Decay of Rotational Airflow with Flow Conditioner in Larger Diameter Ducts for Dust Concentration Measurement using Isokinetic Sampling

Within this study the decay speed of centrifugal device induced rotational airflow in round ducts and its conditioning for dust concentration measurement was investigated. Tests were conducted under room conditions at average Reynolds numbers within the range of 0.7–1.14x10. Duct diameters were 152 mm and 203 mm. Airflow in the ducts was found nonuniform and unsteady within the axial location to diameter ratio (x/D) of 10.5. The nonuniformity of axial airflow was nearly independent of the average Reynolds number. The study proved that it is impractical to decay the rotational airflow simply by using straight extension duct. A flow conditioner with minimal particle loss is needed to make the air flow uniform and steady. As a case study, a simple flow conditioner was tested. It decayed the rotational airflow to an acceptable level within x/D=2. The measured dust concentrations at 5 radial locations at x/D=2 were uniform: σ/μ<1% for total mass concentration, and σ/μ<10% for all the particles smaller than 10 μm.