Internal flow measurement in centrifugal pump by high speed photography and error analysis

Abstract: High speed photography is an effective method to study the flow in the centrifugal pump. However, compared to other object movement, the internal flow in the centrifugal pump has its own particularity. It is necessary to carry out the research on the measurement method in the pump with high speed photography and its error analysis. To reveal the internal flow in centrifugal pump and improve the measurement accuracy, high speed photography system for measuring the internal flow in the centrifugal pump was elaborated in this study. On January 2015, the internal flow measurement in centrifugal pump by high speed photography was carried out at Nanjing Tech University. The external performances of centrifugal pump and the images of internal flow were recorded. The pump rotate speed was 1 000 r/min, the flow rate was 16 m3/h, and the head was about 8.6 m. The shooting region and the arrangement of model pump and high speed camera were investigated. When the recorded rate was less than 3 000 frame/s, it did not need to add light source and the shooting effect was good. The method of determining the main shooting parameters was studied. With the increase of the rotate speed of centrifugal pump, the recorded rate should be increased, and the image scale decreases accordingly. The matters needing attention were elaborated when setting the external calibration. The causes of the speed error and the error in the centrifugal pump were analyzed in detail and the methods for controlling the error were proposed. The results showed that under the working condition, the average image scale was 1.560?10-4 m/pixel, and the standard deviation of image scale was 3.533?10-7m/pixel. The average velocity near the volute inlet was 8.41 m/s, and the standard deviation of velocity was 0.301 m/s. Selecting appropriate shooting distance and improving recognition accuracy could control the speed error in essence. When the recorded rate was more than 2 000 frame/s, reducing the recorded rate could improve the image resolution, so that the image scale could be reduced, which played an important role in controlling the speed error. When the recorded rate was less than 2000 frame/s, the image resolution did not change with the recorded rate. In this case, a higher recorded rate should be chosen to obtain the details of the flow as detailed as possible. At the radius of 0.125 m, when the interpretation intervals increased from 4 frames to 8 frames, the speed relative error caused by inputting error decreased from 0.6% to 0.3%, and the speed relative error caused by arc chord error increased from 0.18% to 0.73% at the recorded rate of 2 000 frame/s. Therefore, when analyzing the flow in the impeller and the spiral case, increasing the image interpretation interval could reduce the speed error caused by inputting error, but increased the speed error caused by arc chord error, so the appropriate image interpretation interval must be selected. When analyzing the flow in the straight pipe section near the pump inlet and outlet, the speed error caused by the arc chord error were negligible, so the measurement error could be reduced by reducing the recorded rate or increasing the image interpretation interval appropriately. When the recorded rate was 2 000 frame/s and the interpretation intervals were 6 frames, the decrease of radius from 0.125 m to 0.02m could result in the measurement error increased from 0.41% to 2.48%. Therefore, the measurement error increased with the decrease of impeller radius. In order to reduce the measurement error, the image interpretation interval should be selected according to the impeller radius. The results can provide valuable information for improving the measurement accuracy of internal flow in the rotary machine.