Characterization of the local hydromechanical stress through experimental and numerical analysis of hydrodynamics under dark fermentation operating conditions

Abstract

In mechanically stirred tanks, turbulent flow conditions have already been shown to impair biohydrogen production through dark fermentation even when the dimensionless Reynolds number (Re) is between 103 and 104. The aim was, therefore, to investigate the local hydrodynamic conditions encountered by bacterial aggregates in a fully baffled 2-L bioreactor equipped with a double stage Rusthton turbine in which dark fermentation had already been extensively studied. Both experimental and numerical tools (namely, Particle Image Velocimetry and Computational Fluid Dynamics) were applied to analyze the flow pattern and determine the main features of the turbulent flow. Experiments and simulations involved two levels of viscosity and three levels of agitation speed, corresponding to Re between 2.2·103 and 1.1·104 and power requirements P/V between 0.6 and 87 W/m3. A good agreement between simulated and experimental data was achieved, including velocity, turbulent kinetic energy and turbulent dissipation rate. Comparing the size of bacterial aggregates to Kolmogorov length scale, experimental and numerical results demonstrated that bacterial aggregates could be exposed to potentially damaging hydromechanical stress under turbulent flow in the vicinity of the impeller when Re = 2.2·103 and that this region rapidly expanded to the whole bioreactor when Re increased, which could explain the rapid fall of biohydrogen production vs.Re in the turbulent flow regime.