Convective heat transfer characteristics of microalgae slurries in a circular tube flow

Abstract

Continuous hydrothermal pretreatment in tubular reactors is a key process for the production of biofuel from microalgae biomass. Complex heat transfer characteristics of microalgae slurries, which consist of micron-sized cells suspended in water, are a challenge for the industrialization of hydrothermal pretreatment systems. In this study, the convective heat transfer characteristics of microalgae slurries flowing through a uniformly heated circular tube were experimentally investigated for the first time. The results revealed that the microalgae cell concentration and cell motion in the solid–liquid two-phase flow were the two main factors affecting the convective heat transfer characteristics of microalgae slurries in laminar tube flow. Considering the effects of the cell concentration, cell motion, viscosity variation with temperature and shear rate, and Peclet number, a novel correlation was proposed to predict the Nusselt number of microalgae slurries in laminar tube flow. In addition, a heat transfer deterioration phenomenon was observed under forced convection subcooled boiling conditions. This study provides guidance for the design and optimization of tubular heat exchangers in continuous hydrothermal pretreatment systems.