The static headspace method using a small enclosed chamber has been extensively used to estimate the emission characteristics of formaldehyde, e.g., emission rates from building materials. The formation of the transient formaldehyde concentration by emission, diffusion and sorption in a small confined chamber was investigated using three-dimensional modelling and numerical analyses. Here, five types of glass desiccators were adopted as the small chamber for the headspace method. The inner geometries of the desiccator with the emission source (the building material) and sorbent (water in a Petri dish) were precisely modelled. Transient numerical analyses were performed to determine the formaldehyde emission from different building materials of the external (evaporative) diffusion control type, molecular diffusion and sorption on the water in the confined desiccators. In order to clarify the effect of the desiccator inner geometry on the formaldehyde emission characteristics, the equivalent diffusion length (Ld) concept, which could be identified as the representative one-dimensional diffusion length scale, was proposed. The results of the numerical analyses showed that Ld and the formaldehyde concentration in the sorbent solution over a 24 h numerical experiment were significantly affected by the desiccator geometry. These results confirmed that the calibration of the emission rate with external diffusion control using Ld is appropriate when measuring the formaldehyde emission rate in an enclosed desiccator with different geometries.
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