the PIV measurement procedure, the velocity field can be estimated only qualitatively not quantitatively. Therefore, in the current section, the velocity measurements obtained at the selected points using Constant Temperature Anemometry technique are discussed.
Referring to the temperature field presented in the previous section, the velocity path-lines are presented around the motor, while the temperature field of the motor external sur-faces is shown in Fig. 4.8. In this picture, the velocity range was limited to the values that occur in the whole numerical domain. In the vicinity of the rotating components, the max-imum reported fluid velocity exceeds the maxmax-imum value in the defined range of the pre-sented range and reaches translational velocity of the rotating components. In the prepre-sented figure, the higher velocities are visible in the pathlines swirling around the motor coupling and in the middle of the bracket area. The pathlines show the air motion from the motor housing walls to the top region of the air block. Some of the pathlines go from the upper to the lower parts of the computational domain in the vicinity of the plexiglass wall. Due to the fact that the acrylic glass walls are of a lower temperature than that of the internal air, the air movement is directed to bottom wall in the vicinity of the side wall and it is consistent with heat transfer theory.
In the following section, a comparison of the velocity results from CFD and measure-ments that were collected outside the motor is presented for the selected operating points.
The velocity results are presented at 28 points measured by 7 anemometers at 4 height levels above both the motor and generator according to the description presented in Section2.1.
The employed instruments allowed to record only one velocity component. Therefore, the velocity values are presented only as a vertical component of the velocity. The sensor num-bering defined in the following results is consistent with the numbers presented in Fig. 2.3.
(a) (b)
(c) (d)
Figure 4.7: The air flow visualisation above the investigated motor using the PIV technique:
frames from (a) to (d) taken in one-second intervals
The error bars were added to the measured values based on the standard deviation calcu-lated on the basis of the recorded measurements.
In Fig.4.9, the velocity results from the numerical model and experimental tests are pre-sented for Operating Point II. According to Fig.4.9, the velocity values obtained at the lowest presented level (height level I) from the CFD model were within the range of the estimated velocity errors. In the next group (height level II), there were 5 values within the error ranges.
In the third group (height level III), only 4 of the measured points were consistent with the modelled values. Finally, the highest level (height level IV) contained 5 points in which the CFD results were in the range of the standard deviations calculated from the experimental values. The average value of the vertical velocity component computed from all the mea-sured points was 0.096m · s−1, while the average values obtained from Thermal Model I was 0.149 m · s−1. Therefore, the CFD results overestimated the experimental values at the
inves-74.7
Figure 4.8: Temperature field of the motor housing walls and the velocity pathlines within the plexiglass walls for Operating Point II
tigated operating point. In Fig. 4.9, most of the CFD results are within the error bar ranges estimated from the experiments. Considering that calculations were conducted in a steady state mode, the agreement between model and experimental results can be treated as sat-isfactory. The high error bars also confirmed that the test results were affected by unstable flow conditions resulting from natural convection phenomenon.
For Operating Point IV, the vertical velocity component values is presented in Fig. 4.10.
In the figure, most of the CFD results are within the error bar ranges estimated from the experimental tests. In the first group of points located on the lowest level (height level I), the CFD model results were in the range of the estimated velocity errors for 6 out of 7 points. At
the next level (height level II), there were 4 values within the error ranges. In the third group (height level III), 6 of the measured points were consistent with the modelled values. Finally, the highest level (height level IV) contained again 5 points in which the CFD results were in the range of the standard deviations computed from the experiments. The average value of the vertical velocity component from all the measured points was 0.08 m · s−1, while the average of these values from Thermal Model I was 0.11 m · s−1.
For Operating Point V, the values of the vertical velocity component were presented in Fig. 4.11. In the figure, most of the CFD results are within the error bar ranges estimated from the measurements. The numerical results were in the range of the estimated velocity errors for 6 out of 7 measured points in the first group at the lowest level (height level I). At the higher level (height level II), there were only 3 values within the error range. However, it is worth noticing that, in this group, two anemometers reached the average values of approx.
-0.05 m · s−1. In the third group (height level III), 5 of the measured points were consistent with the modelled values. Finally, at the highest level (height level IV) again 5 out of 7 points showed that the CFD results were in the range of the standard deviations calculated from the experiments. The average value of the vertical velocity component from all the measured points was 0.089 m · s−1, while the average of these values obtained from Thermal Model I was 0.146 m · s−1. Considering that calculations were conducted in a steady state mode, the agreement between model and measurement can be treated as satisfactory.
For Operating Point VIII, according to Fig. 4.12, in the first group at the lowest level (height level I) , the CFD model results were in the range of the estimated velocity errors for only 2 out of 7 measured values. It is worth adding that two of the measured values reached more than 0.2 m · s−1. This value was more than two times higher than the average for thie considered operating point. In the next group (height level II), there were 5 values within the error ranges. In the third group (height level III), 5 of the measured points were also consis-tent with the modelled values. Finally, the highest level (height level IV) contained 6 values in which the CFD results were in the range of the standard deviations calculated from the measurements. The average value of the vertical velocity component from all the measured points was 0.062 m · s−1, while the average of these values from Thermal Model I was 0.127 m · s−1.
As it was expected, the highest velocity values were reached at Operating Point II which was characterised by the highest temperature of the motor. The numerical results showed a tendency of the decreasing velocity values with decreasing motor temperature. In the exper-imental results, that tendency was disrupted at Operating Points VIII and IV. However, the temperature of the motor housing varied between these operating points only by approx.
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Figure 4.9: Vertical velocity component comparison between CFD and experimental results at the load of 4 resistors and a rotational speed of 3500 rpm (Operating point II)
1 3 5 7 9 11 13 15 17 19 21 23 25 27
Figure 4.10: Vertical velocity component comparison between CFD and experimental re-sults at the load of 3 resistors and a rotational speed of 2500 rpm (Operating point IV)
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Figure 4.11: Vertical velocity component comparison between CFD and experimental re-sults at the load of 3 resistors and a rotational speed of 3500 rpm (Operating point V)
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Figure 4.12: Vertical velocity component comparison between CFD and experimental re-sults at the load of 2 resistors and a rotational speed of 3500 rpm (Operating point VIII)
3 K. This level of the temperature differences accompanied by high-velocity measurement uncertainty could lead to some inconsistencies. According to the presented results, most of the CFD results were in the error bar ranges estimated from the measurements. However, it is worth noticing that the error bars are wide for many points. This can be explained by the character of the natural convection flow that caused high instability of the experimental results.