## 5 Multi Slot Jet Analysis
### H/B = 4  
In this section the cases with three and two slot jets H/B = 4, are analyzed and correspond to the slot close to the target wall. In Figure 11 the velocity contours of the three slots (upper) and two slots (down) configurations are shown. \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value** \\ \hline Re\({}_{\text{outlet}}\) & 5500 \\ T\({}_{\text{outlet}}\) & 30 \({}^{\circ}\)C \\ B & 3.175 mm \\ slot length & 152.4 mm \\ u\({}_{\text{outlet}}\) & 26.6 m/s \\ \(\hat{\text{r}}\) & 0.015 kg/s \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Fundamental parameters of the multi slot jet configurations. Figure 10: Calculation domain for the multi slot jet (three jets), H/B = 4. Since the slots are close to the wall, the fluid cannot mix considerably with the surrounding medium, so the jet is well developed in the wall proximity. The single jet physics is therefore kept in this case, but the interaction between the jets is also present and the wall jets meet. Away from the jet axis, the wall velocity decreases inside the wall jet because of the boundary layer development, leading to an unfavorable pressure gradient \(\partial\rho/\partial\kappa>0\). In the mixing area between the wall jets there is a local pressure increase and the flow tends to curve upwards to leave the upper outlet (where pressure ambient is set).In this regards the pressure contours (with the surface streamlines projected) of the three slots (upper) and two slots (down) configurations are shown in Figure 12, while in Figure 13 the velocity vector in the mixing zone has been reported. The main differences between the three and two slot configurations concern the distance between the jets; in the two jets it is almost doubled, so the wall jets have more space to reduce their velocity (interacting with the surrounding medium) before their interaction and their mixing. The turbulent kinetic energy contours for the two configurations (Figure 14) highlight these differences for the heat transfer. In the descending path of the jet towards the wall, there is a shear layer that is not too strong (due to the jet proximity to the wall). A turbulent kinetic energy production is evident in the wall jet areas where the flow accelerates and interacts with the surrounding fluid. The zone with the maximum production corresponds to the interfering zones of the wall jets. This maximum is lower in the case of two jets because of the higher jet distance, and the turbulence production is more uniformly distributed. Figure 11: Velocity contours of multi slot jet configuration with H/B = 4, Re = 5500 and realizable k-\(\epsilon\) model: three slots (**upper**) and two slots (**down**). Figure 12: Pressure contours and surface streamlines of multi slot jet configuration with H/B = 4, Re = 5500 and realizable k-\(\epsilon\) model: three slots (**upper**) and two slots (**down**). In Figure 15 the convective heat transfer coefficient distributions from the CFD simulations are compared with the experimental data; in Table 5 the average values are reported together with the corresponding percentage error with respect to the experiments. Figure 14: Turbulent kinetic energy contours of multi slot jet configuration with H/B = 4, Re = 5500 and realizable k-\(\epsilon\) model: three slots (**upper**) and two slots (**down**). Figure 13: Velocity vector focus on the meeting zone between two wall jets. Figure 15: Comparison between numerical and experimental data of the convective heat transfer coefficients for the multi slot configurations with H/B = 4 and Re = 5500: 3 slots (**left**) and 2 slots (**right**). The heat exchange peaks of Figure 15 are striking in correspondence of the jets for all the numerical simulations and the experimental measurements. There are intermediate peaks for both configurations, where the wall jets merge.