Towards a new method for experimental determination of aerosol sampler aspiration efficiency in small wind tunnels

Abstract

At present, testing of the performance of aerosol samplers can be reliably carried out only for full-scale sampling systems in wind tunnels large enough to accommodate personal samplers mounted on life-sized mannequins and for particles with aerodynamic diameter in the range up to and exceeding 100 μm. But such tests are very difficult because of the problems of achieving uniform aerosol spatial and temporal distributions in the working sections of such large facilities. Typically aerosol size-specific concentrations in such large facilities vary by as much as ±20% both spatially and temporally. In this paper, we articulate an approach to move away from testing in large wind tunnels. This methodology uses scaling relationships and existing empirical equations for aspiration efficiency to prescribe experimental conditions and small-scale sampler designs that can be tested in small wind tunnels that are physically equivalent to testing full-scale samplers in large wind tunnels. We examine, from first principles, the basic assumptions which underlie the measurement of aerosol sampler performance, and the quantitative effects of errors arising from spatial and temporal non-uniformities in aerosol distributions. We show that aspiration efficiency is strongly dependent on the spatial distributions of air velocity and particle concentration far upstream of the sampler. This has important implications for aerosol sampler research, and for testing the performances of practical devices as used in workplace and ambient atmospheric environments, where velocity and concentration profiles are frequently non-uniform. Such non-uniformities might account for some of the large variability in the measurements made using such samplers in the field. Finally, we describe an experimental setup in a small wind tunnel that provides highly uniform velocity and concentration profiles, reproducible test conditions, and also allows rapid acquisition of large amounts of experimental data for aerosol sampler aspiration efficiency. Such a system can be used to rapidly perform detailed tests on scaled down aerosol samplers, and thus characterize them. These models can then be scaled up for use in full-scale applications without further expensive testing.