A comparative study of wind tunnel and flux chamber for diffuse emission sampling: Experimental evaluation and theoretical approach

One of the most complex challenges in the field of industrial odour emissions is the characterization of passive area sources, which emit contaminants even if not endowed with their own gas flow. Two primary sampling devices have been developed for this scope: Wind Tunnel (WT), with a unidirectional airflow without vertical mixing, and Flux Chamber (FC), with a turbulent well-mixed flow. Essentially, two are the debated issues about the sampling instrumentation. The first concerns which device is more suitable for accurately describing the real fluid dynamic behaviour at the liquid-gas interface. The second issue involves the extent to which the emission rates, estimated by the two sampling systems, differ. This study addresses these debates providing experimental data on the real atmospheric motion near liquid surfaces, aiding the choice between WT and FC. Field tests reveal that atmospheric conditions can make the fluid dynamics similar to either WT or FC, with the general scenario showing an intermediate wind behavior. Thus, it is not possible to a priori determine the most suitable device. Concerning the second issue, this research presents an experimental comparison of VOCs (Volatile Organic Compounds) emission rates between a recently developed WT and a US-EPA design FC. These trials indicate that for soluble compounds, WT and FC yield comparable results. However, for gas-phase controlled species (low-solubility VOCs), WT tends to overestimate emissions compared to FC, with the ratio of specific emission rates ranging from 2 to 4.5. The developed theoretical mass transfer model aligns well with observed values, except for toluene due to its non-ideal mixture with water.