Investigation of effect of tracer species on tracer mixing using CFD.

Tracer-gas techniques are widely used for measurement of airflow in buildings and their accuracy depends critically on the uniformity of tracerlair mixing. However, tracer mixing is still an unsolved problem and the effect of many factors remains unclear. This paper presents a study of the effect on mixing of tracer species. The investigation concentrated on tracer mixing involved in the decay technique, which is the most widely used version of the tracer gas method. The distribution and history of tracer concentration during air flow measurements were examined using Computational Fluid Dynamics (CFD). It was found that for single-zone tracer decay tests, three tracer gases, sulphur hexafluoride, nitrous oxide and carbon dioxide have virtually identical mixing patterns and thus there is no difference between them in terms of flow rate measurement results. However, for multi-tracer gas tests where there is interzonal tracer movement, the three tracer gases with different binary diffusivities exhibit significantly different mixing behaviour. In these situations, the choice of tracer will impact the accuracy of air flow measurement.