Development of Polydisperse Aerosol Generation and Measurement Procedures for Wind Tunnel Evaluation of Size-Selective Aerosol Samplers

Abstract

Accurate development and evaluation of inlets for representatively collecting ambient particulate matter typically involves use of monodisperse particles in aerosol wind tunnels. However, the resource requirements of using monodisperse aerosols for inlet evaluation creates the need for more rapid and less-expensive techniques to enable determination of size-selective performance in aerosol wind tunnels. The goal of recent wind tunnel research at the U.S. EPA was to develop and validate the use of polydisperse aerosols which provide more rapid, less resource-intensive test results which still meet data quality requirements necessary for developing and evaluating ambient aerosol inlets. This goal was successfully achieved through comprehensive efforts regarding polydisperse aerosol generation, dispersion, collection, extraction, and analysis over a wide range of aerodynamic particle sizes. Using proper experimental techniques, a sampler's complete size-selective efficiency curve can be estimated with polydisperse aerosols in a single test, as opposed to the use of monodisperse aerosols which require conducting multiple tests using several different particle sizes. While this polydisperse aerosol technique is not proposed as a regulatory substitute for use of monodisperse aerosols, the use of polydisperse aerosols is advantageous during an inlet's development where variables of sampling flow rate and inlet geometry are often iteratively evaluated before a final inlet design can be successfully achieved. Complete Standard Operating Procedures for the generation, collection, and analysis of polydisperse calibration aerosols are available from EPA as downloadable files. The described experimental methods will be of value to other researchers during development of ambient sampling inlets and size-selective evaluation of the inlets in aerosol wind tunnels.

Figure 1.

EPA wind tunnel test system with air flow direction, distances in meters, and important sections labeled: (A) honeycomb, (B) polydisperse aerosol generator, (C) counter flow mixing fan, and (D) oscillating, cross-flow mixing fans, reference samplers R1 and R2, and the candidate sampler. Schematic not drawn to scale.

Figure 2.

Apparatus used for dispensing bulk Arizona Test Dust in a controlled manner and producing discrete, charge neutralized particles. The 2-stage sonic nozzle assembly consists of two identical sonic nozzles configured in series. Schematic not drawn to scale.

Figure 3.

Schematic of system used to produce an ionized airstream for charge neutralization of the generated aerosol. Schematic not drawn to scale.

Figure 4.

Photograph of the 100 Lpm sharp-edged isokinetic reference nozzles for tests at 2, 8, and 24 km/hr (l to r).

Figure 5.

Upstream view of a typical sampler setup in the wind tunnel’s sampler test section. The four oscillating cross-flow mixing fans and the stationary counter-flow mixing fan are visible in the exposure tunnel’s test section.

Figure 6.

Ratio of the two 100 Lpm isokinetic reference samplers’ measured effectiveness (R1/R2) as a function of aerodynamic particle size and ambient wind speed.

Figure 7.

Ratio of the centrally located 100 Lpm isokinetic reference sampler to the mean of the two 100 Lpm isokinetic reference samplers.

Figure 8.

Measured sampling effectiveness of the 16.7 Lpm isokinetic nozzle during its 24 km/hr wind tunnel evaluation. Whiskers represent +/−1 standard deviation of the 13 replicate tests.

Figure 9.

Photograph of the Texas A&M LVTSP sampler during its 24 km/hr size-selective wind tunnel evaluation.

Figure 10.

Measured sampling effectiveness of the LVTSP sampler at 2 km/hr. Whiskers represent +/−1 standard deviation of 15 replicate tests.

Figure 11.

Measured sampling effectiveness of the LVTSP sampler at 24 km/hr. Whiskers represent +/−1 standard deviation of six replicate tests.