## 4 Results and Discussion
The bubble velocity from four-sensor probe by Equation (8) and visual measurement by Equation (15) are compared in Figure 12. Because bubble moves as an entity, it is worth noting that the bubble velocity measured at different radial locations should remain constant when the probe moves from tube axis to tube wall. However, the black square dots in Figure 12 representing probe measured velocity demonstrate fluctuation feature. This is mainly caused by error and it can be seen  
Figure 11: IAC from probe and visual measurement. Figure 10: Void fraction from probe and visual measurement. the error increases towards the tube wall. Nevertheless, its arithmetic mean value shown by the dashed line agrees well with the visual measurement, with an overestimation of 9.3%. The chord length from four-sensor probe by Equation (9) and visual measurement are compared in Figure 10.0 Both the probe measurement and visual measurement show the same trend with the chord length emerging in the middle with maximum value and decreasing towards both sides. The largest deviation between them is 8.7% at -2.5 mm, and the deviation decreases towards the pipe axis. Although the interface direction is required to calculate the interface displacement velocity in the interfacial measurement theorem expressed by Equation (7), it is rarely verified against other measurement techniques in the accessible literature. Instead of directly using of the interface normal vector, the interface direction can be represented more conveniently by the angle between the interface normal vector and the tube axis. The angle from four-sensor probe by Equation (10) and visual measurement are compared, as shown in Figure 10.1 Figure 14a,b shows the comparisons at front interface and rear interface, respectively. As can be seen, both measurement methods show very close results with the deviation between them growing distinct towards the tube wall, and the largest errors are 22.4% and 3.1% for the front and rear interfaces, respectively. The quantitative comparisons of the measured parameters, the deviation of the probe measurement from the visual measurement are shown in Tables 2 and 3. From the above, it is demonstrated that the measurement of bubbly flow by the four-sensor probe can give agreeable flow parameters with visual measurement techniques in aspects of void fraction, IAC, bubble velocity, bubble chord length and interface direction. Thereby, it also proves the correctness of the proposed method. Figure 10.2 Bubble velocity from probe and visual measurement. Figure 10.3 Bubble chord length from probe and visual measurement. It is common knowledge that the bubble condensation is a kind of typical enhanced heat and mass transfer method encountered in numerous industrial processes such as steam-air mixture injected into subcooled zone, subcooled flow boiling and direct contact condensation [27]. Further knowledge of the bubble interface phenomena and the accurate measurement of the local flow parameters at the multi-scale interfaces, such as the void fraction, IAC, bubble velocity, bubble chord length and interface direction, are important parameters to study the process and mechanism of heat and mass transfer in gas-liquid two-phase flow. As previously reported in the literature [27; 45; 46; 58], during the gas-liquid two-phase heat and mass transfer, the interface structure, void fraction, IAC, bubble equivalent diameter, their velocity, etc. show nonlinear variations, resulting in that they are difficult to be measured quickly and accurately. Although the double-sensor probe with multiple assumptions can also be employed to measure these parameters, it is hard to obtain realistic local data in multi-dimensional two-phase flow. As a result, the method proposed in the present study is expected to promote the solution of this problem.