Shorter Flame Research Articles

Effects of turbulent flow regime on perforated plate stabilized piloted lean premixed flames

An experimental study of the effects of turbulent flow regime on the flame structure is conducted by using perforated-plate-stabilized, hydrogen-piloted, lean premixed methane/air turbulent flames. The underlying turbulent flow field was investigated using two-dimensional three-component particle imaging velocimetry (2D3C-PIV) with two perforated plates of different blockage ratios. The flow data allowed a separation of the turbulent flow regime into axial velocity stream dominated and vortex dominated flows. A plate with 62% blockage ratio was used to establish the stream-dominated flow regime and another with 86% blockage ratio was used to establish the vortex-dominated flow regime. OH laser-induced fluorescence was used to study the effects of the turbulent flow regime on the mean progress variable, flame brush thickness, flame surface density, and global consumption speed. In comparison with the stream-dominated flow, the vortex-dominated flow makes a wider and shorter flame. Also, the vortex-dominated flow has a thicker horizontal flame brush and a thinner longitudinal flame brush. Especially, the spatial variation of the horizontal flame brush thickness for the vortex-dominated flow does not follow the turbulent diffusion theory. The vortex-dominated flow shows a relatively constant flame surface density and the stream-dominated flow shows a decreasing flame surface density along the streamwise direction. The flame surface density for the vortex-dominated flow is higher than the one for the stream-dominated flow. Lastly, the vortex-dominated turbulent flow shows a much higher consumption speed in comparison with that of the stream-dominated turbulent flow with identical velocity fluctuation levels.

Study on the transition mechanism for broadleaf foliage from smoldering to flaming combustion under external radiant heat flux

This paper explores the behaviors of piloted ignition of fresh broadleaf foliage under external radiant heat flux. A total of 12 plant species were adopted, with volatile matter content varying from 15.4 to 41.4%. Measurements were conducted using a cone calorimeter with the radiant heat flux set at 35, 55, 70, and 85 kW m−2. Under the same radiant heat flux, the samples underwent combustion in two major modes. Some of the samples appeared short time flame and followed by smoldering, whereas the others maintained smoldering throughout the measurements. With an increase in the radiant heat flux or volatile matter content of a sample, the combustion mode of the samples may shift from smoldering to flaming combustion at initial stage of measurement. The critical heat release rate of a sample to form flaming combustion is between 20.9 and 22.5 kW m−2. Further analysis confirmed that the flaming combustion formed at initial stage of measurement mainly relies on the pyrolysis process of the sample surface layer. The initial volatile matter content of a sample and the rate of temperature rise at the surface layer are major factors governing the mass flux of volatiles generated and subsequently determining whether the sample undergoes flaming combustion.

Experimental Investigation and Computational Fluid Dynamics Simulation of a Novel Flash Ironmaking Process Based on Partial Combustion of Natural Gas in a Reactor

A large‐scale bench reactor (LSBR) is operated at the University of Utah to develop a novel flash ironmaking process. The reactor vessel is a refractory‐lined furnace with 0.8 m in diameter and 2.1 m in length, with a capacity of processing 1–7 kg h−1 of magnetite concentrate. Natural gas is partially oxidized with oxygen in a uniquely designed burner, producing a short flame that provides heat (1423–1873 K) and a reducing gas mixture (hydrogen and carbon monoxide). The reducing gas reacts with magnetite (Fe3O4) concentrate particles to produce metallic iron. A computational fluid dynamics (CFD) model is developed to simulate the LSBR runs, in which the kinetics of magnetite concentrate reduction, separately determined in a drop‐tube reactor, is incorporated. The reduction degrees and composition of the off‐gas obtained from the CFD model show reasonable agreements with the experimental results.

Effect of solid inertants and sample inclination angle on fire hazard of metallic powder layers

Compared to non-metallic powders, metallic powders usually have a greater layer fire hazard because of high flame temperatures. In this paper, the effects of solid inertants and sample inclination angle on the fire hazard of three metallic powder (iron, titanium, and magnesium) layers were investigated. Flame spread velocity (FSV) was mainly dependent on the particle size and enthalpy of combustion of metallic powders. For example, FSV of iron powders jumped from 0.4 to 4.7mm/s when the particle size decreased from micro scale to nano scale. The FSV of micro Ti powder layers reached 12.8mm/s, which was much higher than the value (1.7mm/s) for micro Mg powder with similar particle size. Inclination angle significantly influenced the FSV of nano Ti powder because of a long flame length resulting from a predominantly homogeneous reaction. For other tested powders, sample inclination angle had little effect on FSV because of a short flame length resulting from a predominantly heterogeneous reaction. Addition of small amounts of solid inertant decreased the layer fire hazard of iron and titanium powder with high melting points, while increasing the fire hazard of magnesium powder because of its low melting point. The melting characteristics of metallic powders should be considered when preventing or mitigating fire hazard using solid inertant technology.

A numerical investigation on the thermo-chemical structures of methane-oxygen diffusion flame-streets in a microchannel

“Flame-street” refers to the interesting combustion phenomenon that a diffusion flame in a narrow channel is separated into a discrete series of flame segments. Although flame-street has been observed experimentally for more than a decade, numerical simulations of this phenomenon with realistic chemistry and detailed analysis of its thermo-chemical structures have yet to be performed. In this paper, the methane-oxygen diffusion flame-street observed by Misse et al. (2005) [36] in a microchannel was numerically reproduced using a reacting flow solver developed based on the open-source framework OpenFOAM. The main purpose of the present work is to investigate the detailed thermo-chemical structures of diffusion flame-street in the anticipation of a deep understanding of the underlying mechanisms responsible for its formation. Our simulation results show that the presence of discrete flame segments can strongly affect the flow field. In addition, the flame-street was found to consist of a bibrachial leading flame with a long diffusion tail and a much weaker fuel-lean premixed branch, and consecutive downstream “new moon”-like flamelets each with a fuel-lean and a fuel-rich premixed branch. Chemical structures of these flame branches were analyzed by examining critical reactions that contribute significantly to the overall heat release. Finally, the effects of channel wall thermal conditions and species mass diffusion on the formation of the flame-street were investigated. It was found that the flame street could be established only at a moderate level of heat loss to the wall: significantly enhanced or reduced heat losses would result in either a single short edge flame or a long continuous edge flame. Increased mass diffusivity was seen to enhance combustion, elongate the leading flame, and even lead to more flame segments that appeared within the channel, although they may exist in a unsteady mode, being repetitively split, moving downstream and being blown-out/extinguished.

Non-linear response of the flame velocity to moderately large curvatures in laminar jet flames of methane–air mixtures

An experiment was set up to investigate the correlation between flame velocity and flame stretch in moderately stretched laminar jet flames of methane–air mixtures at different equivalence ratios. Gas flow velocities were obtained by Particle Image Velocimetry (PIV) from images of axial sections of seeded flows, and the location of the flame front was related to the gradient of luminosity. Flame stretch was determined and shown to be mainly due to flame curvature, with a small contribution from the gas strain rate. Inlet gas velocities not much larger than the planar flame velocity led to short rounded flames for which the classical Markstein relation was experimentally validated at any point of the flame fronts. However, the Markstein relation ceases to be uniformly valid for larger inlet gas velocities, in the order of ten times the planar flame velocity, for which the flame adopts a slender shape. These tall flames are nearly conical and obey the Markstein relation far from their tips, but a superlinear increase of flame velocity with stretch is observed when the tip is approached. Furthermore, the local velocity of these flames depends not only on the stretch but also on the inlet velocity of the gas. This behavior has been analyzed in terms of an empirical correction to the Markstein relation that is quadratic in the flame stretch with a coefficient that depends on the gas inlet velocity. The experimental results suggest that this coefficient is zero below a certain threshold value of this velocity and increases as the square root of the excess of gas inlet velocity above this threshold. This kind of sharp transition controlled by the velocity of the incoming gas is reminiscent of a typical critical behavior and is tentatively taken to reveal a stability exchange between rounded (weakly curved) and slender (strongly curved) flames.

Measurements on flame structure of bluff body and swirl stabilized premixed flames close to blow-off

The structure of unconfined lean premixed CH4/air flames stabilized with an axisymmetric bluff-body and a swirler was studied for conditions far from and close to blow-off. Blow-off was approached by gradually reducing the equivalence ratio while keeping bulk velocity constant at about 20 m/s. The objective is to examine the critical regions of flame structure that initiate and develop to blow-off for relatively short flames. OH-PLIF and CH* chemiluminescence were adopted to capture the instantaneous flame front and time-averaged flame structure. Statistics of the flame root-lift-off, flame front curvature and flame surface density (FSD) were obtained. Local burning velocity was estimated based on the FSD field. The burning intensity was also demonstrated with CH* chemiluminescence. As blow-off was approaching, the flame structure was seen to be more compacted. Lift-off at the flame roots was observed to be more frequent and the lift-off height was increased at leaner conditions. The local burning intensity was significantly decreased at upstream stages including the flame root close to blow-off. The wrinkling ratio was growing obviously at downstream, with finer flame structure observed through the short and intensive probability density function (PDF) of curvatures. The enhanced wrinkling by the recirculating flow at downstream is assumed to be favorable for the flame stabilization prior to the final blow-off. This indicates minor effects of downstream on blow-off. Comparatively, upstream of the flame including the main shear layer and the flame root is more unstable and unfavorable for ignition and flame stabilization, which dominantly promotes the blow-off. The measurements in present study are useful for model validation in terms of flame structures, and also significant for exploring the blow-off behavior of shorter flames as proposed in previous studies.

Design evaluation of diesel-oxygen diffusion flame burner for start-up of high-pressure coal gasifier

Commercial entrained-flow coal gasifiers operate at high temperatures and pressures for increased efficiency. For reduced operation costs, the start-up burner (SUB) of a gasifier requires fast heating and pressurization by intensive combustion, minimizing damage to the refractory lining and to itself. We evaluated different designs of the SUB adopting diesel-oxygen diffusion flame under the stoichiometric condition to improve the performance of the existing premixed flame burner that has been subjected to frequent failures. Using computational fluid dynamics, the combustion characteristics of the SUB with a thermal input up to 2.28MWth and pressure up to 8 bar were predicted for tests in a pilot-scale cylindrical gasifier. The results showed that the injection of diesel and oxygen through separate multiple inlets can intensify the combustion, leading to short and stable turbulent diffusion flames. Varying the design of the burner tip and inlet arrangement reduced the influence of pressure on the flame length while achieving high heat flux to the wall, which is essential for a fast and stable start-up. Further investigations are required to understand the influence of the reactor geometry on the circulation flow of high-temperature gas back to the burner tip in the actual gasifier.

Backward-Inclined Diffusion Jet Flames in Crossflow at Low Jet-to-Crossflow Momentum Flux Ratios

In the design of gas turbine combustors, efforts are engineered toward reducing the combustion pollutant emission levels. The pollutant emissions can be reduced by premixing the fuel and the air prior to ignition. However, the main challenges encountered with premixing are flame flashback and blowout, thus, the preference of diffusion flames. In this study, flame behavior, flow patterns, and thermochemical fields of backward-inclined diffusion jet flames in crossflow at low jet-to-crossflow momentum flux ratio of smaller than 0.04 were studied in a wind tunnel. The backward-inclination angle was varied within 0–50 deg. The flames presented three characteristic modes: crossflow dominated flame (low backward inclination angle) denoted by a large down-washed recirculation flame, transitional flame (mediate backward inclination angle) identified by a recirculation flame and a tail flame, and jet dominated flame (high backward inclination angle) characterized by a blue flame base, a yellow tail flame, and the absence of a recirculation flame. Short flames are detected in the regime of the crossflow dominated flames—an indication of improved fuel–air mixing. The findings suggest that for low exhaust emissions which are vigorously pursued in the aviation and thermal power plant industries, especially during low-load operations, the jet dominated flames are the preferable flames as they generate low unburned hydrocarbon, carbon monoxide, and nitric oxide emissions compared to the other flames.

Scaling relations for the length of coaxial oxy-flames with and without swirl

An extensive experimental study is carried out to analyze scaling laws for the length of methane oxy-flames stabilized on a coaxial injector. The central methane fuel stream is diluted with N2, CO2 or He. The annular air stream is enriched with oxygen and can be impregnated with swirl. Former studies have shown that the stoichiometric mixing length of relatively short flames is controlled by the mixing process taking place in the vicinity of the injector outlet. This property has been used to derive scaling laws at large values of the stoichiometric mixture fraction. It is shown here that the same relation can be extended to methane oxy-flames characterized by small values of the stoichiometric mixture fraction. Flame lengths are determined with OH* chemiluminescence measurements over more than 1000 combinations of momentum ratio, annular swirl level and composition of the inner and outer streams of the coaxial injector. It is found that the lengths of all the flames investigated without swirl collapse on a single line, whose coefficients correspond to within 15% of flame lengths obtained for fuel and oxidizer streams at much larger stoichiometric mixture fractions. This relation is then extended to the case of swirling flames by including the contribution of the tangential velocity in the flow entrainment rate and is found to well reproduce the mixing degree of the two co-axial streams as long as the flow does not exhibit a vortex breakdown bubble. At higher swirl levels, when the flow features a central recirculation region, the flame length is found to also directly depend on the oxygen enrichment in the oxidizer stream.

Resources and Problems of the Mathematical Simulating Thermo-Technological Processes

The paper presents the combustion of natural gas in a short flame in entourage of the very hot air had produced extremely elevated temperature of the flame, reaching 2300...2500 °C at the front of burning. In the stagnant part of the gas space beyond the short flame, the temperature of gases had gone down, approaching gradually to temperature of the molten-glass surface. As it is well known, a relatively low temperature level of the stagnant gas space allows zones of glass cooling to be organized at the end of the molten-glass basin of the glass furnaces with returned gas flow.

AN INVESTIGATION OF ARGON GAS ADDITIVES TO LPG ON THE TURBULENT LEAN PREMIXED FLAME CHARACTERISTICS FOR EV BURNER

A sets of experiments were done on a vertical environmental (EV) burner with fixed coflow air of 873 Lit /min to ensure turbulent lean premixed flow in order to study the effect of additon of argon mixture to the Liquefied Petroleum Gas on the temperature distributions and flame structure. The environmental (EV) burner assembly was fixed vertically in the test rig to scan the flame radially and axially. The flow rate of fuels, diluent and air were measured using rotameters.The whole supply lines were calibrated. The mixture of gases, air were injected through mixing pipes controlled with solenoid valves handled with a Labview program.The combustion flame was in room atmospheric conditions. The turbulent flame axial, radial temperature profile was measured using a shielded Platinum thermocouple type S. Flame images were taken using high resolution Canon 6D digital camera with high resolution lens .The fuel used was mixtures of different fuels of LPG, Argon. The choice of the different investigated cases was based on flame stability. The axial, radial distributions of mean gas and exhaust temperature,smoke number,NOx and combustion efficiency, are presented and analyzed for selective cases.The results obtained indicate the following: for pure LPG flames the images show short light blue flames at low flow rates and with progressive increase of the fuel flow rate this color changed to dark blue flame then to yellowish and white luminous zone.The exhaust temperature ranges was increasing with increasing of fuel flow rate of LPG. By Ar addition the flame luminosity decreases with the increase of Ar dilution, also the flame length increased in all runs with the progressive increase of Argon flow rate while decreasing the soot formation, while the NOx trend, exhaust temperature and combustion efficiency decreased by increasing Argon percentage addition.

Numerical Study of Ignition Process in Turbulent Shear-less Methane-air Mixing Layer

In this work, ignition process in a turbulent shear-less methane-air mixing layer is numerically investigated. A compressible large eddy simulation method with Smagorinsky sub-grid scale model is used to solve the flow field. Also, a thickened flame combustion model and DRM-19 reduced mechanism are used to compute species distribution and the heat release. Non-reacting mean and RMS axial velocity profiles and mean mixture fraction are validated against experimental data. Instantaneous mixture fraction contours show that the large bursts penetrate from the fuel stream into that of the oxidizer and vice versa and a random behaviour in the cross-stream direction. Flame kernel initiation, growth and propagation are analysed and compared with the experimental data. The ignition results show that the flame is not stable and blow-off occurs, but a more detailed investigation shows that local and short time flame stabilization exist during blow-off. During these local stabilization, heat release increased at the upstream edge of the flame. Most_upstream flame edge scalar analysis shows that the methane mass fraction has a dominant role in the local flame stabilization. OH, HO2, CH2O and heat release contours demonstration reveal that HO2 and CH2O mass fraction as well as the heat release reach a maximum on the border of the flame, but the maximum OH concentration is located in the middle of flame kernel.

스월 동축형 인젝터에 의해 형성되는 메탄-산소 확산화염의 연소특성

본 연구에서는 모델연소실 내부 메탄-산소 확산화염의 연소특성을 분석하기 위해 추진제의 다양한 분사조건에서 연소실험을 수행하여 연소안정한계 측정 및 화염의 형상변이를 관찰하였다. 연구 결과, 추진제가 이론당량비 조건으로 접근할수록 높은 산소 Reynolds 수(<TEX>$Re_o$</TEX>) 구간에서도 안정적인 부상화염이 관찰되었으며, 가시화염의 길이는 증가하고, 부상높이는 감소하는 경향이 나타났다. 스월 동축형 인젝터에 의한 추진제의 스월효과로 인해 폭이 넓고, 길이가 짧은 화염이 생성되어 연소실 크기가 제한적인 추력기에서의 사용이 적합함을 확인하였다. In order to analyze combustion characteristics of methane-oxygen diffusion flame in a model combustor, combustion experiments were carried out under various spray conditions of propellant scrutinizing combustion stability limit and flame shapes. As the propellant approached the theoretical equivalence ratio condition, a stable detached flame was observed even under high oxygen Reynolds number. And the length of the visible flame increased and the lift-off distance of the flame exhibited a tendency toward decrease. Due to the swirl effect of the propellant by the swirl-coaxial injector, a wide and short flame was produced. Thus, it may be appropriate to employ the swirl-coaxial injector in thrusters having a limited physical dimension.

KESAN PEMBAKARAN PADA KEADAAN NISBAH KESETARAAN ɸ=0.8333 MENGGUNAKAN DWI PEMUSAR ALIRAN JEJARIAN

Formation of nitrogen oxide (NOx), carbon monoxide (CO) and other emissions is increasing dramatically in the atmosphere. Due to this pressing issue, a study on combustion performance was conducted using a double radial swirler. In this study, a weak swirler with an angle of 30º is set as a primary swirler and strong swirlers each with an angle of 40º, 50º and 60º are set as secondary swirler. Combinations of these swirlers have increased internal recirculation of hot air and help to complete the mixing of fuel and air during combustion. Results show that the combination of 30º/60º swirler produced the best, more stable and shorter flame than the other combinations. Formation of NOX from the 30º/60º swirlers at equivalence ratio of 0.8333 is 27.3% lower than that from the combined 30º/40º swirlers. Other emissions such as CO, CO2 and UHC (Unburned Hydrocarbons) also show a reduction of 12.71%, 10.6% and 5.3%, respectively in the 30º/60º swirlers compared to those from the 30º/40º swirlers.

Large eddy simulations of partially premixed ethanol dilute spray flames using the flamelet generated manifold model

The paper presents Large Eddy Simulations (LESs) for the Sydney ethanol piloted turbulent dilute spray flames ETF2, ETF6, and ETF7. The Flamelet Generated Manifold (FGM) approach is employed to predict mixing and burning of the evaporating fuel droplets. A methodology to match the experimental inflow spray profiles is presented. The spray statistical time-averaged results show reasonable agreement with mean and RMS data. The Particle Size Distribution (PSD) shows a good match downstream of the nozzle exit and up to x/D = 10. At x/D = 20 and 30 the PSD is under-predicted for droplets with mean diameter D10 > 20μm and over-predicted for the smaller size droplets. The simulations reasonably predict the reported mean flame structure and length. The effect of increasing the carrier velocity (ETF2–ETF7) or decreasing the liquid fuel injection mass flow rate (ETF2–ETF6) is found to result in a leaner, shorter flame and stronger spray–flow interactions. Higher tendency to local extinction is observed for ETF7 which is closer to blow-off compared to ETF2 and has higher scalar dissipation rates, higher range of Stokes number, and faster droplet response. The possible sources of LES-FGM deviations from the measurements are discussed and highlighted. In particular, the spray time-averaged statistical error contribution is quantified and the impact of the inflow uncertainty is studied. Sensitivity analysis to the pre-vaporized nozzle fuel mass fraction show that such small inflow perturbations (by ± 2% for the ETF2 flame) have a strong impact on the flame structure, and the droplets’ dynamics. Conditional scatter plots show that the flame exhibits wide range of mixing conditions and bimodal mixing lines particularly at upstream locations (x/D < 20), where the injected droplets are still penetrating the centerline. This is relaxed further downstream as droplets gradually evaporate and burn in a diffusion like mode.

KESAN SUDUT PUSARAN TERHADAP PEMBAKARAN MENGGUNAKAN PEMUSAR JEJARIAN DWI ALIRAN

Development of combustion systems which involves retrofitting or design of new burners, is made to reduce the formation of pollutant emissions. The reduction of this pollutant emission results from the complete mixing of fuel and air during the combustion process. Meanwhile, non-complete mixing of fuel and air during combustion process can cause ignition problem and create problems in terms of flame stability and combustion efficiency. This article describes a study on the effects of swirling flow generated by a double radial swirler on flame characteristics that is related to the emission of NO. The double radial swirlers used in this study have the angles of 30°/40°, 30°/50° and 30°/60°. Diesel is used as a fuel in this study. The results show that all double radial swirlers used have different effects on the flame characteristics and temperature profile. From all these double radial swirlers, the one with an angle of 30°/60° produces flame with high temperature, short flame length with blue colour and wide spread.

Effect of Flame Angle Using Various Swirler Angle in Combustion Performance

The abbreviation N is commonly used as a reference to all oxides of nitrogen and the examples of oxides of nitrogen are NO, N and O. These emission gases mostly come from the combustions of fossil fuels and biofuels in industrial activities and also in road traffics globally. N as a pollutant deserves special attention because of its various issues on human’s health and environment. Some of these issues are global warming, acid rain, ozone layer depletion and photochemical smog. To address these issues, some studies are concentrating on the reduction of NO emission from the combustion process. This paper describes a study in the effects of swirling flow generated by using a radial swirler on flame characteristics that is related to the emission of NO. The radial swirlers used in this study have the angles of 40°, 50° and 60°. Diesel is used as a fuel in this study. The results show that all radial swirlers used have different effects on the flame characteristics. From all these radial swirlers, the one with an angle of 60° produces flame with high temperature, short flame length with blue colour and wide spread. The results also show a short time residence during combustion process that could reduce the formation of CO and N .

Combustion characteristics of premixed propane flame with added H2 and CO on a V-shaped impinging burner

The combustion characteristics of H2/C3H8/air and CO/C3H8/air premixed flames on a V-shaped burner have been investigated experimentally for future blended fuels with flow visualization, chemiluminescence and analysis of exhaust gases. The lean flammability of H2/C3H8/air is expanded to 0.38 and that of CO/C3H8/air is expanded to 0.50 on the V-shaped burner. The flame configurations, flow structures, distributions of chemiluminescence intensity of CH* and OH*, flame temperatures and CO emissions under varied mixture conditions were recorded during the combustion. Two flame configurations were observed – of types M and hill. The addition of H2 and CO to the fuel altered the flame pattern from hill-type to M-type (shorter flame) because the burning velocity, temperature and flame intensity increased with the addition of H2 and CO in the fuel. Analysis of flow visualization and chemiluminescence revealed the impinging effect, in which the collision of mixture jets decreases the main flow velocity and increases the residence time. It results in a greatly decreased leakage of the unburned mixture with accumulated heat acting as a well stirred combustible mixture for improved flame stabilization. At ɸ = 0.6, the flame temperature of H2/C3H8/air with M-type flame is 1.37 times that of CO/C3H8/air with hill-type flame, while the C3H8/air flame is extinguished. The combustion of CO that occurs in the impinging region resulted in decreased CO emissions when the addition of CO to the fuel surpassed 73.7%. These findings about the combustion characteristics, mechanisms and effects of fuel variability provide industries with an effective concept of burning blended fuels, and are applicable to syngas combustion.

Flow Field and Flame Dynamics of Swirling Methane and Hydrogen Flames at Dry and Steam Diluted Conditions

The majority of recent stationary gas turbine combustors employ swirling flows for flame stabilization. The swirling flow undergoes vortex breakdown (VB) and exhibits a complex flow field including zones of recirculating fluid and regions of high shear intensities. Often, self-excited helical flow instabilities, which manifest in a precession of the vortex core, are found in these flows and may influence the combustion process in beneficial and adverse ways. In the present study, we investigate the occurrence and shape of self-excited hydrodynamic instabilities and their impact on heat release fluctuations and mixing characteristics over a wide range of operating conditions. We employ high-speed stereoscopic particle image velocimetry (S-PIV) and simultaneous OH*-chemiluminescence imaging to resolve the flow velocities and heat release distribution, respectively. The results reveal four different flame shapes: A detached annular flame, a long trumpet shaped flame, a V flame, and a very short flame anchored near the combustor inlet. The flame shapes were found to closely correlate with the reactivity of the mixture. Highly steam-diluted or very lean flames cause a detachment, whereas hydrogen fuel leads to very short flames. The detached flames feature a helical instability, which, in terms of frequency and shape, is similar to the isothermal case. A complete suppression of the helical structure is found for the V flame. Both the trumpet shaped flame and the very short flame feature helical instabilities of different frequencies and appearances. The phase-averaged OH*-chemiluminescence images show that the helical instabilities cause large-scale heat release fluctuations. The helical structure of the fluctuations is exploited to use a tomographic reconstruction technique. Furthermore, it is shown that the helical instability significantly enhances the mixing between the emanating jet and the central recirculation zone.