# Downdraft Assessment of Glass Curtain Wall Buildings with a Radiant Floor Heating System
## 4 CFD Simulation
### Boundary Conditions  
To evaluate the temperature and velocity of the indoor airflow, whose parameters impact the downdraft, and the distribution of the draft rate, which is an indicator of a downdraft, a simulation analysis of the indoor environment was performed using the commercial CFD software STAR-CCM+ 12.02. STAR-CCM+ accurately predicts heat transfer in fluids and solids using specialized convection, conduction and radiation models. The automated meshing tools and parts-based simulation option allow validating thermal designs faster [29]. The physical condition of the simulation is described in Table 3. The simulation is interpreted by applying the K-\(\varepsilon\) turbulence model under a steady-state condition. To accurately analyze the ascending effect of the indoor airflow caused by the RFHS, the simulation was performed by applying the Boussinesq model. The details about the numerical models of the CFD simulation are shown in Figure 4.  
The mesh was composed of polyhedral elements, which were a non-regular lattice that can cope with complex shapes. The number of surface meshes analysis was 23,914, and the volume mesh analysis was 72,432. The layer and the immediate space of the window, the edges of the room, and the area, where the window and the room were connected, were formed by a finer mesh. Figure 5 presents the mesh constructed for the simulation. The mesh geometry was appropriately constructed without collapsing or tangling. Moreover, no errors occurred, including the fine-meshed parts. \begin{table}
\begin{tabular}{c c c} \hline \hline \multicolumn{2}{c}{**Classification**} & \multicolumn{2}{c}{**Description**} \\ \hline Target space area & \(6\,\text{(m)}\times 7\,\text{(m)}\times 3\,\text{(m)}\,\text{(w}\times\text{d} \times\text{h)}\) \\ \hline Temperature conditions & Outdoor: –11.3 \({}^{\circ}\)C/Indoor: 20 \({}^{\circ}\)C \\ \hline Window conditions & Type & general dual-pane glass/Low-E dual-pane glass \\ \cline{2-3}  & \(U\)-value & \(2.75\,\text{W}/\text{m}^{2}\cdot\text{K}/1.75\,\text{W}/\text{m}^{2}\cdot \text{K}\) \\ \hline \multirow{2}{*}{Radiant floor heating system conditions} & Heating area & 1.0 m, 3.0 m, 7.0 m distance from Window \\ \cline{2-3}  & \multirow{2}{*}{Surface Temp.} & According to a heating area (Perimeter zone: Maximum 41 \({}^{\circ}\)C; Interior zone: Maximum 29 \({}^{\circ}\)C) \\ \hline Wall condition & & Adiabatic \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Conditions of target space. \begin{table}
\begin{tabular}{c c c} \hline \hline \multicolumn{2}{c}{**Physics Model**} & \multicolumn{2}{c}{**Description**} \\ \hline \multirow{4}{*}{Physics} & Time & Steady state \\  & Turbulence model & K-\(\varepsilon\) Turbulence \\ \cline{1-1} \cline{2-3}  & Radiation & Surface-to-surface Radiation \\ \cline{1-1} \cline{2-3}  & Equation of state & Ideal Gas (Including Boussinesq Model) \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Model of simulation. The analysis results of the perimeter zone standard for perimeter zone application of the RFHS showed that the maximum 1.0 m distance from the window was presented as the perimeter zone in ISO 11588 [30] and a 3.0 m distance from the window (or 1/3 degree of actual depth during the perimeter zone system design) was used. Thus, the radiation area of the floor surface was set for the case in which the RFHS was applied to the perimeter zone. It was assumed that the RFHS was heated up to the load to be removed and that all heating loads generated in the room would be removed through the RFHS. In other words, the thermal output of the RFHS was equal to the heat loss by conduction through the building envelope (i.e., in this study, the window). This is shown in Equation (5). The thermal output can be expressed as the relationship of heat flow intensity and the surface area of the radiant heating  
Figure 4: The numerical models of the computational fluid dynamics (CFD) simulation. Figure 5: Generation of mesh model. panel as shown in Equations (6) and (7).