## 5 Conclusions  
The void fraction, IAC, bubble size, velocity, etc. of two-phase flow have great influences on the heat and mass transfer characteristics, the reaction efficiency and the operation safety of multiple chemical applications. To obtain detailed knowledge of two-phase flow, a miniaturized four-sensor probe was made firstly in this study. Then, a new method based on vector calculation to get the interface direction and a new method to get the bubble velocity magnitude were proposed, by which other local flow parameters can be obtained from the signals produced by four-sensor probe. For the purpose of verification of the probe made and the elaboration algorithm developed, an experimental facility was also built and air-water two-phase bubbly flow was tested. Besides the measurement by probe, the flow parameters were also obtained by video recording and image process techniques. For the first time, the direction of bubble interface was compared with visual image of bubble. The calculation of interface velocity vector was shown to be realizable using legitimate interface velocity direction. The comparison between the results obtained from probe measurement and visual measurement indicates their good agreement. However, since the number of ineffective bubbles increases moving towards tube wall, the measurement error by probe also increases while the probe moves away from tube axis. The averaged values across the tube are over predicted by 8.4%, under predicted by 1.7% and over predicted by 9.3% with probe measurement for void fraction, IAC and bubble velocity, respectively, in comparison to the visual measurement counterpart. The chord length and angle between tube axis and normal to the interface show great consistency with visual results, with maximum deviations of 8.7% and 22.4%, respectively. After more validation tests against multiple kinds of two-phase flow parameters, it can be concluded that the methods proposed in this study are promising in the characteristics and mechanism studies of the gas-liquid heat and mass transfer, the direct measurement of gas-liquid two-phase flow and providing significant database for the improvement of two-phase flow models. **Author Contributions:** Conceptualization, [PERSON] ([PERSON]) and Y.Z. ; methodology, [PERSON] ([PERSON]), Q.G. and Y.Z. ; software, [PERSON] ([PERSON]) and X.Q. ([PERSON]); validation, [PERSON] ([PERSON]), Q.G. and Y.Z. ; formal analysis, [PERSON] ([PERSON]), Q.G., L.L. and Y.Z. ; investigation, [PERSON] ([PERSON]) and Y.Z. ; data curation, [PERSON] ([PERSON]), Y.Z. and L.L. ; writing--original draft preparation, [PERSON] ([PERSON]), and Y.Z. ; writing--review and editing, Q.G. and X.Q. ([PERSON]); visualization, [PERSON] ([PERSON]), Y.Z., X.Q. ([PERSON]) and L.L. ; and supervision, Q.G. and Y.Z. All authors have read and agreed to the published version of the manuscript. **Funding:** This research was funded by the National Natural Science Foundation of China (Grant Nos. 51806128 and 51879154) and the Natural Science Foundation of Shandong Province (Grant Nos. ZR2019 BEE008 and ZR2018 LE011). **Acknowledgments:** The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (Grant Nos. 51806128 and 51879154) and the Natural Science Foundation of Shandong Province (Grant Nos. ZR2019 BEE008 and ZR2018 LE011). **Conflicts of Interest:** The authors declared that there is no conflict of interest. **Nomenclature**  
The following abbreviations are used in this manuscript: