## 3 Experimental Setup
### Experimental Conditions and Procedure  
Firstly, the experiment was conducted to measure the velocity profiles on single-phase liquid (water) flow at a superficial liquid velocity \(U_{L}\) of 200 mm/s and 300 mm/s, respectively (Reynolds numbers: 3699 and 5549). The Developed-UVP performance was compared using the PIV and the Original-UVP (autocorrelation) methods. A summary of the experimental conditions is shown in Table 4. The Developed-UVP, Original-UVP and PIV parameters were set for compatibility with experimental conditions as shown in Tables 5 and 6. \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value** \\ \hline Shutter speed & 1:500 s \\ Number of pictures & 16,281 \\ Spatial resolution & 640:480 pixels \\ Method & Cross-correlation \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Parameter configuration of Particle Image Velocimetry (PIV). \begin{table}
\begin{tabular}{c c} \hline \hline
**Equipment** & **Description** \\ \hline Transducer & 4 MHz, Model: TX-4-5-8, \\ MFG: Met-Flow, Lausanne, Switzerland \\ \multirow{2}{*}{Pulser/receiver} & Model: RPR-4000, \\  & MFG: RITEC, Rhode Island, USA \\ \multirow{2}{*}{Function generator} & Model: AFG-31051, \\  & MFG: RSPRO, Northamptonshire, UK \\ \multirow{2}{*}{Digitizer} & Model: NI USB 5133, \\  & MFG: National Instrument, Texas, USA \\ \multirow{2}{*}{Operating software} & Model: LabVIEW 2011, \\  & MFG: National Instrument, Texas, USA \\ \multirow{2}{*}{Computer} & Model: Vostro, \\  & MFG: Dell, Texas, USA \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Equipment list for the Ultrasonic Velocity Profiler (UVP) system. \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value** \\ \hline Fluid temperature & 25 \({}^{\circ}\)C \(\pm\) 2 \({}^{\circ}\)C \\ Pressure & 1 atm \\ Sound velocity in water & 1493 m/s at 25 \({}^{\circ}\)C \\ Particle & Nylon particles (80 \(\upmu\)m) \\ Pipe material & Acrylic resin \\ Pipe diameter & 20 mm \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Summary of experimental conditions. \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value** \\ \hline Basic frequency (\(f_{0}\)) & 4 MHz \\ Incidence angle (\(\theta\)) & 45\({}^{\circ}\) \\ Emission pulse shape & Gaussian sin \\ Emission voltage & 150 Vp-p \\ Receiving gain & 45 dB \\ Pulse repetition frequency (\(f_{PRF}\)) & 8 kHz \\ Number of repetitions (\(N_{REP}\)) & 128 \\ Number of cycles per pulse (\(n\)) & 4 \\ Channel width (\(w\)) & 0.74 mm \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Parameter configuration of UVP. Secondly, for separating the velocity of the bubble and liquid phases in bubbly flow, the Doppler amplitude threshold must be set. To set the threshold value, Doppler amplitude testing was carried out on two reflector conditions dispersed in fluid flow: (1) nylon particles 80 \(\upmu\)m (liquid phase); and (2) a large number of microbubbles at diameter <1 mm (bubble phase). The testing was conducted at a \(U_{L}\) of 200 mm/s and 300 mm/s, respectively. The evaluation was then obtained by observing the amplitude value of 30,000 Doppler signals in each condition. Thirdly, an experiment was conducted to measure the velocity profile of the two-phase bubbly flow. In this study, the bubbles coalescence and break-up should not be occurred in the measurement volume; therefore, to avoid a complex bubbles behavior on measurement volume, the experiment was conducted in the homogeneous flow regime. The homogeneous flow regime (associated with small superficial gas velocities, \(U_{G}\)) is referred as the flow regime where only \"non-coalescence-induced\" bubbles exist [30]. The \"homogeneous flow regime\", can be classified into mono-dispersed homogeneous flow regime and pseudo-homogeneous flow regime depending on the prevailing bubble size distributions. Coalescence and break-up kernels can be neglected in certain cases (i.e., mono-dispersed homogeneous flow regime and the pseudo-homogeneous flow regime, when C&B are restricted to small areas in the domain) [31].