## 4 Results and Discussion
### Hydrodynamic Performance of TLC Reactor  
#### 4.2.1 Velocity Magnitude  
Microalgae cultivation is velocity sensitive. Thus, an estimation of proper velocity is essential neither to provide the settlement of algae cells at the bottom of the reactor nor to be sheared. Liquid velocity is the measure of liquid flow and the extent of turbulence in the reactor. Some degree of turbulence is required in the reactor to ensure that all cells are frequently exposed to light for effective photosynthesis [33]. With the increase in mass flow rate and slope angle, an increase in velocity is observed (Figure 5a,d). The increase in liquid volume with the increase in water depth and AR causes a substantial reduction in the flow velocity (Figure 5b,c). The increase in velocity is more significant at a higher mass flow rate and particularly at a high slope angle where a sharp increase in velocity is observed. This trend is confirmed in Figure 5d. Theses finding indicate that with increased flow velocity (either due to slope or mass flow rate), better mixing can be achieved [11; 34]. Therefore, to achieve high velocities for the good mixing of algae cells, TLC reactors with a small AR and water depth and inclined at a higher slope angle are a good choice. Figure 4: (**a**) Effect of mass flow rate on residence time at a water depth of 5.6 mm and slope angle of 1\({}^{\circ}\). (**b**) Residence time for various water depths at a mass flow rate of 2.4 kg/s and slope angle of 1\({}^{\circ}\). (**c**) Residence time for different channel width/channel depth ratios at a mass flow rate of 2.4 kg/s, water depth of 5.6 mm, and slope angle of 1\({}^{\circ}\). Water depths, channel width, and the velocity of the suspension (i.e., Reynolds number) are the very important parameters for analyzing the fluid pattern in the thin-layer cascade reactor. Velocity contours at constant mass flow rate with the variation in water depth and channel width are presented in Figures 6 and 7. For better visualization, normalized velocity (U/U\({}_{\text{max}}\)) has been plotted. U (m/s) represents the fluid velocity, and U\({}_{\text{max}}\) (m/s) is the maximum fluid velocity in the reactor. Figure 5: Velocity magnitude (**a**) for different mass flow rates at a water depth of 5.6 mm and slope angle of 2\({}^{\circ}\), (**b**) for various channel width/channel depth ratios at a mass flow rate of 2.4 kg/s and slope angle of 1\({}^{\circ}\), (**c**) for various water depths at a mass flow rate of 2.4 kg/s and slope angle of 1\({}^{\circ}\), and (**d**) for different mass flow rates at a water depth of 5.6 mm and slope angles of 1\({}^{\circ}\), 2\({}^{\circ}\), and 3\({}^{\circ}\). Figure 6: \(Cont.\)  
With the increase in water depth and AR, a gradual decrement in velocity magnitude is evident. The increase in liquid volume with the increase in channel width and water depth causes a substantial  
Figure 6: Velocity vector plots at a constant mass flow rate of 2.4 kg/s, slope angle of 1\({}^{\circ}\), and various water depths: (**a**) 5.6 mm, (**b**) 7.5 mm, and (**c**) 10 mm. Figure 7: Velocity vector plots at a constant mass flow rate of 2.4 kg/s, slope angle of 1\({}^{\circ}\), and various channel width/channel depth ratios: (**a**) channel width/channel depth = 180, (**b**) channel width/channel depth = 260, and (**c**) channel width/channel depth = 340.  
reduction in the velocity magnitude. The TLC reactor with the smallest water depth and channel width represents the highest velocity magnitude for all the cases considered in this study. The cascade is represented by a uniform velocity profile [8]. However, at the inlet section of the lower channel, an abrupt increase in velocity is observed. This increase in velocity magnitude is attributed to the geometrical shape of the flow reversal module, which increases the velocity of fluid. The fast-moving fluid failed to follow the geometrical shapes; a recirculation zone is created on the lower channel. The velocity magnitude reaches its maximum value in this zone, as represented by the arrow plots.