Numerical study of flow and heat transfer characteristics of microalgae slurry in a solar-driven hydrothermal pretreatment system

Abstract

Solar-driven hydrothermal pretreatment is an efficient approach for the pretreatment of microalgae biomass for biofuel production. The flow and heat transfer characteristics of microalgae slurries in a solar-driven hydrothermal pretreatment system, which are not clear yet due to the complex thermo-physical properties and non-uniform heat flux, significantly affect the performance of hydrothermal pretreatment. Herein, a flow and heat transfer model of microalgae slurries with non-uniform heat flux was established, and the effects of direct normal irradiation and volume fraction of microalgae biomass on the flow and heat transfer characteristics were investigated. The results showed that secondary flow occurred on the cross section of absorber tube. With the increasing direct normal irradiation, the local Nu firstly decreased and following increased on axial, while the average wall shear stress firstly increased and then decreased on axial. With the increasing volume fraction, the local Nu decreased, while the wall shear stress increased. The average viscosity of microalgae slurry near wall had a significant effect on local Nu and average wall shear stress. The understanding of flow and heat transfer characteristics of microalgae slurry in non-uniform heat flux will be beneficial to the design and optimization of the solar-driven hydrothermal pretreatment system.