Modeling Oxidation of Soot Particles Within a Laminar Aerosol Flow Reactor Using Computational Fluid Dynamics

Figures & data

FIG. 1 (a) Schematic diagram of the TDMA system measuring soot oxidation rate (Higgins et al. 2002). (b) Schematic showing the modeling domain in CFD-ACE⁺. Flow enters a pre-reactor section at 0.22 ms^{− 1} and 300 K. Both the furnace and post-reactor have a parabolic velocity profile, while the pre-reactor has plug flow as shown by the dotted lines. Axial length (m) scale in the z direction along the modeling domain is also shown.

FIG. 2 Grid configuration applied to the simulated geometry in CFD-ACE⁺. The axial direction is scaled by a factor of 0.01 to make the circular cross section more visible.

FIG. 3 Iso-contours of temperature and axial velocity for “Case 1” (above) and “Case 2” (below) at the centre of the tube. The axial direction is scaled by a factor of 0.01.

FIG. 4 Comparison of predicted standard deviations of 5000 particle trajectories of 20 nm particles in the radial Y direction as a function of elapsed time after particle injection in the modeling domain. Theoretical predictions shown by the dotted line used the Stokes-Einstein relation as discussed in the text.

TABLE 1 Comparison of particle penetration efficiencies for 2500 particles of 10 nm diameter with and without pre-reactor

			Lagrangian penetration%
Flow profile	Geometry	Length m	Velocity ms^− 1	No pre-reactor	With pre-reactor	Eulerian penetration efficiency%
Plug	Rectangular channel	0.9	0.050	77.40	76.28	78.08
		0.6		81.12	80.24	82.22
		0.3		86.32	87.04	87.64
		0.9	0.025	66.44	66.84	68.46
		0.6		73.20	73.12	74.28
		0.3		81.16	80.88	81.89
		0.9	0.010	47.00	47.92	50.15
		0.6		56.72	57.60	59.29
		0.3		69.20	69.00	71.30
Parabolic		0.6	0.050	76.40	89.76	90.41
		0.6	0.025	70.44	82.84	84.27
		0.6	0.010	56.80	67.96	71.48
Parabolic	Circular tube	1.2	0.217	70.00	88.54	87.42

FIG. 5 Comparison of CFD predictions against measured outlet particle size distribution for (a) “Case 2” at various temperatures (b) “Case 1” and “Case 2” at furnace set-point temperature of 950°C. Both the cases fail to predict the long tail towards smaller particle sizes at 950°C as shown in (b).

FIG. 6 Effects of modeling parameters for “Case 2” simulations on predicted particle size distribution at the outlet of the reactor at a furnace set-point temperature of 950°C. (a) Thermophoresis with 90 nm initial size particles. (b) Diffusion with 90 nm initial size particles. (c) Thermophoresis and diffusion for 40 nm initial size particles. “Th+Diff”: Both thermophoresis and diffusion turned on, “NoTh+Diff”: Thermophoresis turned off but diffusion turned on, “Th+NoDiff”: Thermophoresis turned on but diffusion turned off, “Exp”. Measured size distributions from (Higgins et al. 2002).

FIG. 7 Radial velocity vectors within the furnace for the “Case 2” simulations at furnace set-point temperature of 950°C using the horizontal (7a–c) and vertical furnace orientations (7d–f). Axial locations correspond to the length scale shown in Figure 1b.

FIG. 8 Particle Trajectories for the “Case 2” simulations at furnace set-point temperature of 950°C for (a) horizontal tube (b) vertical tube.

FIG. 9 Comparison of outlet size distribution for 90 nm particles at furnace set-point temperature of 950°C for “Case 2” simulations with horizontal and vertical tube configurations.

Higgins , K. J. , Jung , H. , Kittelson , D. B. , Roberts , J. T. and Zachariah , M. R. 2002 . Size–Selected Nanoparticle Chemistry: Kinetics of Soot Oxidation . J. Physical Chemistry A , 106 : 96 – 103 .

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