Modeling radiative transfer in photobioreactors for algal growth

Abstract

Simulations of radiative transfer within an air-lift photobioreactor (PBR) are demonstrated by coupling it to the fluid hydrodynamics and employing wavelength dependant properties for the participating media. The radiative properties of the algal media are determined by matching the numerical predictions against measurements of radiative intensity distributions. To assist towards the design, scale-up and optimization of such reactors, a parametric investigation of the relative importance of the angular resolution of the radiation calculations, scattering phase functions of the bubbles, air mass flow rate and the bubble size are investigated by examining the 3D distributions of radiation within the PBR. Two hundred and eighty eight solid angles in the finite volume (FV) formulation of the radiative transfer equation (RTE) provide an optimum combination of speed and accuracy in the resulting calculations. While scattering results in a more effective redistribution of the energy, the results from employing isotropic and forward-scattering phase functions for the bubbles are found to be similar. The importance of bubble scattering diminishes at algal concentrations (>0.5g/L) where the radiation attenuation is dominated by the absorption coefficient of the algal media. While 1μm sized bubbles more effectively redistribute the radiation downstream of the radiators compared to larger sized bubbles, the differences in the radiation profiles obtained from 10μm and 100μm-sized bubbles were small within this reactor. The radiation distributions are also influenced by the mass flow rate of air. The calculations demonstrate the need to rigorously account for the air flow rate, bubble size and the scattering effects of bubbles through fully coupled numerical solutions to the fluid flow and radiative transfer equations and provide some best practice guidelines for increasing the fidelity of radiation simulations in PBR’s.