A Predictive Model for Aerosol Transmission through a Shrouded Probe

Abstract

Shrouded aerosol sampling probes utilize an aerodynamic decelerator (shroud) placed about an inner probe. A model has been developed for predicting the transmission ratio (T) of aerosol from a free stream to the exit plane of the inner probe. This expression, T ) FA s A pr (1 -WL), is based on use of an existing empirical model to characterize the aspiration ratios of the shroud (A s ) and inner probe (A pr ) and based upon new models to characterize the wall loss ratio in the inner probe (WL) and to relate the concentration in the core region of the shroud to the mean concentration predicted by the existing aspiration model through a correlation function, F. Extensive computational results provide a data base for specification of the correlation function. The need for the correlation function results from the phenomenon that particle enrichment in a subisokinetic shroud is non-uniform, with the concentration higher near the wall than in the center region. However, the concentration in the core region of the shroud, which is the aerosol that is ultimately sampled, is quite uniform, albeit at a level that is somewhat higher than the concentration in the free stream. This correlation function depends on particle Stokes number and the velocity ratio between free stream and shroud inlet. The predictive equation was verified by comparing its results with data from physical experiments conducted in aerosol wind tunnels with several sizes of shrouded probes. The standard error of experimental data of aerosol transmission about the predictive equation was 7.7%. The model was also evaluated in-depth by examining its ability to predict the overall aspiration of aerosol from the free stream to the inlet plane of the inner probe, wall loss ratio, and transmission of aerosols from the free stream to the exit plane of the inner probe. The results show that the model underestimates the aspiration by approximately 2%. The model for wall loss ratio underpredicts the experimental values by 8% (which influences the transmission ratio by about 2%), and transmission ratio prediction is within 1% of average experimental data. Applications of shrouded probes involve sampling air from turbulent flows, and the model is based on conditions that simulate those encountered by shrouded probes in typical stack flows. The model takes into account turbulent, inertial, and gravitational effects. It is assumed that the shrouded probe is oriented parallel to the direction of flow and the inner probe is sufficiently small such that it only samples from the core region of the shroud.

Particle deposition in ventilation ducts

“…Values of C shroud are expected to be greater than those of C shroud,c because C shroud includes the flow regions near the shroud walls, where concentrations are higher. Gong et al (1996) …”

Anisokinetic Shrouded Nozzle System for Constant Low-Flow Rate Aerosol Sampling from Turbulent Duct Flow

“…The nozzle inlet also needed to be small enough to avoid extreme sub-isokinetic sampling, and d inlet was set at 8 mm to meet this criterion. Aerodynamic testing and numerical flow simulations of previous shrouded nozzles showed air speeds through the waist area to be about 90% of the free stream air speed when the waist area is 30%-40% of the shroud cross-sectional area (McFarland et al 1989;Gong et al 1996). For the present design, the shroud diameter was selected so that the waist area would be about 40% of the shroud cross-sectional area, similar to the previous designs.…”

“…The shroud decelerates the flow; therefore, it samples sub-isokinetically leading to an enrichment of large particles within the shroud. As described by Gong et al (1996), this enrichment is not uniform within the shroud; it is expected to be greater near the shroud walls where fluid streamlines diverge more than those near the centerline.…”

“…Values of C shroud are expected to be greater than those of C shroud,c because C shroud includes the flow regions near the shroud walls, where concentrations are higher. Gong et al (1996) developed a correlation for the ratio of C shroud,c to C shroud based on the results of numerical simulations of shrouded nozzles with Reynolds numbers in the range 8900-54,000 (based on the shroud diameter). They expressed this ratio, F, as…”

Performance Testing of a Multimetals Continuous Emissions Monitor

“…9 The developers did, however, anticipate the utility of the shrouded nozzle for possible future utilization in continuous emissions monitoring. 10 The exacting requirements of the present MMCEM application are very similar to those for which the shrouded nozzle was originally intended and is ideally suited. Accordingly, the shrouded nozzle has been adopted as a strategic replacement for a Method 5-type sampling probe that proved to be inadequate for sustaining reliable MMCEM operation.…”