Enhancing turbulent hydrothermal pretreatment of non-Newtonian microalgae slurry utilizing static mixers with secondary flow generators

Hydrothermal pretreatment is an efficient process to convert microalgae slurry into biodiesel. The conversion efficiency largely depends on the heat transfer performance of microalgae slurry in various mixers. Compared with previous studies on laminar heat transfer performance of microalgae slurry in horizontal empty tubes, this study numerically investigates the turbulent heat transfer performance of microalgae slurry in vertical Lightnin static mixer and Kenics static mixer with different aspect ratios. The generalized Reynolds number is in the range of 530–5850. The effects of inlet velocity and mass fraction for microalgae slurry on the enhanced heat transfer performance of static mixers are evaluated. Results indicate the Performance evaluation criteria of Kenics static mixers with the aspect ratios of 1, 1.5, and 2 are higher than those of Lightnin static mixers by 9.32–17.81 %, 10.38–20.51 %, and 3.19–14.95 %, respectively. The empirical correlations of microalgae slurry for Nusselt number and Fanning friction coefficient are proposed. Additionally, the entropy generations of Kenics static mixer are higher than that of Lightnin static mixer by 21.34–27.37 %, 10.80–20.57 %, and 12.31–20.46 %, when the aspect ratios are 1, 1.5, and 2, respectively. Therefore, the Lightnin static mixer is recommended when the enhanced heat transfer performance is mainly considered. Additionally, 5 wt% microalgae slurry is recommended for hydrothermal pretreatment.