Burner Outlet Research Articles

Instability Control by Premixed Pilot Flames

Premixed flames of swirl-stabilized combustors (displaced half-cone) are susceptible to thermo-acoustic instabilities, which should be avoided under all operating conditions in order to guarantee a long service life for both stationary and aircraft gas turbines. The source of this unstable flame behavior can be found in a transition of the premix flame structure between two stationary conditions that can be easily excited by fuel fluctuations, coherent structures within the flow, and other mechanisms. Pilot flames can alleviate this issue either by improving the dynamic stability directly or by sustaining the main combustion process at operating points where instabilities are unlikely. In the present study, the impact of two different premixed pilot injections on the combustion stability is investigated. One of the pilot injector (pilot flame injector) was located upstream of the recirculation zone at the apex of the burner. The second one was a pilot ring placed at the burner outlet on the dump plane. A noticeable feature of the pilot injector was that an ignition device allowed for creating pilot premixed flames. The present investigation showed that these premixed pilot flames were able to suppress instabilities over a wider fuel/air ratio range than the conventional premixed pilot injection alone. Furthermore, it was possible to prevent instabilities and maintain the flame burning near the lean blowout when a percentage of the fuel was premixed with air and injected through the pilot ring. NOx emissions were significantly reduced.

Determination of flow pattern and its effect on NOx emission in a tangentially fired single chamber square furnace

Tangentially fired furnaces are vortex combustion units which have become more attractive in the field of power station firing systems in recent years. Although the application of tangentially fired furnace continuously increases, they have not yet been adequately investigated. The present work provides a numerical study of flow pattern and its effect on NOx emission in a single chamber square tangentially fired furnace. Details of the flow field, along with temperature and species concentration contour maps are obtained from the solution of the conservation equations of mass, momentum, and energy, and transport equations for scalar variables in addition to the equations of the turbulence model. Four cases with different inlet air velocities are studied. Combustion in a natural gas-fired horizontal furnace with circular cross-section is simulated to verify the simulation methodology and the solution algorithm. Results are compared with those of the existing references and good agreement is observed. Calculations for the tangentially fired furnace show that while the vortex created in the center of the furnace becomes stronger as the burner outlet air velocity increases, its size remains almost unchanged. Highest-temperature regions are favorably far from the furnace walls so that erosion and local over-heating can be minimized. According to the results, higher inlet air velocities lead to more uniform temperature distributions with lower peak temperatures, which in turn result in remarkable reduction of NOx emission of the furnace.

Numerical Investigations of the Noise Sources Generated in a Swirl Stabilized Flame

Compressible Large Eddy Simulations (LES) have been performed to study the reacting flow field of a typical industrial double concentric swirl burner. A turbulent flame speed closure (TFC) was implemented into the code to model the premixed flame. The results have been analysed with respect to the acoustic sources used in Lighthill's acoustic analogy. It is shown, that the noise sources generated by the combustion process are significantly larger than the sources caused by the aerodynamics. A spectral analysis allowed to detect two dominant frequencies in the swirling flow field, which are suspected to contribute to the noise emissions. The frequencies are caused by the eigenfrequency of the burner and an oscillating coherent structure near the burner outlet.

Melting temperature distribution characteristic of ash deposits of black liquor coal slurry on furnace wall

To investigate the distribution characteristic of melting temperature of ash deposits deposited on different locations of furnace wall, comparison test on two combustion work conditions was made in trial furnace of black liquor coal slurry. Analysis was made on melting temperature, chemical composition, and mineral phase of ash deposits on furnace wall along journey of fume. The result shows that melting temperature of ash deposits at 155 mm from outlet of burner on furnace wall is the lowest, and that of the highest is in the regions of 1000–1150 mm from outlet of burner. Melting temperature of ash deposits on furnace wall assumes the distribution characteristic of elevation at the start followed by degradation gradually. Such distribution characteristic is correlated directly with transportation and deposition of mineral elementals, and related with combustion heat load to a certain degree.

Spatiotemporal Characterization of a Conical Swirler Flow Field Under Strong Forcing

In this study, a spatiotemporal characterization of forced and unforced flows of a conical swirler is performed based on particle image velocimetry (PIV) and laser Doppler anemometry (LDA). The measurements are performed at a Reynolds number of 33,000 and a swirl number of 0.71. Axisymmetric forcing is applied to approximate the effects of thermoacoustic instabilities on the flow field at the burner inlet and outlet. The actuation frequencies are set at the natural flow frequency (Strouhal number Stf≈0.92) and two higher frequencies (Stf≈1.3 and 1.55) that are not harmonically related to the natural frequency. Phase-averaged measurement are used as a first step to visualize the coherent flow structures. Second, proper orthogonal decomposition (POD) is applied to the PIV data to characterize the effect of the actuation on the fluctuating flow. Measurements indicate a typical natural flow instability of helical nature in the unforced case. The associated induced pressure and flow oscillations travel upstream to the swirler inlet where generally fuel is injected. This observation is of critical importance with respect to the stability of the combustion. Harmonic actuation at different frequencies and amplitudes does not affect the mean velocity profile at the outlet, while the coherent velocity fluctuations are strongly influenced at both the inlet and outlet. On one hand, the dominant helical mode is replaced by an axisymmetric vortex ring if the flow is forced at the natural flow frequency. On the other hand, the natural flow frequency prevails at the outlet under forcing at higher frequencies and POD analysis indicates that the helical structure is still present. The presented results give new insight into the flow dynamics of a swirling flow burner under strong forcing.

Mode switching of combustion instabilities associated with flame transfer function non-linearities

Studies of combustion instabilities indicate that the frequency of oscillation changes during the growth of oscillations before a limit cycle is reached. It is also found that a system can switch from one mode to another when the amplitude exceeds a certain threshold. It is shown that these features can be linked to the flame response non-linearity. The analysis is carried out by examining combustion instabilities appearing on a multipoint injection burner, placed a the end of a cylindrical manifold. This element acts like a resonator and its length can be modified easily. Experiments indicate that mode switching takes place between two resonant modes of the cavity and this is shown to be caused by the non-linearity of the flame response. The phenomenon can be analyzed by considering the non-linear evolution of the flame transfer function. It is possible to combine this information with a stability analysis of the burner, and define conditions which give rise to mode switching. It is found that this behavior is related to a change in phase difference between heat release rate fluctuations and velocity perturbations at the burner outlet. This phase shift takes place as the amplitude of oscillation increases.

P.3.a.009 The bone mineral density in the Korean male patients with chronic schizophrenia and chronic alcoholism

The present study attempts to quantify the effect of radiation on nonpremixed, turbulent diffusion flame temperature and NOx emissions for flames subjected to different overall heat extraction. Comprehensive in-flame data have been generated for two flames of similar strain field and luminosity in order to facilitate flame comparison. The flame produced in the refractory-lined furnace (flame 1) is shorter than that of the water-cooled confinement (flame 2) by around 30%, and the extent of combustion increases by 20% at a distance 27-mm downstream of the burner outlet. The density-averaged mean flame temperatures were 1850 and 1675 K, and furnace NOx emissions were 49 and 33 ppm for flames 1 and 2, respectively.The total radiative intensities calculated spectrally using the Exponential Wide Band model are in verygood agreement with the measurements and indicate that the amount of radiant energy emitted directly by both flames is similar. Radiation plays a crucial role in extracting the energy from the postflame zone, and therefore, it affects the enthalpy of the products entrained from the external recirculation zone. Flame temperatures and NOx emissions are strongly dependent on the energy of the flow entrained by the jets. It is concluded that the radiant fraction cannot be used as an exclusive NOx scaling parameter to describe the effect of radiation on emissions of confined turbulent diffusion flames. Proper scaling parameters should incorporate the energy entrained (from external recirculation zone) into the flame, and therefore, these parameters have to take into account the amount of energy extracted from the postflame zone.

Dynamics of confined premixed flames submitted to upstream acoustic modulations

Dynamics of inverted conical flames anchored on a central bluff-body is investigated in confined and unconfined conditions. The flames are modulated by a driver unit placed upstream. Flame dynamics is strongly coupled to the evolution of large scale structures induced by the modulation and shed from the injector nozzle lip. To study the influence of confinement on the flame motion, the response is measured for different diameters of the duct bounding the flame. This induces changes in the confinement parameter defined as the ratio of the injection duct diameter to the combustor diameter. Natural emission imaging, transfer function measurements and PIV data serve to determine the effect of confinement on the flame response for a set of modulation amplitudes. It is found that when the confinement ratio is low, the time delay between the velocity signal at the burner outlet and heat release perturbations remains essentially constant indicating that the flame is weakly affected by the presence of the wall. As the confinement ratio is increased, the time delay and the gain of the flame transfer function are modified. It is found that interaction with the wall strongly influences the dynamics of the flame tip, the flame wrinkling geometry in the vicinity of the boundary, and to a great extent the amplitude of response. A numerical simulation provides additional information on the fresh stream dynamics inside the tube, and on the dynamical interactions between the flow and the flame.

STEREO-PIV ANALYSIS OF THE LOW-SWIRL FLOW ABOVE A PARTIAL OXIDATION BURNER

A partial oxidation burner is introduced as a cleaning system for the tar content of (bio) fuel. The low-swirl (S ≈ 0.2) flow at the burner outlet is studied using Stereo Particle Image Velocimetry. After calibration the experimental setup is capable of resolving all three velocity components. The presented results show that the averaged coreflow vortex has the characteristics of a solid body rotation. After leaving the burner outlet the swirl number rapidly decreases from about 0.2 to 0.07. The turbulent kinetic energy has also been resolved and shows that the fluctuations are concentrated around the flow center line at levels of 5–10%. The velocity field is homogeneously mixed at about 40 mm above the burner outlet.

Prediction of Pressure Oscillations in a Premixed Swirl Combustor Flow and Comparison to Measurements

The most common and reliable technique used for flame stabilization of industrial combustors with high thermal loads is the application of strongly swirling flows. In addition to stabilization, swirl flames offer the possibility to influence emission characteristics by simply changing the swirl intensity or the type of swirl generation. Despite of these major advantages, swirling flows tend to evolve flow instabilities, that considerably constitute a significant source of noise. In general, noise generation is substantially enhanced, when such a swirling flow is employed for flames. Thus, the minimization of the resulting noise emissions under conservation of the benefit of high ignition stability is one major design challenge for the development of modern swirl stabilized combustion devices. The present investigation makes an attempt to determine mechanisms and processes to influence the noise generation of flames with underlying swirling flows. Therefore, a new burner has been designed, that offers the possibility to vary geometrical parameters as well as the type of swirl generation, typically applied in industrial devices. Experimental data has been acquired for the isothermal flow as well as swirl flames by means of 3-D-LDV-diagnostics comprising the components of long-time averaged mean and rms-velocities as well as spectrally resolved velocity fluctuations for all components. The noise emission data was acquired with microphone probes resulting in sound pressure levels outside the zone of the perceptible fluid flow. Along to the experiments, numerical simulations using RANS and LES have been carried out for isothermal cases with different burner outlet geometries. The results of the measurements show a distinct rise of the sound pressure level, obtained by changing both the test setup from the isothermal into the flame configuration as well as the geometrical parameters. This is also resembled by the LES simulation results. Furthermore, a physical model has been developed from experiments and verified by the LES simulation, that explains the formation of coherent flow structures and allows to separate their contribution to the overall noise emission from ordinary turbulent noise sources.

Effect of Operation Parameters on the Slagging near Swirl Coal Burner Throat

Fluid flow, heat transfer, coal combustion, and slagging processes had been numerically simulated near a swirl burner throat. The effect of the ratio distribution of each burner air, their swirling numbers, and the coal character on the slagging process had been analyzed. The computation results indicate that the maximal sticking-particle numbers occur at the uppermost waterwall, while the sticking-particle number at neither waterwall near the swirl burner outlet is very small. The swirling number has a significant effect on the number of the sticking particle. The sticking-particle number increases rapidly with the increment of the outer secondary air and the primary air-swirling numbers, respectively, because it can strengthen the flow entrainment ability to carry more particles to the waterwall. The inner secondary air has a complicated influence on the slagging process. When the inner secondary air-swirling number is about middle intensive degree (about 0.9), the sticking-particle number reaches maximum. If the inner secondary air-swirling number continues increasing, then the coal particles will combust completely and reduce the particle concentration, thus decrease the sticking-particle number. The ratio of each air has a slight influence on the sticking-particle number relative to the swirling number. The coal particles with small mean diameter combust completely, which can reduce the sticking-particle number.

Combustion dynamics of inverted conical flames

An inverted conical flame anchored on a central bluff-body in an unconfined burner configuration features a distinctive acoustic response. This configuration typifies more complex situations in which the thermo-acoustic instability is driven by the interaction of a flame with a convective vorticity mode. The axisymmetric geometry investigated in this article features a shear region between the reactive jet and the surrounding atmosphere. It exhibits self-sustained oscillations for certain operating conditions involving a powerful flame collapse phenomenon with sudden annihilation of flame surface area. This is caused by a strong interaction between the flame and vortices created in the outer jet shear layer, a process which determines the amplitude of heat release fluctuation and its time delay with respect to incident velocity perturbations. This process also generates an acoustic field that excites the burner and synchronizes the vortex shedding mechanism. The transfer functions between the velocity signal at the burner outlet and heat release are obtained experimentally for a set of flow velocities fluctuations levels. It is found that heat release fluctuations are a strong function of the incoming velocity perturbation amplitude and that the time delay between these two quantities is mainly determined by the convection of the large scale vortices formed in the jet shear layer. A model is formulated, which suitably describes the observed instabilities.

Gas-phase synthesis of nanoparticles: scale-up and design of flame reactors

The scale-up and design of flame aerosol reactors are investigated for synthesis of silica and titania nanoparticles. In specific, coflow burners of different dimensions are studied at various precursor, fuel, and oxygen flow rates. The influence of the flame enthalpy content on product primary particle size is investigated by changing the fuel from methane to propane or hydrogen. Operation lines relating product particle size with reactant outlet conditions, burner size, and flame enthalpy are developed, showing how the different reactors can produce silica or titania nanoparticles of the same size. A scale-up procedure developed for fumed silica is extended to the synthesis of titania nanoparticles covering production rates of 2–200 g/h. At high fuel-oxidant velocity difference at the burner outlet, the operation of diffusion flame reactors converges to that of premixed ones.

Pulverized coal combustion characteristics of high-fuel-ratio coals

It is strongly desired for coal-fired power plants in Japan to utilize not only low-rank coals with high moisture and high ash contents, but also high-rank coals with high fuel ratio for diversifying fuel sources and lowering cost. In this study, pulverized coal combustion characteristics of high-fuel-ratio coals are experimentally investigated using an approximately 100 kg-coal/h pulverized coal combustion test furnace. The combustion characteristics are compared to those for bituminous coal. The coals tested are six kinds of coal with fuel ratios ranging from 1.46 to 7.10. The results show that under the non-staged combustion condition, the minimum burner load for stable combustion rises as fuel ratio increases. To improve the stability, it is effective to lengthen the residence time of coal particles in the high gas temperature region close to the burner outlet by using a recirculation flow. The conversion ratio of fuel nitrogen to NOx and unburned carbon fraction increases with increasing the fuel ratio. In addition, as the fuel ratio increases, NOx reduction owing to the staged combustion becomes small, and unburned carbon fraction increment becomes significant. The numerical simulations conducted under the staged combustion condition show that although the numerical results are in general agreement with the experimental ones, there remains room for improvement in NOx reduction model for high-fuel-ratio coals.

超低NO_Xバーナによる微粉炭燃焼場の数値シミュレーション : 瀝青炭使用時の石炭性状が燃焼特性に与える影響検討(FP2 燃料多様化・環境技術1)

A three-dimensional numerical simulation is applied to a pulverized coal combustion field in a test furnace equipped with an advanced low NO_X burner called CI-α burner. The applicability of the numerical simulation to the prediction of NO_X concentration in exhaust gas and combustion efficiency for bituminous coals is investigated, and their results are compared with experimental results. The results show that the present numerical simulation is generally applicable for the bituminous coals. The accuracy of the numerical simulation is improved by faithfully duplicating the configuration of the burner, and by using this detailed model it can be verified that a recirculation flow is formed in the high- gas-temperature region near CI-α burner outlet, and this lengthens the residence time of coal particles in this high-temperature region, promotes the evolution of volatile matter and the progress of char reaction, and produces an extremely low-O_2 region for effective Nox reduction.

Numerical analysis of pulverized coal combustion characteristics using advanced low-NO x burner

A three-dimensional numerical simulation is applied to a pulverized coal combustion field in a test furnace equipped with an advanced low-NO x burner called CI-α burner, and the detailed combustion characteristics are investigated. In addition, the validities of the existing NO x formation and reduction models are examined. The results show that a recirculation flow is formed in the high-gas-temperature region near the CI-α burner outlet, and this lengthens the residence time of coal particles in this high-temperature region, promotes the evolution of volatile matter and the progress of char reaction, and produces an extremely low-O 2 region for effective NO reduction. It is also found that, by lessening the effect of NO reduction in Levy et al.'s model and taking the NO formation from char N into account, the accuracy of the NO prediction is improved. The efficiency factor of the conversion of char N to NO affects the total NO concentration downstream after the injection of staged combustion air.

Self-induced combustion oscillations of laminar premixed flames stabilized on annular burners

Self-induced instabilities of laminar premixed flames stabilized over an annular burner have been studied in a set of experiments. A method was developed to determine the stability map of these systems using the response of both the burner and the flame to forced oscillations of the flow. This method is detailed for a well controlled example. The natural unstable motion of the flame is analyzed by measuring velocity fluctuations at the burner outlet, pressure fluctuations inside the burner and variations of the spontaneous light emitted by the flame. The burner's response to external pressure modulations is first characterized without flow and combustion. The flame's response to forced oscillations of the flow at the burner outlet is then used to determine the flame transfer function over the range of frequencies of interest. Using these elements, a mechanism is proposed for the onset of instability, which is shown to result from a coupling of the flame's response to flow oscillations with the bulk resonance mode of the burner. The driving mechanism leading to self-sustained oscillations of the flame front is produced by strong variations of the flame's surface area due to cyclic annihilations of neighboring elements in the flame front. During the collapse of large portions of the flame, a pressure pulse is released, which when properly phased with the burner acoustics, leads to resonance. A theoretical model for this instability is proposed and it is shown that at resonance, pressure fluctuations inside the burner and heat release fluctuations outside the burner must be in phase, in agreement with the Rayleigh criterion. Modeling predictions are compared to measurements using a diagram combining the acoustical response of the burner and measurements of the flame transfer functions. Predictions of the potentially unstable flow operating conditions are in good agreement with measurements. A criterion for the onset of instability is derived based on a balance of energy provided to and dissipated by the system. It is found that the gain of the flame transfer function at the resonant frequency of the burner must be sufficient to compensate for the losses of the system. The combined analysis of the burner acoustics and of the flame response to forced modulations of the flow provides a suitable description of instability modes observed experimentally.

Influence of Four Kinds of Gradient Magnetic Fields on Hydrogen-Oxygen Flame

To explore the possibility of flame control by magnetic force, the influence of four kinds of gradient magnetic fields on OH density distribution in a hydrogen-oxygen jet diffusion flame was investigated by numerically solving the equations of reactive gasdynamics and magnetism. Vertically decreasing (case 1), vertically increasing (case 2), horizontally decreasing (case 3), and horizontally increasing (case 4) gradients of magnetic field were considered as model configurations. According to the numerical analysis, because the mass density and the magnetic susceptibility of O 2 gas are much higher than those of other chemical species, the surrounding air containing a lot of O 2 gas is strongly influenced by the magnetic force, and the magnetically induced airflow changes the OH distribution in the flame indirectly. The direction and magnitude of the OH density change significantly depends on the configurations of the gradient magnetic field. Specifically, in cases 1, 3, and 4, the OH distribution migrated to the inside of the flame, but in case 2, it moved toward the outside. However, when the horizontally decreasing gradient of the magnetic field was set up near the burner outlet, as in case 3, the OH density change becomes largest among those in the four cases.

Rice Straw Smoke Generation System for Controlled Human Inhalation Exposures

A unique burner system was designed for the purposes of exposing human subjects to rice straw smoke under highly controlled conditions. The system burns individual straws and has produced particle concentrations up to 900 w g m m 3 in a temperature- and humidity-controlled exposure chamber. Burner operation is fully automatic and programmable. Ignition is by radiant heating in a manner similar to the field. Particle concentration is controlled by the firing interval between straws and by the ratio of burner outlet flow mixing with the main purified air supplied to the exposure chamber. Steady-state particle concentrations are achieved within 5 min from start. Repeatability of particle concentration is excellent at the two exposure levels used, 200 and 500 w g m m 3 . Measurements of particle size distributions suggest that approximately 80% of mass is in the size fraction below 1 w m. Although particle concentrations are controlled at target levels with good precision, emission factors are higher than ach...

A133 微小火炎の局所自発光分光計測

A spectroscopy measurement of local chemiluminescence of a micro flame was conducted in the present study in order to know effects of atmosphere temperature and wall temperature on radical distribution in the vicinity of surface. Local chemiluminescence of a micro flame was measured by Cassegrain optical system and ICCD spectroscope system with changing burner diameters and flow rate. Intensity of chemiluminescence of OH radical was relatively stronger in the vicinity of a small scale burner outlet in the cases of premixed supply of oxygen and methane.