# CFD Analysis of Subcooled Flow Boiling in 4 x 4 Rod Bundle
## 2 Subcooled Boiling Experiment in \(4\times 4\) Rod Bundle  
In this study, an experiment was conducted in the subcooled boiling water flow with a \(4\times 4\) rod bundle heater. Instruments such as Coriolis flow meters, static pressure gauges, differential pressure gauges, thermal couples, and power meters were equipped in the facility, and control valves for precisely controlling the flow rate and preheater to maintain the target inlet temperature were installed. The measurement instrument used in the experiment and the uncertainty are listed in Table 1. The pressure could be adjusted with a pressurizer connected to the water storage tank consisting of a test loop. A separator for separating steam and water was installed at the outlet of the test section, and a condenser and heat exchanger for heat removal from steam and water, respectively, were installed at the rear end of the water separator. An additional heat exchanger was installed at the outlet of the test section to prevent the flashing of high-temperature water. To minimize heat loss from the flow test channel during the experiment, insulating material was applied to the outside wall of the flow channel. The test section was designed as a square channel of which the length of the sides was 85 mm. Each heater rod was 16 mm in outer diameter and 590 mm in heated length (Zh). The pitch-to-diameter ratio was 1.3 in the bundle arrangement; therefore, the hydraulic diameter (DH) of the whole subchannel was 14 mm. A two-sensor optical fiber probe (2S-OFP) consisting of one front and one rear sensor was applied to measure local two-phase flow parameters in the rod bundle. The local two-phase flow parameters measured in the experiment were the void fraction (\(\overline{\alpha}^{t}\)), interfacial area concentration (\(\overline{a}_{i}^{t}\), IAC), interfacial velocity (\(\overline{v_{ij}}^{-t}\)), and bubble diameter (\(\overline{D_{sm}}^{t}\)). The local void fraction is the ratio of passage time of bubbles per unit time at a local point in the two-phase flow condition. The IAC, defined as the \begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameter** & **Measurement Instrument** & **Measurement Uncertainty** \\ \hline Mass flow rate & Coriolis mass flow meter (Micromotion CMF100S) & 0.25 g/s \\ Temperature & K-type thermocouple (OMEGA) & 0.66 K \\ Pressure & Pressure transmitter (Rosemount 3051S) & 0.8 kPa \\ Power & Power meter \& Shunt (Yokogawa WT330) & 0.57 kW \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Measurement instrument with each uncertainty. interfacial area per unit volume, is a parameter of prime importance in terms of mass, momentum, and energy transfers between phases. In this study, the interfacial velocity is the velocity of the actual velocity of the bubbles along the axial direction of a flowing channel. The detailed equations for each parameter are as follows. \[\overline{\alpha}^{\mathrm{t}}=\frac{1}{\mathrm{T}}\sum_{\mathrm{j}}^{\mathrm{ N_{b}}}(\mathrm{t_{F2}}-\mathrm{t_{F1}}) \tag{1}\]  
\[\overline{\mathrm{a_{i}}}^{\mathrm{t}}=2\mathrm{N_{b}}\frac{\overline{1}^{ \mathrm{t}}}{\mathrm{v_{ij}}}\mathrm{I}\left(\phi_{\mathrm{j,max}}\right) \tag{2}\]  
\[\overline{\mathrm{v_{ij}}}^{\mathrm{t}}=\frac{\mathrm{N_{b}}}{\sum_{\mathrm{j }}^{\mathrm{N_{b}}}\frac{(\mathrm{t_{F1}}-\mathrm{t_{F1}})}{\mathrm{S}}} \tag{3}\]  
where T is the total measurement time, \(\mathrm{N_{b}}\) is the number of bubbles in contact with local optical fiber sensor, \(\mathrm{t_{F2}}\) is the time when the bubble completely passes through the front sensor, \(\mathrm{t_{F1}}\) is the time when the bubble starts to pass through the front sensor, \(\mathrm{t_{R1}}\) is the time when the bubble starts to pass through the rear sensor, \(\mathrm{I}\left(\phi_{\mathrm{j,max}}\right)\) is the angle between the mean flow direction vector and the velocity vector of the interface, proposed by [PERSON] et al. (1998)[26], and superscript t is the time average. The bubble diameter is six times the void fraction over the interfacial area concentration.