Measurement of air flow in duct by velocity measurements

Abstract

To measure a flow in a closed duct, one of the available methods is to explore the velocity field. With this method, the quality of the flow measurement is very dependent on the location of the velocity measurement points in the duct section. Recommendations about velocity schemes are proposed in international standards (ISO 3966, ISO 7145, EN 12599 …) for circular and rectangular ducts. These recommendations assume that a turbulent flow profile is established. This requires flow profiler and/or long straight lengths upstream and downstream the measurement section. On site, these recommendations are difficult to apply strictly because conditions of straight lengths are often not available. Secondly, the velocity measurement schemes proposed in standards are often time consuming and users prefer sometimes to simplify them. In this case, the estimation of the measurement error is not known. A numerical study has been carried out to investigate the influence of the velocity measurement scheme on the flow measurement when the distance between disturbances and the measurement section is small in the case of circular and rectangular ducts. The results are presented in term of measurement error according to the shape of the duct, velocity scheme, number of velocity measurements, distance between disturbances and measurement section.