Structure of Propylene Turbulent Diffusion Flames in Cross-Flow Near Smoke Point

Abstract

An experimental study of a propylene diffusion flame at its smoke point in a cross-flow with velocities ranging from 2 to 4 m/s and a series of diluted conditions was conducted. A gas jet flame from a circular tube burner (ID = 3.2 mm) with a range of exit velocities (4.2 to 34 m/s) corresponding to a Reynolds number range of 520 to 6065 was studied. Nitrogen was added to the fuel stream to eliminate smoking when the fuel flow rate was lower than the flow rate of pure fuel at smoke point condition (which is defined as the Critical Fuel Mass Flow Rate, CFMFR). The curve of N2 flow rate with fuel flow rate at the smoke point showed a skewed bell shape with two distinct regions. In the first region, the diluent flow rate increased with the fuel flow rate, and in the subsequent region the trend was reversed. These two regions were separated by a transition region. Our previous studies on flames in quiescent conditions concluded that these two regions were controlled by jet momentum and chemical kinetics, respectively. This study presents flame structure details such as transverse temperature and concentration profiles in typical flames representing these two regimes. Most of the temperature profiles show a dual peak structure, where the peak nearer to the burner was higher than the other. Furthermore, the peaks in the transition region flame were more distinct than those in the momentum dominated flame. Most of the flames in the 2 m/s cross-flow had lower O2 concentrations than the flames in the 3 and 4 m/s cross-flow. The temperature profiles, and the concentration profiles of O2 and soot change significantly when cross-flow velocity was changed from 2 to 4 m/s. Findings from this study enable us to understand industrial flares that are commonly used in petroleum refineries and chemical plants.