## 3 Results  
Images of the spray flames provide insights into the flame dynamics and interaction with the pilot flame. Typical spray flame images for the Series 1 flames are shown in Figure 2. In Series 1, the pilot flame is varied and the spray conditions are kept constant. The lowest pilot equivalence ratio (p40) corresponds to the highest oxygen flow entrain \begin{table}
\begin{tabular}{l c c c c c c c} \hline \hline  & **Pilot** & **Pilot** & **Pilot** & **Atomizing** & **Feed** &
\begin{tabular}{c} **Feed** \\ **Coal Tar** \\ \end{tabular} \\ \cline{2-7}
**Name** & \(\Phi\) & \(\mathbf{C_{2}H_{4}}\) & \(\mathbf{O_{2}}\) & \(\mathbf{N_{2}}\) & \(\mathbf{N_{2}}\) & **Toluene** & **Distillate** \\ \cline{2-7}  & & **(SLPM)** & **(SLPM)** & **(SLPM)** & **(SLPM)** & **(mL/min)** & **(mL/min)** \\ \hline \multicolumn{7}{c}{Series 1: 65\% toluene—35\% coal tar distillate by volume; increasing pilot equivalence ratio} \\ \hline p40t65d35 & 0.40 & 1.65 & 12.36 & 10.4 & 9.9 & 1.95 & 1.05 \\ p43t65d35L & 0.43 & 1.65 & 11.46 & 10.4 & 9.9 & 1.95 & 1.05 \\ p45t65d35 & 0.45 & 1.65 & 11.01 & 10.4 & 9.9 & 1.95 & 1.05 \\ p50t65d35 & 0.50 & 1.65 & 9.89 & 10.4 & 9.9 & 1.95 & 1.05 \\ \hline \multicolumn{7}{c}{Series 2: Pilot equivalence ratio = 0.43; increasing coal tar distillate volume percent} \\ \hline p43t100d0 & 0.43 & 1.65 & 11.46 & 10.4 & 10.8 & 3.00 & 0 \\ p43t65d35H & 0.43 & 1.65 & 11.46 & 10.4 & 10.8 & 1.95 & 1.05 \\ p43t40d60 & 0.43 & 1.65 & 11.46 & 10.4 & 10.8 & 1.20 & 1.80 \\ p43t25d75 & 0.43 & 1.65 & 11.46 & 10.4 & 10.8 & 0.75 & 2.25 \\ \hline \hline \end{tabular} Notes: Feed is liquid at a total flow rate of 3 mL/min. Sheath N\({}_{2}\) gas co-flow is 120 SLPM for all flames.  
\end{table} 
Table 1: Pilot and spray flame conditions. ment to the spray because it is there that the most excess oxygen available. The p40 flame shows the tallest area of flame luminescence because the higher oxygen entrainment may result in a higher spray flame temperature. As the pilot oxygen flow rate is reduced, the equivalence ratio increases such that less excess oxygen is available for the spray flame. In general, the flame temperature of burner-stabilized flames decreases with decreasing flow rate and increases with increasing equivalence ratio. These two competing factors affect the appearance of Series 1 flames. Tall flame heights are maintained with an increasing equivalence ratio, perhaps due to increasing pilot flame temperature. Decreasing flame luminosity due to less available oxygen is also observed and the p50 pilot flame corresponding to the lowest oxygen is relatively short and dim. All flame images are captured at 1/3200 s shutter speed to prevent saturation in the brightest flame images and for comparable images across all flames. The observed range of flame conditions is expected to impact the production of flame-formed carbon black particles. In Series 2 flames, the pilot flame is kept constant and the spray composition is modified to include a greater contribution of coal tar distillates. Typical flame spray images for Series 2 flames are shown in Figure 3. Under this condition, the pure toluene spray flame shows lower visible emissions from the spray carbon flame particles. The addition of more coal tar distillate to the spray feed significantly increases the flame luminosity and the flame height. The greatest flame height observed for the 25% toluene/75% distillates spray flame may be due to the more carbon dense feedstock that induces a longer and stronger spray flame for the same oxygen entrainment. The presence of heavier precursor components is evidenced by bright burning droplets extending into the pilot flame for increasing coal tar distillate feed. The series of flame images imply that the formed carbon undergoes a range of growth times, growth temperatures and oxidation conditions, and these effects are discussed in terms of observed particle properties below. Figure 2: Flame images of Series 1 flames; t65d35 with p40 (**a**), p43 (**b**), p45 (**c**) and p50 (**d**).