Structures and stabilization of low calorific value gas turbulent partially premixed flames in a conical burner

Experiments are carried out on partially premixed turbulent flames stabilized in a conical burner. The investigated gaseous fuels are methane, methane diluted with nitrogen, and mixtures of CH₄, CO, CO₂, H₂ and N₂, simulating typical products from gasification of biomass, and co-firing of gasification gas with methane. The fuel and air are partially premixed in concentric tubes. Flame stabilization behavior is investigated and significantly different stabilization characteristics are observed in flames with and without the cone. Planar laser induced fluorescence (LIF) imaging of a fuel-tracer species, acetone, and OH radicals is carried out to characterize the flame structures. Large eddy simulations of the conical flames are carried out to gain further understanding of the flame/flow interaction in the cone. The data show that the flames with the cone are more stable than those without the cone. Without the cone (i.e. jet burner) the critical jet velocities for blowoff and liftoff of biomass derived gases are higher than that for methane/nitrogen mixture with the same heating values, indicating the enhanced flame stabilization by hydrogen in the mixture. With the cone the stability of flames is not sensitive to the compositions of the fuels, owing to the different flame stabilization mechanism in the conical flames than that in the jet flames. From the PLIF images it is shown that in the conical burner, the flame is stabilized by the cone at nearly the same position for different fuels. From large eddy simulations, the flames are shown to be controlled by the recirculation flows inside cone, which depends on the cone angle, but less sensitive to the fuel compositions and flow speed. The flames tend to be hold in the recirculation zones even at very high flow speed. Flame blowoff occurs when significant local extinction in the main body of the flame appears at high turbulence intensities.     

Laser-based investigations in gas turbine model combustors

Dynamic processes in gas turbine (GT) combustors play a key role in flame stabilization and extinction, combustion instabilities and pollutant formation, and present a challenge for experimental as well as numerical investigations. These phenomena were investigated in two gas turbine model combustors for premixed and partially premixed CH₄/air swirl flames at atmospheric pressure. Optical access through large quartz windows enabled the application of laser Raman scattering, planar laser-induced fluorescence (PLIF) of OH, particle image velocimetry (PIV) at repetition rates up to 10 kHz and the simultaneous application of OH PLIF and PIV at a repetition rate of 5 kHz. Effects of unmixedness and reaction progress in lean premixed GT flames were revealed and quantified by Raman scattering. In a thermo-acoustically unstable flame, the cyclic variation in mixture fraction and its role for the feedback mechanism of the instability are addressed. In a partially premixed oscillating swirl flame, the cyclic variations of the heat release and the flow field were characterized by chemiluminescence imaging and PIV, respectively. Using phase-correlated Raman scattering measurements, significant phase-dependent variations of the mixture fraction and fuel distributions were revealed. The flame structures and the shape of the reaction zones were visualized by planar imaging of OH distribution. The simultaneous OH PLIF/PIV high-speed measurements revealed the time history of the flow field–flame interaction and demonstrated the development of a local flame extinction event. Further, the influence of a precessing vortex core on the flame topology and its dynamics is discussed.     

An Experimental Investigation of the Turbulence Structure of a Lifted H2/N2 Jet Flame in a Vitiated Co-Flow

Measurements of mean velocity components, turbulent intensity, and Reynolds shear stress are presented in a turbulent lifted H₂/N₂ jet flame as well as non-reacting air jet issuing into a vitiated co-flow by laser doppler velocimetry (LDV) technique. The objectives of this paper are to obtain a velocity data base missing in the previous experiment data of the Dibble burner and so provide initial and flow field data for evaluating the validity of various numerical codes describing the turbulent partially premixed flames on this burner. It is found that the potential core is shortened due to the high ratio of jet density to co-flow density in the non-reacting cases. However, the existence of flame suppressed turbulence in the upstream region of the jet dominates the length of potential core in the reacting cases. At the centreline, the normalized axial velocities in the reacting cases are higher than the non-reacting cases, and the relative turbulent intensities of the reacting flow are smaller than in the non-reacting flow, where a self-preserving behaviour for the relative turbulent intensities exists at the downstream region. The profiles of mean axial velocity in the lifted flame distribute between the non-reacting jet and non-premixed flame both in the axial and radial distributions. The radial distributions of turbulent kinetic energy in the lifted flames exhibit a change in distributions indicating the difference of stabilisation mechanisms of the two lifted flame. The experimental results presented will guide the development of an improved modelling for such flames.     

Observations on Upstream Flame Propagation in the Ignition of Hydrocarbon Jets

A variety of investigators have attempted to characterize the mechanisms of how reaction zones stabilize, or propagate, against incoming reactants, particularly in stable lifted jet flames both laminar and turbulent. In this paper, experiments are described that investigate the characteristics of upstream flame propagation in turbulent hydrocarbon jet flames. An axisymmetric, gaseous turbulent jet mixing in air has been selectively ignited at downstream positions to assess the upstream propagation of the bulk reaction zone. The farthest axial position that permitted the reaction zone to propagate upstream after application of the ignition source, referred to as the “upper propagation limit”, or UPL, is determined for a variety of jet and air co-flow parameters. There is an inverse relationship between the upper propagation limit position and the jet Reynolds number. Conversely, there is a direct relationship between the upper propagation limit and the co-flow velocity. Interpretation of the results is related to the velocity at the stoichiometric surface. Global discussion is made as to what these results imply about the stabilization and propagation of turbulent lifted jet flames.     

Finite Rate Chemistry Effects in Highly Sheared Turbulent Premixed Flames

Detailed scalar structure measurements of highly sheared turbulent premixed flames stabilized on the piloted premixed jet burner (PPJB) are reported together with corresponding numerical calculations using a particle based probability density function (PDF) method. The PPJB is capable of stabilizing highly turbulent premixed jet flames through the use of a small stoichiometric pilot that ensures initial ignition of the jet and a large shielding coflow of hot combustion products. Four lean premixed methane-air flames with a constant jet equivalence ratio are studied over a wide range of jet velocities. The scalar structure of the flames are examined through high resolution imaging of temperature and OH mole fraction, whilst the reaction rate structure is examined using simultaneous imaging of temperature and mole fractions of OH and CH₂O. Measurements of temperature and mole fractions of CO and OH using the Raman–Rayleigh–LIF-crossed plane OH technique are used to examine the flame thickening and flame reaction rates. It is found that as the shear rates increase, finite-rate chemistry effects manifest through a gradual decrease in reactedness, rather than the abrupt localized extinction observed in non-premixed flames when approaching blow-off. This gradual decrease in reactedness is accompanied by a broadening in the reaction zone which is consistent with the view that turbulence structures become embedded within the instantaneous flame front. Numerical predictions using a particle-based PDF model are shown to be able to predict the measured flames with significant finite-rate chemistry effects, albeit with the use of a modified mixing frequency.     

Experimental Characterization of Gelled Jet A1 Spray Flames

Gelled propellants provide energetic performance similar to conventional liquid propellants and safety during storage and handling like a solid propellant. Experiments on unconfined gelled Jet A1 spray flames and the comparison with ungelled spray flames are reported for the first time in this paper in terms of the global features, burning regimes, stability limits, visible flame height, emission spectra, natural luminosity, and CH ^? chemiluminescence. Propellants were atomized by an internally impinging two-fluid atomizer, developed specifically for efficient atomization of non-Newtonian gels. Swirling and non-swirling spray flames were successfully stabilized on a burner incorporating bluff body and annular jet of combustion air over a wide range of operating parameters. Structural features of the atomizer impart high momentum to the (central) spray jet, such that the recirculation zone could be penetrated under all conditions. Long-exposure smoke and high-speed visualizations were employed to study cold flow structures and droplet-vortex interactions. Short-exposure direct and backlit imaging were used to observe global features of spray flames. Stability limits and visible flame heights were mapped for different thermal inputs, swirl numbers, and flow rates of atomizing and combustion air jets. Non-swirling stable anchored, partially blown off, and neck-blown off flames were observed. Lifted, and a transition regime, in which the flame could burn in stable and lifted mode repetitively, were observed for the swirling flames. Interactions between central and annular jets are important in these regimes, determining flame shape, symmetry, and flame height. Jet-like propagation zone determines the flame height through its dependence on momentum of spray jet. The length of this zone is affected by variations in thermal input, gas-liquid ratio, and air-fuel ratio. The gelled Jet A1 flames are remarkably shorter despite having a larger average droplet size than ungelled Jet A1. This experimental observation directly supports theoretical predictions reported in literature. These flames are more luminous than ungelled Jet A1, especially at the base and the neck regions. While, majority of the heat is released in the jet-like propagation zone for both the flames, significant heat is released in the neck zone of ungelled Jet A1 spray flame in comparison to ungelled Jet A1 spray flame due to intense turbulence and smaller droplet size.     

LES-CMC Simulations of Different Auto-ignition Regimes of Hydrogen in a Hot Turbulent Air Co-flow

Large-Eddy Simulation (LES) results in combination with first-order Conditional Moment Closure (CMC) are presented for a hydrogen jet, diluted with nitrogen, issued into a turbulent co-flowing hot air stream. The fuel mixes with the co-flow air, ignites and forms a lifted-like flame. Global trends in the experimental observations are in general well reproduced: the auto-ignition length decreases with increase in co-flow temperature and increases with increase in co-flow velocity. In the experiments, the co-flow temperature was varied, so that different auto-ignition regimes, including low Damköhler number situations, were obtained (no ignition, random spots, flashback and lifted flame). All regimes are recovered in the simulations. Auto-ignition is found to be the stabilizing mechanism. The impact of different detailed chemistry mechanisms on the auto-ignition predictions is discussed. With increasing air temperature, the differences between the mechanisms considered diminish. The evolution of temperature, H₂O, H, HO₂ and OH from inert to burning conditions is discussed in mixture fraction space.     

A Comparison of the Blow-Off Behaviour of Swirl-Stabilized Premixed, Non-Premixed and Spray Flames

Confined short turbulent swirling premixed and non-premixed methane and heptane spray flames stabilized on an axisymmetric bluff body in a square enclosure have been examined close to the blow-off limit and during the extinction transient with OH* chemiluminescence and OH-PLIF operated at 5 kHz. The comparison of flames of different canonical types in the same basic aerodynamic field allows insights on the relative blow-off behaviour. The flame structure has been examined for conditions increasingly closer to blow-off. The premixed flame was seen to change from a cylindrical shape at stable burning condtions, with the flame brush closing across the flow at conditions close to blow-off. The PLIF images show that for the gaseous non-premixed flame, holes appear along the flame sheet with increasing frequency as the blow-off condition is approached, while the trend is less obvious for the spray flame. Non-premixed and spray flames showed randomly-occurring lift-off, which is further evidence of localised extinction. The mean lift-off height increased with increasing fuel jet velocity and decreased with increasing air velocity and approaches zero (i.e. the flame is virtually attached) just before the blow-off condition, despite the fact that more holes were evident in the flame sheet as extinction was approached. It was found that the average duration of the blow-off event, when normalised with the characteristic flow time d/U _b (d being the bluff-body diameter and U _b the bulk velocity) was in the range 9–38 with the spray flame extinction lasting a shorter time than the gaseous flames. Finally, it was found that correlations based on a Damköhler number collapse the blow-off velocity data for all flames with reasonable accuracy. The results can help the development of advanced turbulent combustion models.     

Radiative heat flux characteristics of methane flames in oxy-fuel atmospheres

Oxy-fuel combustion is a promising alternative for power generation with CO₂ capture, where the fuel is burned in an atmosphere enriched with oxygen and CO₂ is used as a diluent. This type of combustion is characterised by uncommon characteristics in terms of thermal heat transfer budget as compared to air supported systems. The study presents experimental results of radiative heat flux along the flame axis and radiant fractions of non-premixed jet methane flames developing in oxy-fuel environments with oxygen concentrations ranging from 35% to 70%, as well as in air. The flames investigated have inlet Reynolds numbers from 468 to 2340. The data collected have highlighted the effects of the flame structure and thermo-chemical properties of oxy-fuel combustion on the heat flux radiated by the flames. It was first observed that peak heat flux increases considerably with oxygen concentration. More generally the radiant fraction increases with both increasing Reynolds number in the laminar regime and oxygen concentration. It was found that despite a difference in flame temperature, the radiative characteristics of the flames (heat flux distributions and radiant fraction) in air were similar to those with 35% O₂ in CO₂. The radiative properties of flames in oxy-fuel atmosphere with CO₂ as diluents appear to be dominated by the flame temperature.     

Non-linear Response of Turbulent Premixed Flames to Imposed Inlet Velocity Oscillations of Two Frequencies

This paper describes an experimental study investigating the non-linear response of lean premixed air/ethylene flames to strong inlet velocity perturbations of two frequencies. The combustor has a centrally-placed bluff body and a short quartz section. The annulus between the bluff body and the flow tube, which also housed the acoustic pressure transducers, allowed the reactants into the combustor. The inlet flow was perturbed using loudspeakers. High speed laser tomography, OH* chemiluminescence and OH Planar Laser Induced Fluorescence (PLIF) have been used for flow visualization, heat release and flame surface density (FSD) measurements respectively. The heat release fluctuations increased initially linearly with inlet velocity amplitude for a single frequency forcing, with saturation occurring after forcing amplitudes of around 15% of the bulk velocity, which was found to occur due to vortex roll up and subsequent flame annihilation. The introduction of energy at the second frequency (i.e, the harmonic) was found to change the vortex formation and shedding frequency, depending on the level of forcing. This resulted in a non-linear flame response transfer function (defined as the amplitude of unsteady heat release divided by the amplitude of velocity perturbation at the fundamental) whose amplitude depended greatly on the amount of harmonic content present in the perturbations. The introduction of higher harmonics reduced the flame annihilation events, which are responsible for saturation, thus reducing non-linearity in the amplitude dependence of the flame response. These results were further verified using sequential time-resolved OH PLIF measurements. The findings from this study suggest that the acoustic response of the flame was mostly due to flame area variation effected by modulation of the annular jet and evolution of the shear layers.     

High-resolution imaging of turbulence structures in jet flames and non-reacting jets with laser Rayleigh scattering

A comparative study of the length scales and morphology of dissipation fields in turbulent jet flames and non-reacting jets provides a quantitative analysis of the effects of heat release on the fine-scale structure of turbulent mixing. Planar laser Rayleigh scattering is used for highly resolved measurements of the thermal and scalar dissipation in the near fields of CH₄/H₂/N₂ jet flames (Re _d  = 15,200 and 22,800) and non-reacting propane jets (Re _d  = 7,200–21,700), respectively. Heat release increases the dissipation cutoff length scales in the reaction zone of the flames such that they are significantly larger than the cutoff scales of non-reacting jets with comparable jet exit Reynolds numbers. Fine-scale anisotropy is enhanced in the reaction zone. At x/d = 10, the peaks of the dissipation angle PDFs in the Re _d  = 15,200 and 22,800 jet flames exceed those of non-reacting jets with corresponding jet exit Reynolds numbers by factors of 2.3 and 1.8, respectively. Heat release significantly reduces the dissipation layer curvature in the reaction zone and in the low-temperature periphery of the jet flames. These results suggest that the reaction zone shields the outer regions of the jet flame from the highly turbulent flow closer to the jet axis.     

Leading-Edge Reaction Zones in Lifted-Jet Gas and Spray Flames

An investigation of the leading edge characteristics in lifted turbulent methane-air (gaseous) and ethanol-air (spray) diffusion flames is presented. Both combustion systems consist of a central nonpremixed fuel jet surrounded by low-speed air co-flow. Non-intrusive laser-based diagnostic techniques have been applied to each system to provide information regarding the behavior of the combustion structures and turbulent flow field in the regions of flame stabilization. Simultaneous sequential CH-PLIF/particle image velocimetry and CH-PLIF/Rayleigh scattering measurements are presented for the lifted gaseous flame. The CH-PLIF data for the lifted gas flame reveals the role that ``leading-edge' combustion plays as the stabilization mechanism in gaseous diffusion flames. This phenomenon, characterized by a fuel-lean premixed flame branch protruding radially outward at the flame base, permits partially premixed flame propagation against the incoming flow field. In contrast, the leading edge of the ethanol spray flame, examined using single-shot OH-PLIF imaging and smoke-based flow visualization, does not exhibit the same variety of leading-edge combustion structure, but instead develops a dual reaction zone structure as the liftoff height increases. This dual structure is a result of the partial evaporation (hence partial premixing) of the polydisperse spray and the enhanced rate of air entrainment with increased liftoff height (due to co-flow). The flame stabilizes in a region of the spray, near the edge, occupied by small fuel droplets and characterized by intense mixing due to the presence of turbulent structures. This revised version was published online in July 2006 with corrections to the Cover Date.     

Investigation of the Syngas Flame Characteristics at Elevated Pressures Using Optical and Laser Diagnostic Methods

The effect of pressure on the characteristics of syngas flames is investigated under gas turbine relevant conditions using planar laser induced fluorescence of OH radicals and OH^* chemiluminescence imaging. An optically accessible combustor fitted with a swirl burner was operated with two different syngas mixtures, preheated air at 700?K, and pressures ranging from 5 to 20?bars. The thermal load varied from 15 to 25?kW/bar at an equivalence ratios 0.5. The OH-PLIF measurements show that the flames under all conditions exhibited two reaction fronts, one at the shear layer between the inner recirculation zone and the fuel inlet, and one between the fuel inlet and the air nozzle. The more or less continuous reaction front at low pressure turned into a highly corrugated flame front at higher pressures, with isolated regions of ignition and extinction. The probability density distribution of the flame curvature for the mixtures studied showed that the inner and outer flame responded differently to the pressure increase, with the mean curvature magnitude also depending on the mixture composition and thermal load. The measurements clearly shows the limitations associated with the use of OH^* chemiluminescence images as a marker for the heat release rate especially in case of syngas mixtures.     

Laser Diagnostic Study of the Mechanism of a Periodic Combustion Instability in a Gas Turbine Model Combustor

To investigate the mechanisms leading to sustained thermoacoustic oscillations in swirl flames, a gas turbine model combustor was equipped with an optically accessible combustion chamber allowing the application of various laser techniques. The flame investigated was a swirled CH₄/air diffusion flame (thermal power 10 kW, global equivalence ratio φ = 0.75) at atmospheric pressure which exhibited self-excited thermoacoustic oscillations at a frequency of 290 Hz. In separate experiments, the flow velocities were measured by laser Doppler velocimetry, the flame structures and heat release rates by planar laser-induced fluorescence of CH and by OH chemiluminescence, and the joint probability density functions of the major species concentrations, mixture fraction, and temperature by laser Raman scattering. All measurements were performed in a phase-locked mode, i.e., triggered with respect to the oscillating pressure level measured by a microphone. The results revealed large periodic variations of all measured quantities and showed that the heat release rate was correlated with the degree of mixing of hot products with unburned fuel/air mixtures before ignition. The thermal expansion of the reacting gases had, in turn, a strong influence on the flow field and induced a periodic motion of the inner and outer recirculation zones. The combination of all results yielded a deeper understanding of the events sustaining the oscillations in the flame under investigation. The results also represent a data base that can be used for the validation and improvement of CFD codes.     

Effect of Multilateral Jet Mixing on Stability and Structure of Turbulent Partially-Premixed Flames

An experimental study was conducted to investigate the effects of multilateral jet mixing, using both three and four side-jets, on the structure and stability of turbulent partially-premixed flames. Particle Image Velocimetry and OH*-chemiluminescence were used to study the effects of geometry and operating conditions on the resulting flow-field and reaction zone structures, respectively. These effects were compared under varying ratios of side-jet to primary flow momentum, whilst keeping the bulk flow constant. It was found that the mixing regimes upstream of the nozzle exit affect the flame characteristics, i.e. an impinging regime is likely to generate a lifted flame whilst a backflow regime is likely to generate an attached flame. Unlike the 4 side-jets cases, the OH* images and v _{r m s} profiles for the 3 side-jets cases show distinct asymmetry, with intense OH* and low velocity fluctuations on the opposite sides of the fuel injection. It was also found that the flow and scalar fields become independent of the upstream conditions, for both 3 and 4 side-jets, after one diameter downstream of the nozzle exit.     

Experimental Study on Lifted Flames Operated with Liquid Kerosene at Elevated Pressure and Stabilized by Outer Recirculation

This study deals with the impact of the operating conditions, e.g. pressure, preheating temperature, pressure drop across the nozzle, nozzle size and stoichiometry, on the reaction zone location and spray evaporation progress in case of a lifted flame. Lifted flames are highly valued for their NO_x reduction potential and for their low susceptibility to flash-back and thermo-acoustic instabilities. These advantageous features arise from the improved homogeneity of the fuel-air mixture provided to the reaction zone. One distinctive feature of the lifted flames is the presence of the so called lift-off zone located between nozzle outlet and main reaction zone. Within the lift-off zone fuel and oxidizer remain a certain time in contact and mix together prior to the onset of the combustion reaction. This leads to a more uniform heat release distribution and lowers the nitrogen oxides emissions at lean conditions by reducing the temperature spikes. In contrast to many other studies the subject of investigation was not a plain jet flame, but a modified version of the airblast nozzle, widely used in industrial applications. The nozzle was operated with liquid kerosene. As liquid fuels are easier to handle than gaseous or solid, it is expected that many efforts in the future will focus on the development of liquid fuels surrogates. Our previous investigations have shown, that the nozzle is well suited to be operated with gaseous fuels as well (Fokaides et al, J Eng Gas Turbine Power 130, 011508 2008). The position of the reaction zone was determined by means of chemiluminescence of the OH^? radicals and from its location the lift-off height was derived. In addition the fuel evaporation progress was measured by means of light scattering, revealing that fuel droplets and main reaction zone are well separated. It was found that the operating conditions have a versatile impact on the length of the lift-off zone and spray cone and thus on the degree of pre-evaporation and premixing. Thus, it may be concluded, that through a proper choice of operating conditions and combustor size a desired lift-off height can be adjusted in accordance with criteria, like available space, required emission levels etc.     

Similarity Issues of Kerosene and Methane Confined Flames Stabilized by Swirl in Regard to the Weak Extinction Limit

One of the most promising methods for reducing NO _x emissions of jet engines is the lean combustion process. For realization of this concept the percentage of air flowing through the combustor dome has to be drastically increased, which implies high volume fluxes in the primary zone of the combustion chamber and represents a substantial challenge in regard to the flame stabilization. Swirl motion is thus applied to the air flux by the swirl generator and decisively contributes to the flame stabilization. The current paper reviews an atmospheric investigation of a burner configuration in regard to the weak extinction limit, comprising a confined non-premixed swirl-stabilized flame. The burner can be supplied with either kerosene or after a small adaption with natural gas (methane). Therefore, a comparison of a kerosene-fuelled flame (spray flame) to a natural gas fuelled one (methane flame) can be performed. Both are realized by almost identical burner configuration and at identical conditions. The main idea of this work is to align the stability characteristics of both flames by means of similarity. However, fundamental differences regarding the flame structures of the flames are detected through in-flame measurements. This determines the limits of the current approach and motivates an appropriate choice of flame modeling.     

Combustion and stabilization characteristics of a branched flame under Helmholtz-type excitations

 The combustion and pollution characteristics of the newly rediscovered “branched flame” are experimentally investigated using a Helmholtz-type excitation. Under specific excitation conditions, high-amplitude Helmholtz excitation induces side jet ejection, which leads to a branched flame. Intense combustion and enhanced heat transfer due to strong oscillation of the flame and hot gases of the branched flame increase the heating effectiveness and fuel saving. Strong velocity oscillation results in accumulation of jet fluid ahead of the ring structure for generation of the side jet. In the side-jet evolution, the strong entrainment of the ring vortex in the initial stages followed by the early growth of the streamwise vortical structures greatly shortens the route to mixing transition of fuel and air in the upstream region of the flame. This enhanced premixing process of the side jet leading to high F probability, which is defined as the probability of the presence of a premixture of fuel and air with concentration within the flammability limits, and low strain rate has significant implications for the stabilization of the branched flame. NOx emission indices for the branched flames can be 30% higher and CO emission indices 50% lower than the unexcited case. Received: 5 June 2000 / Accepted: 21 March 2001     

Spatial resolution of a chemiluminescence sensor for local heat-release rate and equivalence ratio measurements in a model gas turbine combustor

The spatial resolution of a Chemiluminescence Sensor, based on focused Cassegrain optics, to detect the location of the reaction zone and heat-release rate in a model gas turbine combustor is reported. The sensor measures simultaneously the chemiluminescent intensities from OH* and CH* excited radicals in flames in order to obtain information on the local flame characteristics. The spatial resolution was evaluated by a combined theoretical and experimental study in laminar and turbulent flames and was supported by detailed chemistry calculations, including the chemiluminescent species, of unstrained one-dimensional flames. The experimental study involved simultaneous measurements of chemiluminescence with the sensor and laser-based reaction rate imaging, using the product of OH and CH₂O radicals obtained from planar laser-induced fluorescence (PLIF), and OH PLIF for the location of the reaction zone. The study quantified the influence of flame shape and dimensions and the direction of traverse of the focal region of the sensor through the flames on the spatial resolution, thereby identifying the limitations and optimising the applicability of the sensor. The sensor was used to obtain local time-dependent measurements of heat-release and equivalence ratio of a reacting mixture, based on the chemiluminescent intensity ratio of OH*/CH*, in a swirl-stabilised model gas turbine combustor and quantified the degree of air–fuel premixedness, probability of reaction and power spectra of pressure and chemiluminescent intensity fluctuations in two unsteady flames.     

LES/CMC of Blow-off in a Liquid Fueled Swirl Burner

Large Eddy Simulations of two-phase flames with the Conditional Moment Closure combustion model have been performed for flow conditions corresponding to stable and blow-off regimes in a swirl n-heptane spray burner. In the case of stable flame (i.e. low air velocity), the predicted mean and r.m.s. velocities and the location and shape of the flame agree reasonably well with experiment. In particular, the presence of localised extinctions is captured in agreement with experiment. Using model constants previously calibrated against piloted jet methane flames (Sandia F) with localised extinction, we obtain that at the experimentally determined blow-off velocity of the swirling spray flame, the predicted flame also blows off, demonstrating that the LES-CMC approach can capture the global extinction point in a realistic configuration.