# CFD Analysis of Subcooled Flow Boiling in 4 x 4 Rod Bundle
## 1 Introduction
The usability of the detailed model used in this study has been evaluated by researchers as follows. [PERSON] (1998) studied simple but reliable correlations for a drag coefficient by comparing experimental data. The correlations were obtained for the drag coefficient of single bubbles under a wide range of fluid properties, bubble diameters, and acceleration of gravity. These were developed according to the balance of forces acting on the bubbles in stagnant liquid, and the available empirical correlations of the terminal rising velocities of single bubbles [16]. It has been shown that in many flows, the inertial force can simply be added to the generalization of the lift force introduced by [PERSON] (1987) [17]. The lift force of [PERSON] (1988) consists of the rotational force and inertial force or additional mass [18]. Owing to the neglect of the wall area force, the predicted pore profile peaked instead of approaching zero at the wall. Therefore, [PERSON] (1985) concluded that a different kind of transverse force is needed to break the bubbles away from the wall [19]. Later, [PERSON] (1991) developed a two-fluid model of multidimensional laminar bubble two-phase flow to analyze vertical pipe flow. In this study, the wall lift force for the dispersed phase (i.e., bubbles) and the wall lubrication force about the wall force were calculated. The results showed good agreement with the data when the considered model was used [20]. [PERSON] et al. (1993) used \(C_{TD}=1\) to predict the lateral phase distribution of a three-dimensional water/air bubble upflow or downflow in a pipe using the k-\(\epsilon\) turbulence model and an algebraic stress law. It can be confirmed [21]. These results are very interesting because a properly formulated multidimensional two-fluid model has the inherent ability to predict the lateral phase distribution phenomena in simple and complex geometric conduits for bubbles of size 1 mm \(<D_{b}<6\) mm. The model of [PERSON] and [PERSON] (1990) was used to determine the influence of bubble wall area fraction properties [22]. This model can estimate the fraction of the wall area affected by the sweeping of the liquid inflow under the bubble departure. In the two-fluid model, active nucleation site density is an important factor for estimating the IAC. Thus, the model by [PERSON] (2003) proposed that the active nucleation site density is a function of the critical cavity size and contact angle [23]. This model clearly demonstrated the dependence of active nucleation site density on the superheated wall by various studies. [PERSON] (1983) reported that the bubble departure diameter was associated with water over a wide range of pressures [24]. Using the correlation proposed by [PERSON], the mean deviation of the experimental data is approximately \(\pm 33\%\). It accurately simulated the available experimental water data, considering various free surfaces. [PERSON] (1960) demonstrated through experimental measurements that the main forces acting on bubbles leaving the surface at high heat flows are the buoyancy and drag forces [25]. The dimensionless relationship was developed by the bubble velocity, bubble diameter, and contact angle at breakoff. There are many other studies. However, the boiling phenomenon of the subchannel is still a field that requires various studies depending on the total length, hydraulic diameter, flow rate, and heat transfer rate. This study analyzes the coolant flow in \(4\times 4\) rod bundles with applied symmetrical conditions. The diameter, pitch, and hydraulic diameter of the rods are 16, 21, and 14.01 mm respectively, and the total length is 590 mm. The cross-sectional area of the coolant is 0.004008 m\({}^{2}\). The grid size has three prism layers total of 0.8 mm near the wall, and 1 mm in other areas. Since the fluid belongs to high-Re, High-y+ wall treatment (applicable to \(30\leq y+\leq 100\)) was applied. This simulation is calculated with steady state and constant density.