Computational Analysis of Particle Nucleation in Dilution Tunnels: Effects of Flow Configuration and Tunnel Geometry

Figures & data

TABLE 1 Flow parameters and boundary conditions for the cross-flow and axial flow tunnels for dilution ratios of 20 and 110

	Cross-flow	Cross-flow	Axial-flow	Axial-flow
	(DR = 20)	(DR = 110)	(DR = 20)	(DR = 110)
Dilution air flow rate (g/s)	20	20	3.27	3.27
Dilution air bulk velocity (m/s)	1.0	1.0	0.25	0.25
Dilution air Reynolds number (ReD)	9000	9000	2250	2250
Dilution air temperature (K)	298	298	298	298
Dilution air relative humidity (%)	10	10	10	10
Sample flow rate (g/s)	1.0	0.18	0.16	0.03
Sample bulk velocity (m/s)	14.90	2.75	2.50	0.47
Sample Reynolds number (ReS)	4100	760	693	130
Sample temperature (K)	514	514	514	514
Sample H2O mole fraction	0.065	0.065	0.065	0.065
Sample H2SO4 concentration (kg/m3)	2.32 × 10^− 6	2.32 × 10^− 6	2.32 × 10^− 6	2.32 × 10^− 6
Sample organic 1 concentration (kg/m3)	33 × 10^− 6	33 × 10^− 6	33 × 10^− 6	33 × 10^− 6
Sample organic 2 concentration (kg/m3)	22 × 10^− 6	22 × 10^− 6	22 × 10^− 6	22 × 10^− 6

FIG. 1. Geometries of (a) the cross-flow and (b) axial-flow tunnels. Only regions of the tunnels near the sample inlet are shown. For reference, the diameters of the cross-flow and axial-flow tunnels are 0.15 m and 0.11 m, respectively.

FIG. 2. Measured (solid line), truncated (dashed line), and sample (dash-dot line) particle number distributions (PND) for the cross-flow tunnel at DR = 110.

FIG. 3. Contours of instantaneous velocity magnitude on a cross-section through the center of the cross-flow tunnel for (a) DR = 20 and (b) DR = 110.

FIG. 4. Instantaneous velocity vectors on a cross-section (a) near the sample inlet (b) in a small region near the sample inlet, and contours of instantaneous velocity magnitude (c) immediately downstream of the swirler (d) near the sample inlet for the axial-flow tunnel at DR = 20.

FIG. 5. Measured (solid line) and predicted (dashed line) PND at the tunnel exits for the cross-flow tunnel at (a) DR = 20, (b) DR = 110, and the axial-flow tunnel at (c) DR = 20, (d) DR = 110.

FIG. 6. (a) Comparison of the measured and predicted total N at the tunnel exits for the cross-flow (CF) and the axial-flow (AF) tunnels at DR 20 and 110, (b) model predictions of total N at the tunnel exit for DR 20 low-flow, DR 110 baseline, DR 20 baseline, and DR 110 high-flow cases of the cross-flow (CF) tunnel, (c) model predictions of total N at tunnel exits for large axial-flow (AF) tunnel, baseline cross-flow (CF) tunnel, and large axial-flow (AF) tunnel with after-mixing, and (d) model predictions of total N at the tunnel exits for DR 20 baseline and DR 110 high-flow cases of the cross-flow (CF) tunnel, and for DR 20 of large axial-flow (AF) tunnel with after-mixing for dilution air relative humidity (RH) of 5%, 10%, and 15%.

FIG. 7. Contours of instantaneous nucleation rate in the tunnels for cross-flow tunnel at (a) DR = 20, (b) DR = 110, and (c) the axial-flow tunnel at DR = 20.

FIG. 8. Contours of instantaneous RH on a cross-section in the center of the cross-flow tunnel at DR = 110, (b) contours of instantaneous H₂SO₄ condensation rate at DR = 20, and (c) contours of instantaneous H₂SO₄ condensation rate at DR = 110.

FIG. 9. (a) Contours of instantaneous nucleation rate, (b) contours of root-mean-squared (RMS) velocity fluctuations for DR = 110 high-flow case, (c) contours of instantaneous nucleation rate, and (d) contours of RMS velocity fluctuations for DR = 20 flow-flow case of the cross-flow tunnel. Legend for nucleation rate is provided in Figure 7.

FIG. 10. Contours of (a) instantaneous nucleation rate (b) instantaneous velocity magnitude for DR = 20 of large axial-flow tunnel, and (c) contours of instantaneous nucleation rate for DR = 20 of the large axial-flow (AF) tunnel with after-mixing. Legend for nucleation rate is provided in Figure 7.