Steady and Dynamic Rheological Properties of Dense Slurries of Chlorella vulgaris

This experimental study reports the steady and dynamic rheological properties of dense slurries of the green algae Chlorella vulgaris. Biofuel production from algae growth is a promising technology that has the potential to serve as a significant component of the world’s revised energy mix. Along with providing a renewable fuel source, algae production acts as a CO₂ sink, potentially reducing net CO₂ emissions. Design and operation of algae biofuel production facilities require accurate knowledge of the flow characteristics of algae slurries and estimation of the pumping and harvesting energy requirements. Reliable rheological data is needed to optimize production processes to lower costs and increase yields. This study reports steady state viscosity measurements conducted using the ARES TA rotational rheometer using the common algae strain Chlorella vulgaris over the packing factor range from 0.1 to 0.8. Viscoelastic data was gathered using oscillatory tests conducted on the rotational rheometer with a double wall coquette fixture geometry. Dynamic frequency sweep tests were used to recover the storage shear modulus (G^′), and the loss shear modulus (G^″), which correspond to the elastic and viscous properties of the fluid, respectively. Apparent viscosity of the cell suspensions increased with increasing packing factors. Packing factors lower than 0.3 exhibited Newtonian characteristics, whereas at larger packing factors the behavior was shear-thinning. The algae suspensions exhibited both viscous and elastic behavior when subjected to oscillatory flow, behaving as a dilute solution. Finally, the frequency of the gel point increased with increasing packing factor.