Typically the headspace gases are flowing, since, even in those bioreactors in which the bed itself is not forcefully aerated, air is typically circulated through the bioreactor such that it moves transversely across the bed surface. In forcefully aerated bioreactors the flow is normal to the bed surface, that is, the air leaving the bed moves perpendicularly away from the bed surface (although in this type of bioreactor, once the air has left the bed, little attention is paid to it). In either case, this bulk flow carries not only energy with it, but also O2, CO2, and water vapor. With this bulk movement of air, conduction and diffusion will typically make negligible contributions to heat and mass transfer within the headspace phase.
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