Blow-off mechanisms of turbulent premixed bluff-body stabilised flames operated with vapourised kerosene fuels

The lean blow-off (LBO) behaviour of unconfined lean premixed blu -body stabilised flames with various fuels was investigated. Methane and vapourised ethanol, heptane, Jet-A1, and an alternative alcohol-derived kerosene (Gevo) were used. OH* chemiluminescence (5 kHz), OH- and Fuel-PLIF (5 kHz), and CH2O-PLIF (10 Hz) were deployed. For all fuels, 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 extinguished. The CH2O seemed to enter into 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 and the duration of this transition was 25 (d=UBO) in the methane and ethanol flames and 60 (d=UBO) in flames operated with heptane and the two kerosenes (where d is the blu -body diameter and UBO the LBO velocity). This large di erence could be due to re-ignitions of partially-quenched fluid inside the RZ during the LBO event. Additionally, for the same bulk velocity, the kerosene flames blow-o at higher equivalence ratios than the single-component fuelled flames, which is possibly due to the higher Lewis number and lower extinction strain rates of these fuels. The results suggest that the blow-o 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.Cambridge Trust

Experimental assessment of the lean blow-off in a fully premixed annular combustor

The behaviour of the flame in an annular combustor with multiple bluff-body injectors with swirl was investigated to provide insights into lean blow-off (LBO) mechanisms when flames interact. Two different configurations, with 12 and 18 burners, and various bulk velocities and equivalence ratios were tested. Flame shape and main features were studied by means of 5 KHz OH$^*$ chemiluminescence imaging and the stability limits were identified and compiled into regime diagrams. As the equivalence ratio of the mixture was reduced the individual flames would first exhibit a transition from a stable ``W-shape" state to a stable ``V-shape" state before becoming unstable close to extinction. In the 18-burner configuration LBO was characterised by random detachment and re-stabilisation of the flame over multiple burners across the chamber, until complete lift-off. In the 12-burner configuration the flame anchors on a few burners in azimuthally symmetric locations, making the overall flame less prone to extinguish. Finally, the stability curves were computed using a correlation based on the Damkholer (Da) number and compared to single burner configurations. The beginning of the blow-off transient was found to be similar to the LBO condition for a single burner in the 12-burner setup, while the 18-burner configuration was less stable for all the conditions investigated. However, it was found that correlations based on single burner extinction data do not fully work for the extinction of interacting flames. The results provide insights into the blow-off of realistic gas turbine engines and can be used for validating models of such processes.EU Marie Skłodowska-Curie Grant Agreement No. 76599

Effect of spark location and laminar flame speed on the ignition transient of a premixed annular combustor

The flame expansion process (``light-round'') during the ignition transient in annular combustors depends on a number of parameters such as equivalence ratio (and hence laminar burning velocity, $S_L$, of the mixture), turbulent intensity, mean flow magnitude and direction, geometry, and spark location. Here, an experimental study on a fully premixed, swirled, bluff-body stabilised annular combustor is carried out to identify the sensitivity of the light-round to these parameters. A wide range of conditions were assessed: two inter-burner spacing distances, two fuels (methane and ethylene), bulk velocities from 10 to 30 m/s, and $\phi$ between 0.75 and 1 for methane and 0.58 and 0.9 for ethylene. The spark location was varied longitudinally ($x/D$ = 0.5 and $x/D$ = 5, where $D$ is the bluff body diameter, expected to lie inside and downstream of the inner recirculation zone of a single burner, respectively) and azimuthally. The propagation of the flame during the ignition transient was investigated via high speed (10 kHz) OH$^*$ chemiluminescence using two cameras to simultaneously image the annular chamber from axially downstream and from the side of the combustor. The pattern of flame propagation depended on the initial longitudinal spark location and comprised of burner-to-burner propagation close to the bluff bodies and upstream propagation of the flame front. The spark azimuthal position\textcolor{red}{, in this horizontal configuration,} had a negligible impact on the light-round time ($\tau_{LR}$), thus buoyancy plays a minor role in the process. In contrast, sparking at $x/D$ = 5 resulted in an increase in $\tau_{LR}$ by $\sim$30-40\% for all the conditions examined. The inter-burner spacing had a negligible effect on $\tau_{LR}$. When increasing bulk velocity, $\tau_{LR}$ decreased. For a constant bulk velocity, $\tau_{LR}$ depended strongly on $S_L$ and it was found that mixtures with the same $S_L$ from different fuels resulted in the same $\tau_{LR}$. Further, the observed propagation speed, corrected for dilatation, was approximately proportional to $S_L$ and was within 30\% of estimates of the turbulent flame speed at the same conditions. These findings suggest that $S_L$ is one of the controlling parameters of the light-round process; hence turbulent flame propagation has a major role in the light-round process, in addition to dilatation and flame advection by the mean flow. The results reported in the study help explain the mechanism of light-round and can assist the development of efficient ignition procedures in aviation gas turbines.EU project ANNULIGHT (765998

Lean blow-off scaling of turbulent premixed bluff-body flames of vaporized liquid fuels

Cambridge Trust

Endogenous Growth and Technological Progress with Innovation Driven by Social Interactions

We analyze the implications of innovation and social interactions on economic growth in a stylized endogenous growth model with heterogenous research firms. A large number of research firms decide whether to innovate or not, by taking into account what competitors (i.e., other firms) do. This is due to the fact that their profits partly depend on an externality related to the share of firms which actively engage in research activities. Such a share of innovative firms also determines the evolution of technology in the macroeconomy, which ultimately drives economic growth. We show that when the externality effect is strong enough multiple BGP equilibria may exist. In such a framework, the economy may face a low growth trap suggesting that it may end up in a situation of slow long run growth; however, such an outcome may be fully solved by government intervention. We also show that whenever multiple BGP exist, the economy may cyclically fluctuate between the low and high BGP as a result of shocks affecting the individual behavior of research firms

Experimental investigation of unconfined turbulent premixed bluff-body stabilized flames operated with vapourised liquid fuels

The structure of turbulent unconfined bluff-body flames of vapourised liquid fuels was investigated at conditions far from and close to blow-off with high-speed (5 kHz) OH-PLIF imaging and 10 Hz CH$_2$O-PLIF imaging. Four different fuels were considered: ethanol, heptane, and two different kerosene blends (a conventional Jet-A and an alcohol-to-jet kerosene, respectively denoted as A2 and C1 following the USA National Jet Fuels Combustion Programme). OH-PLIF images of ethanol flames far from blow-off display a high intensity of OH-LIF signal along the shear layer. In contrast, the OH-LIF signal was evenly distributed throughout the recirculation zone (RZ) of the heptane and kerosene 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 ($\Sigma_{2D}$) 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 as the flames approached blow-off. No significant variation in $\Sigma_{2D}$, curvature PDF, and local turbulent consumption speed was observed between the different fuel types. The average CH$_2$O-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, heptane and kerosene flames showed more pockets of CH$_{2}$O-LIF signal within the RZ as compared to ethanol, suggesting that considerably more partially-combusted fluid enters the RZ of the former than the latter. High-speed particle image velocimetry was performed to measure the local velocity fields and place various regions of the flame 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. However, the fact that the fuel type results in differences in some structural features near blow-off suggests that flames produced with heavy hydrocarbon fuels involve chemistry effects at blow-off that are not fully characterized by laminar flame properties.Cambridge Trust

Experimental investigation of unconfined turbulent premixed bluff-body stabilized flames operated with vapourised liquid fuels

The structure of turbulent unconfined bluff-body flames of vapourised liquid fuels was investigated at conditions far from and close to blow-off with high-speed (5 kHz) OH-PLIF imaging and 10 Hz CH2O-PLIF imaging. Four different fuels were considered: ethanol, heptane, and two different kerosene blends (a conventional Jet-A and an alcohol-to-jet kerosene, respectively denoted as A2 and C1 following the USA National Jet Fuels Combustion Programme. OH-PLIF images of ethanol flames far from blow-off display a high intensity of OH-LIF signal along the shear layer. In contrast, the OH-LIF signal was evenly distributed throughout the recirculation zone (RZ) of the heptane and kerosene 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 (Σ2D) 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 as the flames approached blow-off. No significant variation in Σ2D, 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, heptane and kerosene flames showed more pockets of CH2O-LIF signal within the RZ as compared to ethanol, suggesting that considerably more partially-combusted fluid enters the RZ of the former than the latter. High-speed particle image velocimetry was performed to measure the local velocity fields and place various regions of the flame 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. However, the fact that the fuel type results in differences in some structural features near blow-off suggests that flames produced with heavy hydrocarbon fuels involve chemistry effects at blow-off that are not fully characterized by laminar flame properties

Experimental Investigation of Soot Production and Oxidation in a Lab-Scale Rich–Quench–Lean (RQL) Burner

Swirl-stabilized, turbulent, non-premixed ethylene–air flames at atmospheric pressure with downstream radially-injected dilution air were investigated from the perspective of soot emissions. The velocity and location of the dilution air jets were systematically varied while the global equivalence ratio was kept constant at 0.3. The employed laser diagnostics included 5 kHz planar laser-induced fluorescence (PLIF) of OH, 10 Hz PAH-PLIF, and 10 Hz laser-induced incandescence (LII) imaging of soot particles. OH-PLIF images showed that the reaction zone widens with dilution, and that regions with high OH-LIF signal shift from the shear layer to the axis of the burner as dilution increases. Dilution is effective at mitigating soot formation within the central recirculation zone (CRZ), as evident by the smaller PAH-containing regions and the much weaker LII signal. Dilution is also effective at halting PAH and soot propagation downstream of the dilution air injection point. The high momentum dilution air circulates upstream to the root of the flame and reduces fuel penetration lengths, induces fast mixing, and increases velocities within the CRZ. Soot intermittency increased with high dilution velocities and dilution jet distances up to two bluff body diameters from the burner inlet, with detection probabilities of < 5% compared to 50% without dilution. These results reveal that soot formation and oxidation within the RQL are dependant on the amount and location of dilution air injected. This data can be used to validate turbulent combustion models for soot

Experimental assessment of the lean blow-off in a fully premixed annular combustor

The behaviour of the flame in an annular combustor with multiple bluff-body injectors with swirl was investigated to provide insights into lean blow-off (LBO) mechanisms when flames interact. Two different configurations, with 12 and 18 burners, and various bulk velocities and equivalence ratios were tested. Flame shape and main features were studied by means of 5 KHz OH$^*$ chemiluminescence imaging and the stability limits were identified and compiled into regime diagrams. As the equivalence ratio of the mixture was reduced the individual flames would first exhibit a transition from a stable ``W-shape" state to a stable ``V-shape" state before becoming unstable close to extinction. In the 18-burner configuration LBO was characterised by random detachment and re-stabilisation of the flame over multiple burners across the chamber, until complete lift-off. In the 12-burner configuration the flame anchors on a few burners in azimuthally symmetric locations, making the overall flame less prone to extinguish. Finally, the stability curves were computed using a correlation based on the Damkholer (Da) number and compared to single burner configurations. The beginning of the blow-off transient was found to be similar to the LBO condition for a single burner in the 12-burner setup, while the 18-burner configuration was less stable for all the conditions investigated. However, it was found that correlations based on single burner extinction data do not fully work for the extinction of interacting flames. The results provide insights into the blow-off of realistic gas turbine engines and can be used for validating models of such processes

Blow off mechanism in turbulent premixed bluff-body stabilised flames with pre-vaporised fuels

The lean blow off (LBO) limits and flame structure of turbulent premixed flames were investigated with pre-vaporised liquid fuels in a bluff-body burner. Ethanol, heptane, and two kerosenes (fuels "A2" and "C1" from the National Jet Fuel Combustion Programme) were used. In order to facilitate comparisons to gaseous-fueled flames, results were also obtained from methane flames. The flame structure was studied with OH* chemiluminescence and CH2 O-PLIF imaging far from and close to blow off. The measured LBO limits indicate that the single-component fuels in this burner are more resilient to blow off than the kerosene fuels. Furthermore, a correlation based on a Damköhler number, which is proportional to the laminar flame speed, does not lead to the successful collapse of the different fuels, indicating that the heavy hydrocarbon fuels experience a different LBO mechanism than the simpler fuels. Average OH* chemiluminescence images of the ethanol and heptane flames are qualitatively similar to that from methane with an M-shape flame brush close to blow off. In contrast, most of the OH* chemiluminescence from the A2 flame is found further above the bluff-body indicating weaker burning near the bluff body, which is not evident in the gaseous-fueled and lighter single-component liquid fueled flames. Both single-and multi-component fuels exhibit significant broadening of their CH2 O-layers as blow-off is approached, with the average thickness increasing from about twice to nearly six times the unstrained laminar flame value. Also, an increasing amount of CH2 O-LIF signal is observed within the recirculation zone, which is consistent with prior and present results obtained from methane flames. Overall, the thickness and appearance of the CH2 O-layers are qualitatively similar between the single-and multi-component fuels; however, the kerosene fuels tend to exhibit wider CH2 O-layers. Additionally, these fuels tend to possess more isolated pockets of CH2 O-LIF signal within the recirculation zone, which suggests that a larger amount of partially-combusted fluid enters it. This result is consistent with the observation that these heavy hydrocarbon flames tend to burn less vigorously near the bluff body than the simpler-fueled flames. The results indicate that at LBO conditions, fuel effects may result in a complex behaviour that can not be simply understood through high-temperature chemistry concepts such as the laminar flame speed