共查询到20条相似文献,搜索用时 31 毫秒
1.
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. 相似文献
2.
Self-sustained jet flapping is observed in a confined, premixed and preheated methane-air turbulent flame, generated in a single-nozzle jet-stabilized gas turbine model combustor designed based on the FLOX ? concept. The flapping frequency and its complex motion within the confinement of the combustor are characterized in detail using proper orthogonal decomposition (POD) of the flow fields measured by particle imaging velocimetry (PIV). The influence of jet flapping on combustion stability is examined using simultaneous PIV/OH chemiluminescence imaging and PIV/planar laser-induced fluorescence of OH radicals (OH PLIF) at 5 kHz repetition rate. By influencing the size and location of the recirculation zones, jet flapping modifies the flame shape and flame lift-off height. It also controls the amount of hot gas entrainment into the recirculation zones. In extreme cases, jet flapping is found to cause temporary local extinction of the flame, due to jet impingement on the combustor wall and partial blockage of burned gas entrainment. The flame is only able to recover after the jet detaches from the wall and reopens the back flow channel. The results suggest that jet flapping could play a key role in the stabilization mechanisms in similar jet-stabilized combustors. 相似文献
3.
CH double-pulsed PLIF measurement in turbulent premixed flame 总被引:1,自引:0,他引:1
The flame displacement speeds in turbulent premixed flames have been measured directly by the CH double-pulsed planar laser-induced
fluorescence (PLIF). The CH double-pulsed PLIF systems consist of two independent conventional CH PLIF measurement systems
and laser beams from each laser system are led to same optical pass using the difference of polarization. The highly time-resolved
measurements are conducted in relatively high Reynolds number turbulent premixed flames on a swirl-stabilized combustor. Since
the time interval of the successive CH PLIF can be selected to any optimum value for the purpose intended, both of the large
scale dynamics and local displacement of the flame front can be discussed. By selecting an appropriate time interval (100–200 μs),
deformations of the flame front are captured clearly. Successive CH fluorescence images reveal the burning/generating process
of the unburned mixtures or the handgrip structures in burnt gas, which have been predicted by three-dimensional direct numerical
simulations of turbulent premixed flames. To evaluate the local flame displacement speed directly from the successive CH images,
a flame front identification scheme and a displacement vector evaluation scheme are developed. Direct measurements of flame
displacement speed are conducted by selecting a minute time interval (≈30 μs) for different Reynolds number (Re
λ = 63.1–115.0). Local flame displacement speeds coincide well for different Reynolds number cases. Furthermore, comparisons
of the mean flame displacement speed and the mean fluid velocity show that the convection in the turbulent flames will affect
the flame displacement speed for high Reynolds number flames. 相似文献
4.
Y. Lafay B. Taupin G. Martins G. Cabot B. Renou A. Boukhalfa 《Experiments in fluids》2007,43(2-3):395-410
The aim of the present work is to compare stability combustion domains, flame structures and dynamics between CH4/air flames and a biogas/air flames (issued from waste methanisation) in a lean gas turbine premixed combustion conditions.
Velocity profiles are obtained by Laser Doppler Anemometry measurements. CH* chemiluminescence measurements and temporal acquisition
of chamber pressure are performed in order to describe flame structure and instabilities. Changes in flame structure and dynamics
when fuel composition is varying are found to strongly depend on laminar flame speed. No clear correlation between the unstable
flame and the reaction zone penetration in the corner recirculation can be found. 相似文献
5.
U. Stopper M. Aigner H. Ax W. Meier R. Sadanandan M. Stöhr A. Bonaldo 《Experimental Thermal and Fluid Science》2010,34(3):396-403
Several laser diagnostic measurement techniques have been applied to study the lean premixed natural gas/air flames of an industrial swirl burner. This was made possible by equipping the burner with an optical combustion chamber that was installed in the high-pressure test rig facility at the DLR Institute of Combustion Technology in Stuttgart. The burner was operated with preheated air at various operating conditions with pressures up to p = 6 bar and a maximum thermal power of P = 1 MW.The instantaneous planar flow field inside the combustor was studied with particle image velocimetry (PIV). Planar laser induced fluorescence (PLIF) of OH radicals on a single-shot basis was used to determine the shape and the location of the flame front as well as the spatial distribution of reaction products. 1D laser Raman spectroscopy was successfully applied for the measurement of the temperature and the concentration of major species under realistic gas turbine conditions.Results of the flow field analysis show the shape and the size of the main flow regimes: the inflow region, the inner and the outer recirculation zone. The highly turbulent flow field of the inner shear layer is found to be dominated by small and medium sized vortices. High RMS fluctuations of the flow velocity in the exhaust gas indicate the existence of a rotating exhaust gas swirl. From the PLIF images it is seen that the primary reactions happened in the shear layers between inflow and the recirculation zones and that the appearance of the reaction zones changed with flame parameters. The results of the multiscalar Raman measurements show a strong variation of the local mixture fraction allowing conclusions to be drawn about the premix quality. Furthermore, mixing effects of unburnt fuel and air with fully reacted combustion products are studied giving insights into the processes of the turbulence–chemistry interaction. 相似文献
6.
Krishna K. Venkatesan Galen B. King Normand M. Laurendeau Michael W. Renfro Benjamin Böhm 《Flow, Turbulence and Combustion》2009,83(1):131-152
Simultaneous high repetition-rate, two-point hydroxyl (OH) time-series measurements with associated PLIF/PIV measurements
are employed to investigate spatio–temporal scales and flame-velocity interactions in turbulent opposed jets sustaining methane-air
double flames. For a fuel-side equivalence ratio, ϕ
B
= 1.2, a rich premixed flame exists on the fuel side while a diffusion flame exists on the air side of the stagnation plane.
The bulk Reynolds number (Re) and strain rate (SR) can be adjusted to generate flames at ϕ
B
= 1.2 with both well separated and completely merged flame fronts. Simultaneous PLIF/PIV measurements highlight distinct
spatial OH structures of the premixed and diffusive fronts corresponding to variations in the flow field. The self-propagating
tendency of the rich premixed front causes large-scale wrinkling, thereby enhancing the OH contour length by 15% as compared
to the diffusive front. Two-point OH time-series measurements are implemented to quantify both spatial and temporal fluctuations
via study of radial length and time scales. In general, these integral length and time scales follow similar trends and reach
a minimum at the axial location of peak [OH]. In comparison to merged double flames having higher Re and SR, greater OH fluctuations
are observed in the rich-premixed front as compared to the diffusive front for a well separated double flame. Because of the
developing turbulence, the OH length scales exhibit reduced axial gradients across the reaction zone for higher Re in comparison
to lower Re. A stochastic time-series simulation, using a state relationship based on a joint mixture fraction and progress
variable, is utilized to extract estimated scalar time scales from those of measured OH. The simulations indicate that the
hydroxyl fluctuations in double flames are only twice those of the underlying conserved scalar.
“Turbulent Opposed-Jet Double Flames” is submitted for consideration as a full length article to Flow Turbulence and Combustion. 相似文献
7.
Rajesh Sadanandan Peter Kutne Adam Steinberg Wolfgang Meier 《Flow, Turbulence and Combustion》2012,89(2):275-294
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. 相似文献
8.
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. 相似文献
9.
High-speed laser diagnostics for the study of flame dynamics in a lean premixed gas turbine model combustor 总被引:1,自引:0,他引:1
Isaac Boxx Christoph M. Arndt Campbell D. Carter Wolfgang Meier 《Experiments in fluids》2012,52(3):555-567
A series of measurements was taken on two technically premixed, swirl-stabilized methane-air flames (at overall equivalence
ratios of ϕ = 0.73 and 0.83) in an optically accessible gas turbine model combustor. The primary diagnostics used were combined
planar laser-induced fluorescence of the OH radical and stereoscopic particle image velocimetry (PIV) with simultaneous repetition
rates of 10 kHz and a measurement duration of 0.8 s. Also measured were acoustic pulsations and OH chemiluminescence. Analysis
revealed strong local periodicity in the thermoacoustically self-excited (or ‘noisy’) flame (ϕ = 0.73) in the regions of the flow corresponding to the inner shear layer and the jet-inflow. This periodicity
appears to be the result of a helical precessing vortex core (PVC) present in that region of the combustor. The PVC has a
precession frequency double (at 570 Hz) that of the thermo-acoustic pulsation (at 288 Hz). A comparison of the various data
sets and analysis techniques applied to each flame suggests a strong coupling between the PVC and the thermo-acoustic pulsation
in the noisy flame. Measurements of the stable (‘quiet’) flame (ϕ = 0.83) revealed a global fluctuation in both velocity and heat-release around 364 Hz, but no clear evidence of
a PVC. 相似文献
10.
Mamoru Tanahashi Shohei Inoue Masayasu Shimura Shohei Taka Gyung-Min Choi Toshio Miyauchi 《Experiments in fluids》2008,45(3):447-460
Flame structures of turbulent premixed flames in a noise-controlled, swirl-stabilized combustor are investigated to clarify
the mechanism of combustion noise reduction by the secondary fuel injection. Planar laser-induced fluorescence (PLIF) is conducted
for several cases with different secondary fuel injection, and 3D flame structure is reconstructed from PLIF results on multiple
planes. The secondary fuel injection suppresses the fluctuation of high-temperature gas in the recirculation zone and reduces
Reynolds stress and entropy terms in the acoustic sound source. In the flame zone, effects of the injection frequency are
discussed by introducing mean progress variable. The flame brush is very wide for the no control case, whereas it becomes
thin and is confined to a narrow space for the secondary fuel injection cases. The investigated combustor gives minimum sound
level at a relevant fuel injection frequency, which is very low compared with the natural acoustic mode of the combustor.
The flame brush becomes very thin, and self-induced oscillations of the flame brush disappear at this relevant frequency.
The oscillation of the flame brush represents large-scale fluctuation of the mean heat release rate. The relations between
characteristics of flame brush and combustion noise are discussed by introducing instantaneous and dynamical effects of flame
front on the entropy term of the sound source. The secondary fuel injection works for the control of the entropy term in the
sound source because the thin flame brush represents suppression of the instantaneous and dynamical effects. 相似文献
11.
This large eddy simulation (LES) study is applied to three different premixed turbulent flames under lean conditions at atmospheric
pressure. The hierarchy of complexity of these flames in ascending order are a simple Bunsen-like burner, a sudden-expansion
dump combustor, and a typical swirl-stabilized gas turbine burner–combustor. The purpose of this paper is to examine numerically
whether the chosen combination of the Smagorinsky turbulence model for sgs fluxes and a novel turbulent premixed reaction
closure is applicable over all the three combustion configurations with varied degree of flow and turbulence. A quality assessment
method for the LES calculations is applied. The cold flow data obtained with the Smagorinsky closure on the dump combustor
are in close proximity with the experiments. It moderately predicts the vortex breakdown and bubble shape, which control the
flame position on the double-cone burner. Here, the jet break-up at the root of the burner is premature and differs with the
experiments by as much as half the burner exit diameter, attributing the discrepancy to poor grid resolution. With the first
two combustion configurations, the applied subgrid reaction model is in good correspondence with the experiments. For the
third case, a complex swirl-stabilized burner–combustor configuration, although the flow field inside the burner is only modestly
numerically explored, the level of flame stabilization at the junction of the burner–combustor has been rather well captured.
Furthermore, the critical flame drift from the combustor into the burner was possible to capture in the LES context (which
was not possible with the RANS plus k–ɛ model), however, requiring tuning of a prefactor in the reaction closure. 相似文献
12.
Matthew J. Dunn Assaad R. Masri Robert W. Bilger Robert S. Barlow 《Flow, Turbulence and Combustion》2010,85(3-4):621-648
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. 相似文献
13.
B. Yan B. Li E. Baudoin C. Liu Z.W. Sun Z.S. Li X.S. Bai M. Aldén G. Chen M.S. Mansour 《Experimental Thermal and Fluid Science》2010,34(3):412-419
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. 相似文献
14.
P. Weigand W. Meier X. R. Duan R. Giezendanner-Thoben U. Meier 《Flow, Turbulence and Combustion》2005,75(1-4):275-292
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. 相似文献
15.
Imaging methods provide new insights into many fundamental combustion processes. Many imaging techniques have been devised in recent years and applied to a range of experiments. One particularly useful method is to seed the flow with oil particles and illuminate the domain of interest with a planar sheet of laser light. The droplets evaporate and vanish when they pass through the flame. The light scattered by the particles may be imaged for example with a CCD camera or with high-speed cinematography to show the structure and dynamics of the flame front. This technique, sometimes called laser tomography, is based on Mie scattering. It provides essentially qualitative information on the geometry and motion of the flame front. Another valuable method relies on spontaneous emission imaging. In this method the light emitted by certain radicals produced by the chemical reaction is detected by a camera and delivered to a computer for further processing. In some circumstances it is possible to deduce from this measurement the spatial distribution of heat release in the reactive flow. More quantitative data may be gathered with planar laser-induced fluorescence (PLIF) imaging. The reactive flow is illuminated with a planar laser sheet delivered by a tunable laser. The laser light excites the fluorescence of a species that is present in the flow, which is then detected with an intensified CCD camera. The data obtained in this way can be processed to obtain spatial measurements of the species concentration. The basic principles, equipment requirements, and experimental aspects of these three imaging techniques are reviewed. Practical applications to turbulent flames are emphasized. It is shown that emission imaging applied to turbulent ducted flames yields interesting information for modeling. A second example of application is the ignition sequence of a multiple-injector combustor, of importance to modern cryogenic rocket engines. Emission and PLIF imaging have been used to obtain data on the development of the initial flame kernel and on its propagation from the first injector to the next. The images gathered in this experiment yield a unique view on the flame patterns that lead to the final stabilization of the reactive fronts. While current imaging methods are essentially qualitative, it is possible to deduce quantitative results from the data, and some of the present limitations may be overcome with more refined measurement procedures. These issues are analyzed, and future developments in this area are evaluated. 相似文献
16.
Planar laser induced fluorescence imaging (PLIF) is shown to be a quantitative method of measuring average values, rms fluctuations, and probability density functions of OH concentration in laboratory scale, H2-air diffusion flames. When compared with single-pulse laser saturated fluorescence (LSF) data, PLIF data show agreement (within a factor of two) for average and rms values in laminar, transitional, and turbulent flames. The unknown temperature dependence of the H2 quenching cross section introduces a factor of two uncertainty in PLIF measurements in rich flame zones. Extensions of PLIF to other molecules and other combustion systems are discussed.A version of this paper was presented at the ASME Winter Annual Meeting of 1984 相似文献
17.
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 NOx 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. 相似文献
18.
Large Eddy Simulations (LES) of a swirl-stabilized natural gas-air flame in a laboratory gas turbine combustor is performed using six different LES combustion models to provide a head-to-head comparative study. More specifically, six finite rate chemistry models, including the thickened flame model, the partially stirred reactor model, the approximate deconvolution model and the stochastic fields model have been studied. The LES predictions are compared against experimental data including velocity, temperature and major species concentrations measured using Particle Image Velocimetry (PIV), OH Planar Laser-Induced Fluorescence (OH-PLIF), OH chemiluminescence imaging and one-dimensional laser Raman scattering. Based on previous results a skeletal methane-air reaction mechanism based on the well-known Smooke and Giovangigli mechanism was used in this work. Two computational grids of about 7 and 56 million cells, respectively, are used to quantify the influence of grid resolution. The overall flow and flame structures appear similar for all LES combustion models studied and agree well with experimental still and video images. Takeno flame index and chemical explosives mode analysis suggest that the flame is premixed and resides within the thin reaction zone. The LES results show good agreement with the experimental data for the axial velocity, temperature and major species, but differences due to the choice of LES combustion model are observed and discussed. Furthermore, the intrinsic flame structure and the flame dynamics are similarly predicted by all LES combustion models examined. Within this range of models, there is no strong case for deciding which model performs the best. 相似文献
19.
Effects of Unmixedness on Combustion Instabilities in a Lean-Premixed Gas Turbine Combustor 总被引:1,自引:0,他引:1
The present experimental study focuses on the effects of the degree of premixing and swirl strength on combustion instabilities
occurring in a lean premixed gas turbine combustor burning natural gas and air. The combustor operated at pressurized conditions
with heated air. Major measurements for the investigation of premixed combustion dynamics include pressure fluctuations, flame
emissions in reacting flow, and acetone fluorescence in non-reacting flow to assess the degree of premixing between fuel and
air. The acetone PLIF results revealed that the degree of premixing improves as mixing time increases. The first and second
longitudinal acoustic modes were the dominant excited modes for most cases of interest. Combustion at a lean premixed condition
becomes more susceptible to instabilities as the degree of premixing becomes poor, and self-excited pressure oscillations
are obviously present under a fully premixed condition, even without equivalence ratio fluctuations in space. For incomplete
premixing cases, local equivalence ratio fluctuations caused by poor premixing may initiate instabilities since reaction rate
is sensitive to equivalence ratio fluctuations at lean conditions. Phase resolved chemiluminescence measurements show that
pressure oscillations are strongly coupled with variations in flame structures. 相似文献
20.
Davide E. Cavaliere James Kariuki Epaminondas Mastorakos 《Flow, Turbulence and Combustion》2013,91(2):347-372
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. 相似文献