Simulation of low pressure impactor collection efficiency curves

Abstract

The collection efficiency of the low pressure impactors has been studied using numerical simulations. Flow field was modeled by solving the equations describing the time-average flow field (RANS) with a commercial CFD solver. Particle tracks were calculated separately using Lagrangian methods. Simulation results were verified against published experimental results. Effect of turbulent velocity fluctuations on the impactor resolution was investigated by comparing the ratio of the simulated to experimental impactor resolutions as a function of the turbulence level of the jet. It was found that the turbulence is the dominant mechanism reducing the resolution when the local Reynolds number is over 1800. Effect of jet-to-plate distance on the resolution of the low pressure impactor was studied in the case of low turbulence level. Highest resolution was achieved when the ratio of jet diameter to jet to plate distance (S/W) is 2. When the ratio is lower or higher, resolution is reduced because there is an increase in nonuniformity of the impaction conditions across the jet.

Introduction

Impactors have been used since the end of the 19th century in aerosol science to classify particles according to their size. One widely used method is to use impactor stages in a cascade configuration to measure the size distribution of the particles. In cascade impactors, several impactor stages with varying cutpoints are placed in series. To achieve lower cutpoints, low pressure impactors have been introduced. In cascade low pressure impactors the pressure is reduced either by having a separate pressure reducing stage (e.g. Hering et al., 1978, Hering et al., 1979) or by gradually reducing the pressures by using high jet velocities (e.g. Berner, 1972, Vanderpool et al., 1990).

Several theoretical and numerical studies have been conducted to evaluate the effects of design and flow conditions on the operation of the impactor. The foundation was laid by Marple (e.g. Marple and Liu, 1974, Marple et al., 1974, Rader and Marple, 1985). Jurcik and Wang (1995) studied the effect of geometry on the shape of the collection efficiency curves. Swanson, Muzzio, Annapragada, and Adjei (1996) modeled a low flow rate single orifice cascade impactor. Asgharian, Zhang, and Fang (1997) made theoretical calculations of the collection efficiency of spherical particles and fibers in an impactor. Vinchurkar, Longest, and Peart (2009) modeled the Andersen impactor and studied the effect of particle charge on the collection efficiency. These studies have assumed constant fluid properties. Much less work has been done on the compressible and partially turbulent flow range where many of the practical low pressure impactors work. Leduc, Fredriksson, and Hermansson (2006) used particle tracking in Fluent software to calculate the collection efficiencies of a low pressure impactor. In their study they found that the method was valid only for low velocities and Reynolds numbers. No successful computational study has been published on the factors defining the shape of the collection efficiency curve of the widely used high jet velocity, low pressure impactor. In this study we will present a modeling method that can accurately predict the cutpoints of the low pressure impactor and within the certain limits also the shape of the collection efficiency curve.

The calculation of the Stokes and Reynolds numbers is not trivial in the case of compressible flow, reduced pressure cascade impactors, since the stagnation conditions vary from stage to stage. Usually the pressure on the impaction plate is assumed to be equal to the stagnation pressure before the jet (Biswas and Flagan, 1984, Flagan, 1982). The flow inside the impactor is assumed to be adiabatic and jet velocities are calculated assuming adiabatic conditions (Hering, 1987, Hillamo and Kauppinen, 1991). A semi-empirical method is conventionally used to scale the impactor cutpoint by changing the pressure conditions or jet velocities once the critical Stokes number of the stage has been defined experimentally. This method requires measurements and does not provide information on the shape of the collection efficiency curve. For example, an important parameter characterizing collection efficiency curve is its steepness and conditions in the flow field that define it. Successful modeling effort can bring new beneficial information, for example when trying to understand the factors affecting the resolution of the impactor.

The objective of this study is to introduce and validate a CFD simulation approach that correctly describes the cutpoints and within certain limits the shape of the collection efficiency curve of the low pressure impactor. Validation of the simulation is based on comparisons of numerically predicted cutpoint diameters of the ELPI and the QCM impactors with the existing experimental data. Simulation is based on the time-averaged flow fields which excludes the direct effects of the turbulent velocity fluctuations on particle tracks. Therefore, the range of flow conditions, where the time-averaged approach works was verified. This was achieved by comparing experimental and simulated collection efficiencies with the modeling parameter that describes the intensity of turbulence in the impactor jet. The model was then applied to predict the flow fields and cut curves of the impactor stage and to characterize the flow field dynamics inside the low pressure impactor. The model is used to characterize the effect of the collection plate distance and the flow field properties on the resolution of the impactor.