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Biogas is a fuel gaining increased interest. To be commercially viable the biogas production process needs to be further improved with advanced industrial standards where the technical, economic and environmental aspects are fully considered.

Understanding fluid dynamics and the microbial reactions in the digestion process is necessary to accurately model and predict the biogas production. In connection with the Swedish company SvenskVäxkraft AB we focus on reactors where part of the produced gas is re-injected at the bottom to generate a strong recirculation with a gas-lift effect with a rising flow in the core. The mixture motion in this type of bio-reactors is entirely induced by the gas.

Computational fluid dynamics (CFD) is used to study the effect of gas plumes of bubbles in the range smaller than 10mm with a maximum local gas volume fraction lower than 10%. This study shows that considering the appropriate models to account for the added agitation and turbulence by the bubbles improves the prediction of the liquid flow characteristics. Neglecting the induced bubble effect leads to erroneous results where the radial dispersion of the gas concentration, the liquid velocity and the turbulence are significantly underestimated.

To validate the model we performed local measurements in an experimental facility where a laboratory water-model is equipped with advanced instruments to measure the gas volume fraction as well as the liquid and gas vertical velocities.

It was found that using the bubble induced turbulence model by Sato et al. [8] with the Tomyami models for the drag and lift forces [3-6], provides predictions in good agreement with the measured quantities.

This study shows that for such processes where the flow is mainly created by the bubbles presence, the pseudo-turbulence (the turbulence induced by the bubbles) and the bubble size distribution need to be properly considered.