Numerical Study on Isokinetic Samplers with Different Nozzle Geometries

When collecting dust samples from coal-fired power plant chimneys, a nozzle specially designed with an inlet shape of circle type is used for constant velocity suction. However, it is cumbersome to use nozzles with different areas depending on the flow rate. In this study, the effect of the nozzle inlet shape of the isokinetic sampler on the performance of constant velocity suction was evaluated through simulation. The simulations were conducted using the realizable k-epsilon model, which is known to be suitable for separation flow analysis for particles of 1-50 mu m. The turbulent flow fully developed before reaching the inlet of the sampling probe, and the flow rate was set under the condition that the uniformity was secured at approximately 92% at least. The aspiration ratio was employed for evaluating the degree of constant velocity aspiration of the isokinetic sampling probe. It was found that the larger the particle diameter and the faster the flow rate, the larger the aspiration ratio for both the circular and ellipsoidal inlets. In particular, compared with the circular inlet, the aspiration ratio of the sampler with ellipsoidal inlet was closer to 1 in the free-flow velocity range, from 5 to 15 m/s. For this reason, if the ellipsoidal inlet nozzle is used by adjusting only the length parallel to the major axis, the maintenance cost is expected to be reduced compared to the circle-type nozzle.