2.2.2. Multi-Dimensional Model of MARS-KS

For modeling the heated section, the multi-dimensional component, namely MULTID, was employed. The connections to the heated sections upstream and downstream were modeled by MULTIPLE JUNCTION components. The system pressure and inlet flow boundary conditions were modeled by TIME DEPENDENT VOLUME and TIME DEPENDENT JUNCTION components, respectively. The MULTID has the capability to model the turbulent stress and energy mixing between channels, and these are proportional to the square of a user-defined Prandtl *mixing length*. The required *mixing length* is given by Equation (1), which is recommended by the user guide [13] as the reference correlation for the channel *mixing length*:

$$\text{Mixing length} = L \left[ 0.14 - 0.08 \left( 1 - \frac{y}{L} \right)^2 - 0.06 \left( 1 - \frac{y}{L} \right)^4 \right] \tag{1}$$

where *L* is channel half-width, and *y* is given by the range from 0 to *L*. In this study, the maximum value of *mixing length* was utilized in the reference model. In addition, a sensitivity study without the *mixing length* term was conducted, and the results will be shown in the following section.

#### 2.2.3. Multi-Dimensional Model of TRACE

In the case of TRACE, the VESSEL component was employed to model the heated section. As with the multi-dimensional model of MARS-KS, the additional hydraulic volumes, which were modeled by VESSEL component as well, were connected to each upstream and downstream segment of the heated section, respectively. Inter-connections between the heated section and the additional volumes were given by VESSEL JUNCTION components. The system pressure and inlet flow boundary conditions were given by BREAK and FILL components, respectively. Since TRACE has no additional turbulent model as MARS-KS, only diversion crossflows were implemented between channels.
