Design and Testing of an Aerosol/Sheath Inlet for High Resolution Measurements With a DMA.
Design and Testing of an Aerosol/Sheath Inlet for High
Resolution Measurements With a DMA.
Chen, D. R.; Pui, D. Y. H.; Mulholland, G. W.;
Fernandez, M. G.
Journal of Aerosol Science, Vol. 30, No. 8, 983-999,
aerosols; field models; fluid flow; measurement
A modified aerosol/sheath inlet was designed for a
differential mobility analyzer (DMA) for high resolution
measurements based on field mode1 calculations which
include fluid flow, electric field, and
convective/diffusive transport. To avoid the predicted
flow recirculation for the current inlet design at an
aerosol-to-sheath flow ratio of 0.05. the slit width is
reduced and aerodynamically shaped so that the sheath
velocity and aerosol velocity more nearly match.
Numerical results are presented comparing the fluid flow
of the old and new inlet. Problems associated with the
old inlet include: flow unsteadiness at a flow ratio of
0.025, voltage shift at the peak particle concentration
as a function of the flow ratio, and the historical
observation that, while performing tandem differential
mobility analyzer measurements (TDMA), the voltage
applied on the second DMA for the peak particle
concentration is higher than that for the first.
Measurements demonstrate that all these problems are
reduced or eliminated with the new inlet design. The
TDMA measurements include flow ratios of 0.1, 0.55,
0.025 and 0.0125 at sheath flows of 166 and 333 cm3s-1
(10 and 20 1 min-1). The challenge of performing
measurements at these low flow ratios will be discussed
including flow calibration, flow matching, and pressure
monitoring. The new inlet is applied to the measurement
of the National Institute of Standards and Technology
0.1 mun Standard Reference Material 1963, and it is
shown that the DMA can accurately measure the standard
deviation of this narrowly distributed aerosol.