Design, simulation, and characterization of a radial opposed migration ion and aerosol classifier (ROMIAC)

Figures & data

Figure 1. (a) Simplified diagram of balanced drag, F_D, and electric, F_E, forces acting on a particle of mobility Z* in the ROMIAC classification region, resulting in a particle streamline that is parallel to the sample flow direction. (b) Simplified diagram of aerosol and cross-flow fluid velocities (u_a and u_cf, respectively) and electrical migration velocity, v_mig, acting on a particle of mobility Z*. Low operation results in fluid streamlines that significantly deviate from vertical. Particles of mobility Z* will thus experience a high advective velocity (relative to the cross-flow velocity) from sample inlet to outlet, increasing the transmission of all particles, and resulting in lower resolution. (c) High operation results in nearly vertical fluid streamlines, discriminating more heavily against the transmission of particles that are not of mobility Z*.

Figure 2. Schematic of ROMIAC aerosol introduction, classification region, and classified aerosol outlet. Particles are introduced via a tangential inlet tube to be azimuthally distributed through a thin knife-edge gap into the classification region, where particles follow characteristic trajectories based on the degree of balance of drag and electric forces experienced. Particles of mobility Z* will be advected toward the center and extracted through the outlet. Key dimensions that define the classification region are indicated. The origin of the coordinate system is designated to be the center of the incoming cross-flow electrode.

Figure 3. (a) Cutaway view and (b) photograph of ROMIAC with key features numbered. Overall exterior dimensions are 10.5 cm in height and 11.4 cm in diameter. (1) Aerosol inlet tube; (2) tangential inlet to distribution racetrack; (3) distribution knife-edge; (4) dielectric spacer; (5) classified aerosol outlet tube; (6) incoming cross-flow port; (7) high voltage porous electrode; (8) electrically grounded porous electrode; (9) outgoing cross-flow port.

Figure 4. Example COMSOL Multiphysics™ solutions for the case of 2 lpm/20 lpm/10/10 nm. Aerosol inlet and outlet tube extremities are cropped out of view. Color scales are restricted in range to distinguish areas of interest. Physical features of interest that may not be obvious are labeled. (a) Section view showing axial component of fluid flow velocity, u_z. Lines show fluid flow velocity streamlines that originate from the cross-flow inlet, and are truncated at the converging region near the aerosol outlet for clarity. Note that the vertical velocity increases approaching the centered aerosol outlet. Dashed lines z₁ and z₂ mark sections corresponding to those in (b). (b) Overhead view showing non-axial component of fluid flow velocity, . Dashed circles z₁ and z₂ correspond to the cut planes in (a). The white ring between z₁ and z₂ indicates the absence of data, not zero velocity. Note that the non-vertical velocity increases approaching the centered aerosol outlet. (c) Section view showing the normalized electric potential solution. Lines show electric field lines that originate from the high voltage electrode surface. (d) Section view showing the particle concentration solution at a voltage corresponding to Z*.

Table 1. Nominal flow conditions and particle diameters for simulations and experiments in this study. Pe_mig and Pe_migβ values are calculated for convenience in using Figures 7–9.

Qa (lpm)	Qcf (lpm)		β
2	13.3	6.7	0.15
2.5	13.3	5.3	0.19
3	13.3	4.4	0.23
2	16.6	8.3	0.12
2.5	16.6	6.6	0.15
3	16.6	5.5	0.18
2	20	10	0.10
2.5	20	8	0.13
3	20	6.7	0.15
2.1	13.9	6.6	0.15
2.25	20	8.9	0.11
2	30	15	0.07
1.7	34.3	20.2	0.05

^SCOMSOL Multiphysics™ simulation conducted for this D_Z.

^ETandem-ROMIAC experiment conducted for this D_Z. (Electrosprayed molecular standards for D_Z < 2 nm and atomized NaCl for D_Z ⩾ 5 nm.)

Figure 5. Experimental setup diagrams for tandem-ROMIAC measurements. (a) Electrosprayed TAAX molecular standards calibration. (b) Atomized NaCl calibration.

Figure 6. Example measurements (points) and transfer function fits (lines) with listed operating conditions and fitting parameters. Top row: COMSOL Multiphysics™ simulation examples for (a) small D_Z at low operation and (b) large D_Z at high operation. Middle row: tandem-ROMIAC TAAX measurement examples for (c) small D_Z at low operation and (d) small D_Z at high operation. Bottom row: tandem-ROMIAC NaCl measurement examples for (e) large D_Z at low operation and (f) large D_Z at high operation.

Figure 7. Simulation results used to estimate f_Z,bias. (a) f_u,z (from COMSOL Multiphysics™-driven kinematic particle trajectories) against β_nom. Best-fit line is f_u,z = 1.01 + 0.11β_nom. (b) f_Z,bias against estimated f_u,z colored by Pe_mig. (c) 1-to-1 comparison of simulation f_Z,bias against those estimated from an empirical function of f_u,z and Pe_mig. For convenience, Pe_mig values are listed in Table 1.

Figure 8. COMSOL Multiphysics™ simulation correction factor relationships, comparison of simulation values to empirical functions, and comparison of experimental values to empirical functions. Error bars in experimental results represent 2 standard deviations of multiple measurements. Top row: (a) simulation f_β against f_u,z colored by Pe_mig. (b) 1-to-1 comparison of simulation f_β against empirical model of f_u,z and Pe_mig. (c) 1-to-1 comparison of experiment-fitted f_β against empirical model of f_u,z and Pe_mig. Middle row: (d) simulation against β_eff. (e) 1-to-1 comparison of simulation against empirical model of β_eff. (f) 1-to-1 comparison of experiment-fitted against empirical model of β_eff. Bottom row: (g) simulation η_trans against Pe_migβ_eff colored by β_eff. (h) 1-to-1 comparison of simulation η_trans against empirical model of Pe_mig and β_eff. (i) 1-to-1 comparison of experiment-fitted η_trans against empirical model of Pe_mig and β_eff. For convenience, Pe_mig and Pe_migβ values are listed in Table 1.

Figure 9. (a) Theoretical (lines) and observed simulation and experimental (markers) resolution obtained with the ROMIAC against V and Pe_mig. Curves and markers for only three values of are displayed for clarity ( 5.3, 10, and 20.2). (b) Theoretical (lines) and observed simulation and experimental (markers) effective relative resolution obtained with the ROMIAC against V and Pe_mig. Markers for all operating conditions are shown, but curves for only three values of are displayed for clarity ( 5.3, 10, and 20.2). Error bars in experimental results represent 1 standard deviation of multiple measurements. For convenience, Pe_mig values are listed in Table 1.

Table 2. Geometry parameters, fitting factors, and typical flow conditions for various DMAs.

Cylindrical	Inner	Outer	Channel								DZ
classifier	radius	radius	length			Leff	Qa	Qsh			range
	(cm)	(cm)	(cm)	f	fZ	(m)	(lpm)	(lpm)			(nm)	Source(s)
TSI-Long	0.937	1.958	44.44	0.707		7.1	1–3	5–20	4–20	0.5–1	8–1000	Fissan et al. (1996)
												Birmili et al. (1997)
												Flagan (1999)
TSI-Short	0.937	1.958	11.11	0.707			1	10	10	0.6–0.8	5–50	Fissan et al. (1996)
												Flagan (1999)
TSI-Nano	0.937	1.905	4.987	0.716		1.4, 2.24	1–1.5	7.5–30	10–30	0.3–1	1.16–50	Chen et al. (1998)
												Jiang et al. (2011)
Grimm nanoDMA	1.3	2	1.3	0.81		3.87	2	21.9	11	0.5–0.6	1.16–1.70	Jiang et al. (2011)
Vienna-Long	2.9	3.7	60	0.867		4.6	1–3	10–12	4–10	0.6–0.8	10–1000	Winklmayr et al. (1991)
												Birmili et al. (1997)
Vienna-Short	2.5	3.3	11	0.837		4.6	2–5	20–28	5.6–10	0.4–0.9	1–150	Winklmayr et al. (1991)
												Birmili et al. (1997)
												Karlsson and Martinsson (2003)
Karlsruhe-Vienna	2.29	3.3	2.5	0.838		11.53	6	61.4	10	0.5–0.7	1.16–1.70	Jiang et al. (2011)
Rosell-Llompart	2.5	3.3	1.6	0.867			1.45–3.6	10.78–202.2	7.4–56	0.17–0.31	1.12–1.90	Rosell-Llompart et al. (1996)
												Flagan (1999)
Radial	Inner	Outer	Gap								DZ
classifier	radius(cm)	radius(cm)	thick-ness (cm)	f	fZ	Leff (m)	Qa (lpm)	Qsh (lpm)			range (nm)	Source(s)
SMEC	∼0.5	6.5	0.4	1.0		16	1	10	10	0.4–1	6–50	Fissan et al. (1996)
Caltech	0.24	5.04	1.0	1.0	0.989–1.07	0.083–1.99	0.3–2	3–20	10	0.2–0.79	1.16–200	Zhang et al. 1995
RDMA												Zhang and Flagan (1996)
												Birmili et al. (1997)
												Jiang et al. (2011)
Caltech	0.24	0.75	1.0	1.0	1.15	0.87	0.6–1.5	6–15	10–17	0.3–0.6	1.16–1.8	Brunelli et al. (2009)
nano-RDMA												Jiang et al. (2011)
ROMIAC	0.24	1.61^a	1.0^a	1.0	1.14, 1.17^b		1.7–3	13.3–34.3	4.4–20	0.4–1.4^c	1.16–20^a	Mui et al. (2013)
												This study

^aNominal value.

^bf_Z,E values for ROMIAC1 and ROMIAC2, respectively.

^c values range from 0.35 to 1.07.

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