Flame Blowout Research Articles

Hetero-/homogeneous combustion and flame stability of fuel-lean propane–air mixtures over platinum in catalytic micro-combustors

The hetero-/homogeneous combustion characteristics and flame stability of lean premixed propane–air mixtures over platinum in micro-combustors were numerically studied. For catalytic micro-combustors in particular, numerical studies focused on key thermal management issues, including impact of combustor materials and heat recirculation through the combustor walls. A global heterogeneous reaction step validated over a wide range of parameters has been established, and was further coupled to a detailed homogeneous reaction scheme. Parametric studies were performed with a two-dimensional numerical model to elucidate the role of key operating conditions (wall thermal conductivity, combustor dimension, inlet velocity, flow rate, and equivalence ratio) in determining flame stability and combustion efficiency. Based on these insights, the roles of heat loss and heat recirculation on the mechanisms of flame stability, blowout and extinction were studied. Simulations indicated that the combustor dimension (which includes gap distance and wall thickness) and wall material are important design variables. An optimum thermal conductivity exists for best flame stability, which is lower than that for the homogeneous counterpart. High conductivity materials should be preferred when high-power systems are desired; contrarily, low-power systems would favor more insulating materials to minimize heat losses. Extinction occurs at lower values of the external heat transfer coefficient. Operation diagram denoting flame stability (stability maps) was constructed, and design recommendations for catalytic micro-combustors were made. Comparisons to homogeneous micro-combustors were also presented.

Transitions and blowoff of unconfined non-premixed swirling flame

The present experimental work reports the first observations of primary and secondary transitions in the time-averaged flame topology in a non-premixed swirling flame as the geometric swirl number SG (a non-dimensional number used to quantify the intensity of imparted swirl) is varied from a magnitude of zero till flame blowout. First observations of two transition types viz. primary and secondary transitions are reported. The primary transition represents a transformation from yellow straight jet flame (at SG = 0) to lifted flame with blue base and finally to swirling seated (burner attached) yellow flame. Time-averaged streamline plot obtained from 2D PIV in mid-longitudinal plane shows a recirculation zone (RZ) at the immediate vicinity of burner exit. The lifted flame is stabilized along the vortex core of this RZ. Further, when SG ∼ 1.4–3, the first occurrence of vortex breakdown (VB) induced internal recirculation zone (IRZ) is witnessed. The flame now stabilizes at the upstream stagnation point of the VB-IRZ, which is attached to the burner lip. The secondary transition represents a transformation from a swirling seated flame to swirling flame with a conical tailpiece and finally to a highly-swirled near blowout oxidizer-rich flame. This transition is understood to be the result of transition in vortex breakdown modes of the swirling flow field from dual-ring VB bubble to central toroidal recirculation zone (CTRZ). The physics of transition is described on the basis of modified Rossby number (Rom). Finally, when the swirl intensity is very high i.e. SG ∼ 10, the flame blows out due to excessive straining and due to entrainment of large amount of oxidizer due to partial premixing. The present investigation involving changes in flame topology is immensely important because any change in global flame structure causes oscillatory heat release that can couple with dynamic pressure and velocity fluctuations leading to unsteady combustion. In this light, understanding mechanisms of flame stabilization is essential to tackle the problem of thermo-acoustic instability.

Properties of inverse nonpremixed pure O2/CH4 coflow flames in a model combustor

In order to evaluate the potential of oxy-combustion of inverse nonpremixed coflow flames using a clean fuel to improve the combustion and emission characteristics compared with the normal coflow flames using air, the fundamental properties of inverse nonpremixed pure oxygen (O2)/methane (CH4) coflow flames in a model combustor are experimentally investigated. The combustion stability (extinction) limits, structure and nitrogen oxide (NOx) emissions of the inverse coflow flames in the quartz-windowed combustion chamber with a single shear coaxial injector and a fully opened exhaust nozzle are measured using OH∗ chemiluminescence, a schlieren imaging system and a gas analyzer. Results show four distinct stability regimes and limits: the attached flame regime, the near-blowout flame regime, blowout limits and blowoff limits. The direct flame and OH∗ chemiluminescence images confirm the cooling effects of chamber walls that cause no steady liftoff flame and only the near-blowout flame. The extinction limits of the inverse CH4/O2 coflow flames are extended compared with the normal CH4/air coflow flames and reasonable levels of NOx emissions are observed, which supports that oxy-combustion of the inverse nonpremixed coflow flames using the clean fuel is acceptable for practical application, providing a useful database for modeling the flames in the confined chamber.

Intermittent Burst Oscillations: Signature Prior to Flame Blowout in a Turbulent Swirl-Stabilized Combustor

This experimental study focuses on characterizing the nonlinear combustion dynamics prior to blowout in a partially premixed combustor with a swirl-stabilized burner. Experiments were performed by varying the global equivalence ratio from 0.92 to 0.33. The acquired time-series data of pressure fluctuations were analyzed using nonlinear time-series analysis. When the combustor was operated at an equivalence ratio of 0.92, broadband low-amplitude pressure oscillations called combustion noise were observed. As the equivalence ratio is decreased to 0.62, the oscillations change their characteristics from low-amplitude broadband oscillations to high-amplitude discrete tones, and the combustor undergoes limit-cycle oscillations. A sharp peak is observed in the amplitude spectra of the acoustic pressure oscillations at this stage. On reducing the equivalence ratio to 0.42 and lower, the periodic motion is interrupted by occasional irregular bursts and the system switches back and forth between the noisy (periodic) and quiet (aperiodic) zones. In the dynamical systems theory parlance, this irregular switching between periodic and aperiodic behaviors is termed intermittency. On reducing the equivalence ratio further, the system reaches blowout condition. Intermittency can be used as the signature prior to flame blowout. The dynamical patterns and inherent nonlinearity during the transition from chaotic behavior to intermittent burst oscillations prior to blowout are shown qualitatively by drawing phase portraits, return maps, and recurrence plots.

Experimental analysis of oxygen-methane combustion inside a gas turbine reactor under various operating conditions

The oxygen-methane diffusion flame taking place in a gas turbine reactor was investigated experimentally with emphasis on flame stability. The oxidizer is a mixture of O2 and CO2 and the oxy-combustion process was studied at different equivalence ratios ranging from Φ = 0.5 to 1.0 and different O2/CO2 mixture composition (100/0, 80/20, 60/40, 50/50, 40/60, 30/70 and 25/75). The flame blowout condition was achieved through the reduction of oxygen percentage in the oxidizer mixture. Measurements were obtained for the flue gas temperature and concentration as well as flame visualization. It was found that the flame is very stable at the equivalence ratio of 0.65. At this ratio, the flame blows out at an O2/CO2 blending ratio of 22/78 for the case of fuel flow rate of 6 L/min and at a blending ratio of 21/79 for the case of fuel flow rate of 9 L/min. Attempts for operating the burner with less than 21% O2 were unsuccessful at all ranges of the operating parameters and resulted in unstable operation and blowout. Moreover, it was observed that the stabilization behavior did not change significantly with the variation of the fuel volume flow rate. It was also found that both flame and flue gas temperatures are reduced with the increase of the equivalence ratio.

Understanding Blowout Phenomena to the Induced Angle of V-Gutter-Stabilized Flames

AbstractThe combustion and flame blowout characteristics are investigated in a vitiated environment by placing the 60°, 90° and 120° V-gutters. The blowout is initiated through varying the equivalence ratio of reactants’ flow rates. The blowout is mainly investigated in ultra-lean condition where the equivalence ratio ranges from 0.18 to 0.8, by varying the length of the combustor; the blowout of flame takes place more quickly; it occurred in all tested gutter angles. On increasing the gutter angle from 60° to 120° the flame blowout takes place at Reynolds number from 2,500 to 6,000, which is inversely proportional to the gutter-induced angle of the gutter. The flame flashback possibility occurs due to aggression of the flame marching towards the blowout. The 120° V-gutter has possible flashback since the blowout takes place at very low Reynolds number. As the length of the combustor increases, it is also evident that flashback phenomenon occurred in the rigorous flame just before the flame blowout.

Successive laser ablation ignition of premixed methane/air mixtures.

Laser ablation has been used to study successive ignition in premixed methane/air mixtures under conditions in which the flow speed leads to flame blow-out. A range of laser pulse frequencies is experimentally mimicked by varying the time interval between two closely spaced laser pulses. Emission intensities from the laser ablation kernels are measured to qualitatively estimate laser energy coupling, and flame CH* chemiluminescence is recorded in a time-resolved manner to capture the flame evolution and propagation. A comparison of the measurements is made between the two successive breakdown ignition events. It is found that the formation of the subsequent ablation kernel is almost independent of the previous one, however, for the successive breakdowns, the first breakdown and its ensuing combustion created temporal regions of no energy coupling as they heat the gas and lower the density. Flame imaging shows that the second ablation event successfully produces another flame kernel and is capable of holding the flame-base even at pulse intervals where the second laser pulse cannot form a breakdown. This study demonstrates that successive ablation ignition can allow for the use of higher laser frequencies and enhanced flame stabilization than successive breakdown ignition.

Investigation of Flame Blow-Out in a Low Bypass Military Turbofan Engine

Flame blow-out is a serious concern for military gas turbine engines, and maintaining a reliable and stable flame in engine throughout the mission is a great challenge. A low bypass military turbo fan engine is investigated for flame blow-out. The history of the engine and its accessories were reviewed. Flight data were analyzed to confirm the blow-out incident. Air flow passages and engine modules were examined for possible foreign object damages or internal damages and found satisfactory. Analysis of atomizer characteristics showed the shifting of spray cone angle and fuel flow rate close to the lower limits. Blockage of atomizer flow passages and liner front end flares could contribute to flame blow-out. One of the filters in the fuel line found damaged. Significant amount of debris were found inside the fuel control valve. From the investigations and evidences, it was concluded that the engine flame blow-out was due to blockage of fuel flow when the air flow was almost steady. Assessment and control of debris in fuel accessories, the periodicity of inspection, and cleaning are very important for maintaining a stable flame throughout the mission. Also understanding the blow-out mechanism in an aero gas turbine engine during maneuvers is necessary to address blow-out issues.

마이크로파가 인가된 화염에서의 화염안정성 및 오염물질 배출특성

The use of electromagnetic energy and non-equilibrium plasma for enhancing ignition and combustion stability is receiving increased attention recently. The conventional technologies have adapted the electrical devices to make the electromagnetic field, which resulted in various safety issues such as high-maintenance, additional high-cost system, electric shock and explosion. Therefore, an electrodeless microwave technology has an advantage for economic and reliability compared with conventional one because of no oxidation. However, the application of microwave has been still limited because of lack of interaction mechanism between flame and microwave. In this study, an experiment was performed with jet diffusion flames induced by microwaves to clarify the effect of microwave on the combustion stability and pollutant emissions. The results show that microwave induced flames enhanced the flame stability and blowout limit because of abundance of radical pool. However, NOx emission was increased monotonically with microwave intensity except 0.2 kW, and soot emission was reduced at the post flame region.

Effects of Electric Fields on Stabilized Lifted Propane Flames

The effects that various charged electrodes, and associated electric fields, have on lifted propane flames have been investigated. Two electrodes were used to provide an electric field with potentials ranging from 0 to 11,000 V. The primary electrode was around the flame and the secondary electrode was the fuel nozzle. Electrode polarity and primary electrode location with various flame field locations (near, mid, far) were varied, resulting in a variety of flame behavior. Results show that the body force resultant from the bulk flow of formed ions, from a positively charged fuel nozzle, and grounded ring electrode, will increase flame liftoff height and, eventually, cause blowout. However, for the opposite polarity (positively charged ring electrode and grounded fuel nozzle), the flame progresses toward reattachment with increasing potentials. Observing the narrow window of flame blowout or reattachment (varying with polarity), it was observed that the lifted flame height fluctuations were increased with the presence of the grounded ring electrode, but reduced when the polarity was shifted to positive configuration (positively charged primary electrode). Flame hysteresis was observed when the ring electrode was positively charged and it was found that the hysteresis regime increased when the potential of the ring electrode was increased to 1500 V but had little changes at lower potentials. While the ring electrode was positively charged, a distinct hole was observed in the center of the flame. Several images are presented that show these flame holes that are present when the electrodes are charged.

On the stability of a turbulent non-premixed biogas flame: Effect of low swirl strength

Biogas like other low calorific value fuels has a very narrow stable region when operating in diffusion flame mode owing to their low burning velocity in conjunction with the unburned flow high velocity. This paper presents an experimental study on the effect of the burner geometry on the stability limits of a turbulent non-premixed biogas flame. The main focus of the study is on the role of the low swirl strength of the co-airflow, and the fuel nozzle diameter. The results revealed that the swirl plays a dominant role on the flame mode (attached or lifted) as well as on its operating/stability limits. However, the results revealed that the swirl effect prevails only at relatively moderate to high co-airflow velocity. That is, the swirl does not have an apparent effect at weak co-airflow when the flame is attached. Whereas, it becomes dominant at relatively high co-airflow velocity where the attached flame lifts off and stabilizes at a distance above the burner. Correlations were proposed to describe the lifted biogas flame blowout limits.

Blowout of turbulent jet diffusion flames

Experiments have been performed to determine the blowout of jet diffusion flames with pure fuels, oxygenated fuels, mixed fuels, and diluted fuels. Stability tests were conducted with pure hydrocarbons at the C2 level to determine the effects of structural differences in the fuels. Diffusion flame blowout models were also used to correlate and interpret the data. Ethylene is more stable than ethane because of the additional heat release from the double-carbon bond and ethane is more stable than dimethyl ether. The blowout pressures of mixtures of ethylene and ethane are not linear contributions of component blowout pressures. Stability tests were conducted with ethylene diluted with air and nitrogen. Since both diluents have similar properties and also have a similar density to ethylene, factors in the blowout process, such as the laminar flame speed and air–fuel mass ratio, were isolated and measured. Stability tests with hydrogen diluted with helium, nitrogen, carbon dioxide, and sulfur hexafluoride were also conducted. The diluted hydrogen diffusion flames become less stable as the complexity of the diluent increases.

Effect of bluff body shape on the blow-off limit of hydrogen/air flame in a planar micro-combustor

We recently developed a micro-combustor with a triangular bluff body, which has a demonstrated 5-time extension in the blow-off limit compared to straight channel. In the present work, the effect of bluff body shape on the blow-off limit was investigated with a detailed H2/air reaction mechanism. The results show that the blow-off limits for the triangular and semicircular bluff bodies are 36 and 43 m/s respectively at the same equivalence ratio of 0.5. Analyses reveal that flame blowout occurs due to the stretching effect in the shear layers for both the triangular and semicircular bluff bodies. Moreover, it is found that the triangular bluff body has a smaller blow-off limit because of the stronger flame stretching as compared with the semicircular case. Calculations indicate that the two cases have negligible differences in heat losses because the reaction zones and high temperature regions are located in the combustor centers. Therefore, the heat losses have a negligible effect on the difference in the blow-off limit of the two micro-combustors.

The effect of the blockage ratio on the blow-off limit of a hydrogen/air flame in a planar micro-combustor with a bluff body

We recently developed a micro-combustor with a bluff body, which has a demonstrated 3- to 5-time extension in the blow-off limit. In the present work, the dimension effect of the bluff body on the blow-off limit (indicated by the blockage ratio, ζ) was investigated with a detailed H2/O2 reaction mechanism. The results indicate that the blow-off limits for ζ = 0.3, 0.4 and 0.5 are 20, 31 and 36 m/s, respectively. Analyses reveal that for ζ = 0.3, flame blowout occurs due to insufficient recirculation zone size. However, flame blowout occurs due to the stretching effect in the shear layers when ζ = 0.4 and 0.5. Calculations indicate that the three cases have negligible differences in heat loss because the high temperature zones are located in the combustor centers; therefore, their effects on the combustor walls are mitigated.

Limits of flame propagation in an aluminum-air mixture flow

The region of stable combustion of ASD aluminum powder in a high-velocity air flow is determined. The dependence of the flame blowout velocity for an aluminum-air mixture flow on the air-to-fuel ratio within α = 0.04–3.0 is obtained.

Responses of V-flames placed in an HF transverse acoustic field from a velocity to pressure antinode

A V-flame stabilized by a vertical cylindrical rod in an annular jet is positioned at different points of an acoustic transverse field. Original mechanisms highlight flame and flow dynamics: vertical mass flow rate modulation resulting from a longitudinal-type wave induced by the pressure fluctuations of the transverse wave; jet and flame dissymmetry caused by acoustic energy effects; CH* emission modulations, related to heat release rate modulations, fluctuating at f0, the forcing frequency, or at f0/2 due to a vortex pairing phenomenon; flame blowout induced by the competition between vertical and lateral displacements due to the induced axial and transverse acoustic velocities, respectively.

Nonlinear self-excited thermoacoustic oscillations: intermittency and flame blowout

AbstractNonlinear self-excited thermoacoustic oscillations appear in systems involving confined combustion in the form of coupled acoustic pressure oscillations and unsteady heat release rate. In this paper, we investigate the nonlinear transition undergone by thermoacoustic oscillations to flame blowout via intermittency, in response to variation in the location of the combustion source with respect to the acoustic field of the confinement. A ducted laminar premixed conical flame, stabilized on a circular jet exit with a fully developed exit velocity profile, was investigated. Transition to limit cycle oscillations from a non-oscillatory state was observed to occur via a subcritical Hopf bifurcation. Limit cycle oscillations underwent a further bifurcation to quasi-periodic oscillations characterized by the repeated formation of elongated necks in the flame that pinch off as pockets of unburned fuel–air mixture. The quasi-periodic state loses stability, resulting in an intermittent state identified as type II through recurrence analysis of phase space trajectories reconstructed from the acoustic pressure time trace. In this state, the flame undergoes repeated lift-off and reattachment. Instantaneous flame images suggest that the intermittent flame behaviour is influenced by jet flow dynamics.

Effect of Partial Premixing on Stabilization and Local Extinction of Turbulent Methane/Air Flames

The stabilization characteristics and local extinction structures of partially premixed methane/air flames were studied using simultaneous OH-PLIF/PIV techniques, and large eddy simulations employing a two-scalar flamelet model. Partial premixing was made in a mixing chamber comprised of two concentric tubes, where the degree of partial premixing of fuel and air was controlled by varying the mixing length of the chamber. At the exit of the mixing chamber a cone was mounted to stabilize the flames at high turbulence intensities. The stability regime of flames was determined for different degree of partial premixing and Reynolds numbers. It was found that in general partially premixed flames at low Reynolds numbers become more stable when the level of partial premixing of air to the fuel stream decreases. At high Reynolds numbers, for the presently studied burner configuration there is an optimal partial premixing level of air to the fuel stream at which the flame is most stable. OH-PLIF images revealed that for the stable flames not very close to the blowout regime,significant local extinction holes appear already. By increasing premixing air to fuel stream successively, local extinction holes grow in size leading to eventual flame blowout. Local flame extinction was found to frequently attain to locations where locally high velocity flows impinging to the flame. The local flame extinction poses a future challenge for model simulations and the present flames provide a possible test case for such study. (Less)

Multiple Sliding Modes with Override Logic: Limit Management in Aircraft Engine Controls

G AS turbine engines are subjected to several operational constraints, both at steady state and in the transient regime. These constraints correspond to mechanical limits such as speed and acceleration; limits in thermodynamic variables such as pressures and temperatures; and limits preventing engine components from undergoing dynamic instabilities, mainly surge, stall and flame blowout. Engine control systems incorporate measures to protect the engine from crossing these limits at all times. Achieving limit protection while ensuring fast transient response represents a design challenge, which was summarized by Spang and Brown in their 1999 paper [1]: “. . .much of the complexity of the control comes from the need to operate the engine as close as possible to its limits.” This paper presents a novel and very effective way to achieve such maximal exploitation of the available limits to enable fast responses in thrust or other relevant variables. The proposed technique is based on thewidely usedmultiregulator architecture with max–min limit protection logic [1,2], which uses linear compensators in its original form. As shown in Fig. 1, this architecture uses a single control input (fuel flow) to drive the desired output (typically fan speed or engine pressure ratio) to a setpoint through a control loop with integral action and a linear compensator. Additional compensators are set up to control limited variables to their permissible values, still using fuel flow as the control input. A max–min selection logic stage decides whether the main output regulation task or any of the limit protection tasks should be made active at anygiven time. Integral action is used to achieve type 1 loops for offset-free regulation. The regulators, thus, provide control input rates. Define two index sets as L f1; 2; . . . lg and H fl 1; l 2; . . . hg, where L contains the indices of the regulators associated with the min selector, while the elements of H are the indices of the regulators associatedwith themax selector. No index is defined for the output of the min selector. As shown in Fig. 1, the control rate applied to the integrator is selected according to the formula

Investigating the dynamics of combustion-driven oscillations leading to lean blowout

The dynamics of combustion-driven thermoacoustic oscillations for a ducted laminar premixed flame has been investigated in lean equivalence ratio conditions. Combustion instability appears in the system as acoustic pressure and flame surface oscillations following a Hopf bifurcation. Further change in the control parameter leads to subsequent bifurcations, causing a rich dynamical behavior such as quasi-periodic and intermittent burst oscillations to appear in the system. During the burst oscillation phase, the system dynamics resembles the flame blowout phenomenon, which is of interest in practical combustion applications.