## 5 Results and Discussion  
### Measurement Accuracy  
Figure 9 shows the DPIV measurement results under the working condition of \(51.9\times 10^{-3}\) m/s-80% (flow velocity: \(51.9\times 10^{-3}\) m/s; water cut: 80%). To reduce the influence of image distortion by oil drops passing the light path, preprocess on the collected oil-water two-phase immiscible flow image is carried  
Figure 8: Optical distortion of the transparent curved surface of the vertical pipe. \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value** \\ \hline Dip angle & 0\({}^{\circ}\) \\ Pipe inner diameter & 125 mm \\ pipe outside diameter & 150 mm \\ object distance & 0.5 m \\ Length of steady flow section & 6.0 m \\ Fluid composition & tap water, diesel oil \\ Fluid Reynolds coefficient & 2638–6595 \\ Flow state & turbulent \\ Diesel density & 0.8 g/cm\({}^{3}\) \\ Light source & LED backlight source \\ Camera & HSVISION EoSens mini2 \\ Frame rate & 1000 frames/s \\ Image resolution & 1472 pixel \(\times\) 1036 pixel \\ Measured area size & 180 mm \(\times\) 144 mm \\ exposure time & 1 μs-1 s \\ Depth of focus & 0.8712 mm \\ Depth of field & 261 mm \\ Water phase velocity error & 0.10\% \\ Oil phase velocity error & 0.10\% \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Experiment parameters. out. Firstly, the regions of interest of images are cropped; then, a series of image processing techniques is adopted, such as Laplace sharpening and contrast enhancement to enhance useful information in the image. The experiment results show that the quality of the image is much better than that of the original image although there is still some distortion. Figure 9a,b show pairs of preprocessed images. The time interval is 0.006 s, and the measurement area size is 124 mm \(\times\) 124 mm. Figure 9c,d shows the flow velocity fields measured by the WIDIM-DPIV algorithm and the IM-WIDIM-DPIV algorithm, respectively. According to the calculation, the average velocity of the WIDIM-DPIV algorithm was \(47.12\times 10^{-3}\) m/s and that of the IM-WIDIM-DPIV algorithm was \(53.81\times 10^{-3}\) m/s. The accuracy of the WIDIM-DPIV algorithm was 90.80% and that of the IM-WIDIM-DPIV algorithm was 96.31%. Therefore, the IM-WIDIM-DPIV algorithm improved the measurement accuracy. To decrease the effect of experiment contingency on measurement accuracy, the DPIV experiment was conducted on the oil-water two-phase immiscible flow under different conditions in the inner diameter 125 mm vertical pipe. The flow velocity ranges from \(9.4\times 10^{-3}\) m/s to \(51.9\times 10^{-3}\) m/s with  
Figure 9: Measurement results at \(51.9\times 10^{-3}\) m/s-80%. (**a**,**b**) Preprocessed image pairs. (**c**) Flow velocity field by the WIDIM-DPIV algorithm. (**d**) Flow velocity field by the IM-WIDIM-DPIV algorithm. intergroup interval of \(4.7\times 10^{-3}\) m/s. Figure 10 shows the measurement results at conditions of 90%, 80%, and 70% water cut. As shown in Figure 10, the measurement results of the IM-WIDIM-DPIV algorithm were closer to the theoretical values for all water cut than those of WIDIM-DPIV algorithm. This qualitatively indicates that the accuracy of the IM-WIDIM-DPIV algorithm was higher than that of the WIDIM-DPIV algorithm. Under different working conditions, the mean error of the WIDIM-DPIV algorithm was \(3.39\times 10^{-3}\) m/s and that of the IM-WIDIM-DPIV algorithm was \(1.43\times 10^{-3}\) m/s. This quantitatively indicates that the mean error of the IM-WIDIM-DPIV algorithm was smaller than that of WIDIM-DPIV algorithm and the measurement results of the IM-WIDIM-DPIV algorithm were closer to the theory  
Figure 10: Measurement results under different water cut: (**a**) 90%, (**b**) 80%, and (**c**) 70%. value under all working conditions. According to all measurement data, the average accuracy of the IM-WIDIM-DPIV algorithm was 94.8%. It was higher than that of the WIDIM-DPIV algorithm, which was 88.3%.