Experimental investigation of unconfined bluff body stabilised flames with vapourised kerosene fuels

To achieve low emission targets, combustion technologies have increasingly implemented lean premixed flames as they facilitate low NOx and soot emissions. In most modern transport vehicles, combustion occurs under highly turbulent conditions. However, stabilizing lean premixed flames within the high Reynolds number conditions of practical devices is difficult because they are prone to blow-off, resulting in reduced efficiency or worse, engine failure. Thus, there is a need to understand the underlying physics of lean blow-off (LBO) so that accurate, yet computationally tractable models can be developed to predict its onset. In this dissertation, the lean blow-off limits and turbulent flame structure of unconfined, pre-vapourised liquid fuels stabilised on a bluff body burner were investigated at two conditions: far from blow-off (φ/φbo = 1.20) and close to blow-off (φ/φbo = 1.01). Four different fuels were considered, two of which comprised of a single component (ethanol and heptane) while the other two were multi-component kerosene blends (A2 and C1 from the National Jet Fuel Combustion Programme). The lean blow-off limit indicates that the ethanol and heptane flames are more resilient to blow-off than the kerosene fuels. To facilitate comparisons with gaseous-fueled flames, results were also obtained from methane flames. Furthermore, a correlation based on a Damköhler number (Da), which is proportional to the laminar flame speed, does not lead to the successful collapse of the different fuels, indicating that the Da correlations based on laminar flame speed are not applicable. The flame structure and lean blow-off behaviour were studied with OH* chemiluminescence and high-speed (5 kHz) OH-PLIF imaging. Additionally, CH2O-PLIF imaging was used to assess the impact of fuel composition on the CH2O-layer thickness. As the flame approached LBO, fragmentation was observed downstream. The two sides of the flame merged at the axis, pockets of OH and CH2O were found in the recirculation zone (RZ), and eventually, the individual fragments were extinguished. The CH2O seemed to enter the RZ from downstream early in the LBO process, with reactants following suit at times closer to LBO. During LBO, the integrated OH* signal decreased slowly to zero. The duration of this transition was ~25 (d/UBO) in the methane and ethanol flames and ~60 (d/UBO) in the flames operated with heptane and the two kerosenes (where d is the bluff-body diameter and UBO the LBO velocity). This large difference could be due to the re-ignition of partially-quenched fluid inside the RZ during the LBO event. Additionally, for the same bulk velocity, the kerosene flames blow-off at higher equivalence ratios than the single-component fuelled flames, possibly due to the higher Lewis number and lower extinction strain rates of these fuels. The results suggest that the blow-off mechanism is qualitatively similar for each of the fuels; however, the complex chemistry associated with heavy hydrocarbons appears to result in a prolonged LBO event. The average OH* chemiluminescence images of the ethanol and heptane flames are qualitatively similar to those of methane: the flame brushes of both exhibit an M-shape when close to blow-off. In contrast, the distribution of OH* signal in the kerosene flames is primarily concentrated in regions further downstream of the bluff body. Also, whilst close to blow-off, the flame fronts on opposite sides of the bluff body in the downstream region of the recirculation zone merge to create a seemingly closed region above the bluff body for all four flames. The OH-PLIF images of ethanol at far from blow-off display a higher intensity of OH-LIF signal along the annular jets, while the OH-LIF signal was more distributed in the heptane- and kerosene-fueled flames. Regardless of the fuel used, close to blow-off the flame becomes shorter with peak OH-LIF signal intensities lying inside the RZ. All four fuels showed a decrease in flame surface density (∑2D) and broadening of the 2-D curvature PDFs as their blow-off limits were approached. An increase in local turbulent consumption speed was observed in the downstream region at close to blow-off. No significant variation in ∑2D, curvature PDF, and local turbulent consumption speed was observed between the different fuel types. The average CH2O-layer thickness was larger than the computed laminar flame value by a factor of two and six for conditions far from and close to blow-off, respectively. Moreover, when LBO is approached, an increased amount of CH2O-LIF signal is observed within the recirculation zone, which is consistent with prior results obtained from methane flames. Overall, the thickness and appearance of the CH2O-layers are qualitatively similar between the single- and multi-component fuels; however, the kerosene fuels tend to exhibit wider CH2O-layers. Additionally, these fuels tend to possess more isolated pockets of CH2O-LIF signal within the recirculation zone, suggesting that a considerable amount of partially-combusted fluid enters it. High-speed particle image velocimetry was performed to measure the local velocity fields and place these flames on the turbulent premixed regime diagram. It was observed that, regardless of fuel type, conditions close to blow-off occupy the same region on the regime diagram. Ultimately, this effort's results highlight the influence fuel-type has on the LBO of bluff-body stabilized flames. Moreover, this work indicates that the LBO behavior of flames produced with complex hydrocarbon fuels can not be fully understood by high-temperature chemistry concepts such as laminar flame speed.Cambridge India Ramanujan Scholarshi