## 4 Results and Discussion
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Items** & **Void Fraction** & **IAC (m\({}^{-1}\))** & **Bubble Velocity (m/s)** \\ \hline Probe measurement & 0.28 & 147.54 & 0.13 \\ Visual measurement & 0.26 & 150.15 & 0.12 \\ Deviation (\%) & 8.42 & \(-\)1.74 & 9.25 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Mean probe and visual measurement of void fraction, IAC and bubble velocity. Figure 14: Angle between interface normal and tube axis by probe and visual measurement. (**a**) Bottom surface direction of the bubble; (**b**) Top surface direction of the bubble. \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline
**Radial** & \multicolumn{3}{c}{**Chord Length (mm)**} & \multicolumn{3}{c}{**Angle at Front Interface (\({}^{\circ}\))**} & \multicolumn{3}{c}{**Angle at Rear Interface (\({}^{\circ}\))**} \\ \cline{2-10}
**Location** & **Probe** & **Visual** & **Deviation** & **Probe** & **Visual** & **Deviation** & **Probe** & **Visual** & **Deviation** \\
2.5 & 5.65 & 5.20 & 8.65 & 28.1 & 35.0 & \(-\)19.74 & 166.0 & 161.0 & 3.10 \\ \(-\)2.0 & 6.10 & 5.73 & 6.46 & 25.7 &  
The interface velocity and its direction obtained using the brief vector-based calculation method proposed in this study are beneficial to further master the heat and mass transfer process at the interface from a macro viewpoint. Meanwhile, the IAC is an important parameter to characterizing the interface transfer phenomenon (heat and mass transfer). Compared with the double-sensor probe, the four-sensor probe is used to measure the IAC to be able to solve the detection error caused by the escape and retreat of bubbles. On this basis, by combining the proposed vector-based signal processing method, a variety of assumptions can be avoided, which will be of great significance to further study the internal relationship between IAC and bubble size, bubble deformation and condensation effect, as well as to modify the calculation model of IAC. This shows that the proposed method can be widely applied in the future research of heat and mass transfer characteristics. Combined with several groups of four-sensor probes and the new vector-based signal processing method, the variation laws of the void fraction, the bubble size and number of bubbles in the condensation area can be measured accurately, quickly and conveniently, and thereby the phase distribution characteristics in the condensation area can be known. Especially during the air-steam mixture bubble condensation, the phase distribution characteristics, the interface velocity and direction can reveal the mechanism affecting the interface heat and mass transfer. Furthermore, some small bubbles with high non-condensable gas concentration appear at the later stage of the air-steam mixture bubbles condensation [58]. According to the previous study [59], when the content of non-condensable air is constant, the volume change rate of bubble increases with the decrease of bubble diameter. This means that the process of small bubble condensation is of great significance for studying the enhancement of condensation heat transfer of air-steam mixture bubbles. Therefore, the proposed method in this study will also be promising for investigating the influence mechanism of non-condensable air at the interface on the tiny steam bubble condensation enhancement.