## 4 Results and Discussion
### Mixing Performance of TLC Reactor  
Mixing in outdoor cultures of microalgae is essential to prevent cells from settling and sticking to the bottom, to break down the diffusional gradients of essential nutrients, and to ensure uniform exposure to sunlight. The mixing length of the algae particles which account for the combined streamwise and transversal movement of the algal cell at any given instant can be calculated using Equation (21) [27]. The mixing length is the approximate distance covered by the algae particles while moving from the inlet to outlet of the reactor at any given instant. To account for the combined streamwise and transversal mixing of algae particles, the mixing efficiency in terms of algae cells received at the outlet boundary of the reactor after going through the mixing process at the last time step were calculated. A high value of mass flow rate increases the degree of mixing, coupled with an increase in the number of algae cells reaching the outlet (Figure 3a). These findings are consistent with the results of [PERSON] et al. [10]. The increase in mass flow rate promotes a larger number of particles to undergo the mixing process due to the beneficial effect of turbulence. This situation shows that, to some degree, the mixing can be managed simply by adjusting the fluid velocities in the reactor [7]. However, high mass flow rates consequently can require high power consumption. Moreover, the concentration of algal cells at the outlet of the upper channel is greater in comparison to the lower channel (Figure 3a). The most possible reason for the acquired results can be attributed to the geometric height to which the suspension is delivered onto the upper channel of the reactor, leading to increased cell concentrations and improved mixing [4]. Figure 2: Comparison of the present study with the experimental results of [PERSON] et al. [11]. A twofold decrease in mixing is observed with the increase in the AR from 180 to 340 (Figure 3b). An increase in channel width decreases the concentration of algae particles at the outlet of the reactor because of the increased reactor volume, thereby indicating that particles find more space to move and mix in the reactor. It can be stated that a variation in geometry cause variations in the hydrodynamic properties of the reactor, which affect the mixing phenomenon of the cells [29]. A TLC reactor with a small AR and faster flow rates are suitable for microalgae cultivation because of the significant increase in the cell mixing, provided that the mechanical structure of the algae cells is not damaged [28,30]. The residence time of algae cells has a vital importance in the design of the photobioreactor. The residence time or retention time of the algae cells is the total time traveled by the particles from the inlet to the outlet of the reactor. The effect of mass flow rate, water depth, and AR on residence time is plotted separately for each compartment of the reactor (Figure 4a-c). The algae particles need more time to move from the inlet to the outlet boundary of the reactor when lower mass flow rates are adopted (Figure 4a). However, faster flow rates reduced the residence time by increasing the flow velocity. The higher flow velocity in the lower channel resulted in a larger residence time of algae particles in the lower channel in comparison with the upper channel. Figure 4: _Cont_. Figure 3: (**a**) Effect of mass flow rate on mixing efficiency at a water depth of 5.6 mm and slope angle of 1\({}^{\circ}\). (**b**) Effect of channel width/channel depth on mixing efficiency of the complete reactor at a mass flow rate of 2.4 kg/s, water depth of 5.6 mm, and slope angle of 1\({}^{\circ}\). The residence time is strongly influenced by the geometrical aspects of TLC, as evidenced by the longer time spent by the algae cells in the reactor with the increased water depth and AR due to the increase in reactor volume (Figure 4b,c).