# Experimental Study of Coflow Propane--Air Laminar Diffusion Flames at Subatmospheric Pressures
## 3 Results and Discussion
### Soot Formation  
The LII signals obtained at pressures ranging from 50 kPa to 101 kPa are shown in Figure 6. Note that the flames at pressures lower than 50 kPa were not measured for the LII signal as it becomes too weak to be distinguished from the noise signal at very low pressures. Given that the intensity of LII signal is proportional to the soot volume fraction, the contour of the relative soot volume fraction (\(f_{\
u}^{\ast}\)) could be obtained, as in Figure 6. The contour of \(f_{\
u}^{\ast}\) showed the general tendency that the soot yield increased and the sooting region moved upstream as the pressure increased. These tendencies coincide with the observations from Figure 4.  
Figure 5: Power law fit for the maximum flame’s cross-section diameter (\(r_{\
u(\mathrm{fuel/air})}=1\) and 0.5, ambient pressure = 30–101 kPa). Figure 6: Signal intensities from LII measurements at various ambient pressures with \(r_{\
u(\mathrm{fuel/air})}=1\) (**top**) and \(r_{\
u(\mathrm{fuel/air})}=0.5\) (**bottom**). HAB (height above the burner). The \(f_{v}^{*}\) value along the center line is presented in Figure 7. Nonmonotonic influence of \(r_{v(\text{fuel/air})}=1\) on \(f_{v}^{*}\) was observed at different pressures. At pressures higher than 60 kPa, the soot yield increased firstly with the increase in the \(r_{v(\text{fuel/air})}\), whereas below 60 kPa, an inverse tendency occurred (see also in Figure 4). [PERSON] et al. (2018) observed that the smoke point increased with the increase in the \(r_{v(\text{fuel/air})}\) to \(<1\). Their conclusions are in agreement with the tendency observed at the high-pressure range in this study. The peak delimits the soot production-dominated region and the soot oxidation-dominated region. Flames at \(r_{v(\text{fuel/air})}=0.5\) have a larger soot production-dominated region than that of the flames at \(r_{v(\text{fuel/air})}=1\). Therefore, the residence time for soot formation increased at lower \(r_{v(\text{fuel/air})}\), which would contribute to the increased relative soot volume fraction (LII signal intensity). The peak relative soot volume fraction (\(f_{v,max}^{*}\)) versus pressure is plotted in Figure 8. A power law relationship was fitted to seek the dependence of \(f_{v,max}^{*}\) on \(p^{n}\), with the exponents of \(n=1.7\) and \(2.8\) for \(r_{v(\text{fuel/air})}=1\) and \(r_{v(\text{fuel/air})}=0.5\), respectively. The soot formation rate decreases at lower pressures, therefore, at both conditions, the peak relative soot volume fraction decreased with the decrease in pressure. However, at a lower \(r_{v(\text{fuel/air})}\), the soot yield decreased more quickly with the decrease in pressure. This was possibly related to the decrease in the fuel burning rate at a lower \(r_{v(\text{fuel/air})}\). The fuel burning rate was mainly controlled by forced convection and buoyancy-induced natural convection. The former was due to the momentum exchange between the fuel and air streams and was stronger at a higher \(r_{v(\text{fuel/air})}\). The latter was significantly determined by the environmental pressure and would decrease to a lower level at low pressures, thus forming a more spherical flame, as in Figure 4. At the normal pressure, 101 kPa, buoyancy-induced convection was strong enough to compensate for the weakness of forced convection and ensure complete mixing and burning of the fuel. However, the mixture was still fuel-rich at the flame base for \(r_{v(\text{fuel/air})}=0.5\) as forced convection was dominant there. Therefore, flames at a lower \(r_{v(\text{fuel/air})}\) have higher sooting propensity and presented a larger soot volume fraction. With the decrease in pressure, the fuel burning rate decreased since forced convection was insufficient to mix all the fuel, and the fuel burning rate decreased more quickly at a lower \(r_{v(\text{fuel/air})}\). Considering that the conversion ratio of carbon into soot was approximately constant, the quick decrease in \(f_{v,max}^{*}\) in Figure 8 was the expected consequence.