H. Büchner

Combustion dynamics of turbulent swirling flames

Abstract In the present paper the influence of a periodic excitation of the mass flow rate, varied in frequency and in amplitude, on the isothermal flow field and on the flame characteristics of swirl burners with different swirl intensities S 0,th is investigated. At first the fluid dynamical conditions for the formation of ring-vortex structures in the burner near flow field are determined. The results indicate that the minimum level of excitation of the mass flow rate for vortex formation decreases hyperbolically with increasing frequency of pulsation and characteristic Strouhal number, respectively. In further studies, the dynamical behavior of lean-premixed swirl flames is investigated, whereas the fuel gas/air mixture mass flow rate at the burner exit was modulated sinusoidally. The dynamical behavior of the investigated flames is found to be dominated by two different effects in certain frequency ranges: For moderate pulsation frequencies (f puls ≲ 50 Hz) the detected periodic heat release rate of pulsated, premixed swirl flames is dominated by an effect that inhibits the strong entrainment of ambient medium in comparison with the corresponding quasi steady-state swirl flames. With increasing frequency (f puls ≳ 50 Hz) this effect will be overlaid by the periodical formation of ring-vortices entraining additional ambient medium and interfering with the combustion process. The physical understanding of the frequency-dependent flame dynamics (flame transfer function) on periodic disturbances is indispensable for the prediction of the formation of combustion-driven oscillations that occur in technical combustion systems (e.g., gas turbines, industrial combustors). It is also evident for the development of methods to prevent or suppress periodic combustion instabilities. The results of the flame transfer measurements presented in this paper will lead to a basic understanding of the formation and reaction of large-scale coherent vortex structures in turbulent flames, that are well known as drivers of combustion instabilities.

Experimental Investigations on the Dynamics of Pulsated Premixed Axial Jet Flames

Abstract The understanding of the origin of self-excited pressure oscillations in technical combustion systems as well as a successful design of pulse combustors depends on the knowledge of the dynamical behavior of the used flame. In this study the mixture mass flow rate through the burner nozzle was modulated sinusoidally to measure the frequency dependent heat release rate of a premixed turbulent jet flame. The dynamical behaviour of the investigated flames was found to be predominantly a function of a Strouhal number formed with the characteristics of the corresponding steady flames and to be very similar to that of an ideal idle-time model. The discrepancy between this model and the experiments at higher frequencies could be explained by the periodical formation of vortex rings.

PREDICTION OF STABILITY LIMITS FOR LP AND LPP GAS TURBINE COMBUSTORS

ABSTRACT The prediction and the systematic suppression of self-sustained combustion instabilities in combustors for gas turbine applications still suffer from the lack of physical models for the dynamic flame characteristics of Lean-Premixed natural gas and Lean-Prevaporized-Premixed kerosene flames. Hence, the experimental determination of the flame transfer functions of LPP swirl flames was achieved using a prevaporization unit for kerosene. A time-independent and spatial homogeneous mixture of kerosene and combustion air was generated at the burner exit. By exchangeable fuel nozzles the flame dynamics were as well detected for natural gas LP swirl flames with identical operation conditions. The flame dynamics are strongly affected by the formation and in-phase reaction of coherent vortex structures, well known as drivers of combustion instabilities. These structures have been visualized with a phase-correlated imaging technique. The results discussed in this paper lead to a basic understanding and quantitative prediction of the frequency-dependent dynamics of LP and LPP swirl flames. Especially, the influence of preheating temperature, air equivalence ratio and the type of fuel on the amplitude responses and phase angle functions were investigated in detail. Based on theoretical considerations concerning the burning velocity of steady-state premixed flames a physical model and – derived from that it–scaling laws for the prediction of unstable operation modes in dependence on main operation parameters of the flame were formulated and validated by the measurements. Thus, it is now possible to scale and quantitatively predict the stability limits for the formation of periodic combustion instabilities of lean-premixed combustors with swirl-stabilized flames.

Effect of co- and counter-swirl on the isothermal flow- and mixture-field of an airblast atomizer nozzle

Abstract Results of the performed experiments describe the influence of co- and counter-rotating airflows through the inner and outer ducts of an airblast atomizer on the turbulent flow and mixture field. Therefore, two different nozzles have been manufactured, which differ only in the orientation of rotation of the inner and outer airflow, and have been confined by a combustion chamber, which provides optical access for laser diagnostics. The isothermal flow field has been investigated using a 3D-LDV-system. The 30° off-axis forward scattering configuration allows a high temporal and spatial resolution of the three velocity components, thus determining not only mean velocities but the total Reynolds stress tensor. Determination of the mixture field has been performed by a suction probe and subsequent analysis of the local concentration by means of conventional gas analysis. Compared to the co-swirl configuration the flow field of the counter-swirl arrangement exhibits a marked increase of the mass flow recirculated in the internal recirculation zone and a reduction of its length in axial direction. The analysis of turbulence quantities shows a considerable attenuation of the turbulent exchange of momentum perpendicular to the main flow direction for counter-rotating airflows. According to Rayleigh’s criterion this effect is attributed to the weaker reduction of the radial profiles of the time mean tangential velocity within the domain of the near-nozzle outer jet boundary in case of the counter-swirl configuration. In analogy to the exchange of momentum the obtained mixture fields feature a reduction of turbulent mass transfer rate in radial direction with counter-rotating airflows.

EFFECT OF CO- AND COUNTER-SWIRL ON THE ISOTHERMAL FLOW- AND MIXTURE-FIELD OF AN AIRBLAST ATOMIZER NOZZLE

ABSTRACT Results of the performed experiments describe the influence of co- and counter-rotating airflows through the inner and outer duct of an airblast atomizer on the turbulent flow and mixture field. Therefore, two different nozzles have been manufactured, which differ only in the orientation of rotation of the inner and outer airflow, and have been confined by a combustion chamber, which provides optical access for laser diagnostics. The isothermal flow field has been investigated using a 3D-LDV-System. The 30° off-axis forward scattering configuration allows a high temporal and spatial resolution of the three velocity components, thus determining not only mean velocities but the total Reynolds-stress tensor. Determination of the mixture field has been performed by a suction probe and subsequent analysis of the local concentration by means of conventional gas analysis. Compared to the co-swirl configuration the flow field of the counter-swirl arrangement exhibits a marked increase of the mass flow recirculated in the internal recirculation zone and a reduction of its length in axial direction. The analysis of turbulence quantities shows a considerable attenuation of the turbulent exchange of momentum perpendicular to the main flow direction for counter-rotating airflows. According to Rayleighs criterion this effect is attributed to the weaker reduction of the radial profiles of the time mean tangential velocity within the domain of the near-nozzle outer jet boundary in case of the counter-swirl configuration. In analogy to the exchange of momentum the obtained mixture fields feature a reduction of turbulent mass transfer rate in radial direction with counter-rotating airflows.

MODELING OF RING VORTICES AND THEIR INTERACTION WITH TURBULENT PREMIXED FLAMES

Depending on amplitude and frequency of self-excited flame/pressure oscillations in technical combustion systems, the formation of large-scale turbulent ring vortices can be observed which interact with the flame. The phenomenon was investigated with numerical simulations using unsteady Reynolds-averaged Navier–Stokes methods. The results were validated against experimental data. The calculation of the forced isothermal flow field yields that the prediction of the transient vortex positions works very well. In contrast, the turbulent diffusion is underestimated. The flame frequency response of a forced premixed turbulent axial methane jet flame (P therm = 40 kW) was calculated using a simplistic turbulent reaction model. The measured flame frequency response was derived via detection of the global radiation of OH radicals and the calculated flame response via analysis of the global heat release rate. Despite the fact that a quantitative comparison of these properties is not possible, a qualitative comparison shows good agreement regarding the representation of amplitude response and phase-angle function.

Measurement and Simulation of Combustion Noise emitted from Swirl Burners

A major uncertaincy, when designing combustors is the influence of geometrical patterns of the design on the combustion noise generated. In order to determine the mechanisms and processes that influence the noise generation of flames with underlying swirling flows, a new burner has been designed, that offers the possibility to vary geometrical parameters. Experimental data (flow field, noise emission) have been determined for this burner. In addition, Large Eddy Simulations (LES) have been performed to study the isothermal and reacting flow of the burner. The results of the measurements show a distinct rise of the sound pressure level, obtained by changing the test setup from the isothermal to the flame configuration as well as by varying geometrical parameters, which is also resembled by the LES simulation results. 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. The computed isothermal and reacting flow fields have been discussed through flow visualization; the computed acoustic pressure has been compared with the experiment and it showed good agreement.

The impact of flame stabilisation and coherent flow structures on the noise emission of turbulent swirl flames

The development of modern jet engine and stationary gas turbine applications is focused on the reduction of pollutants and, increasingly, on the reduction of noise emissions including combustion noise. This requires the minimization of noise sources, namely noise from the turbulent flow, combustion noise and noise caused by periodic flow and combustion instabilities or fluctuations of the ignition zone. This has to be achieved under conservation of the benefits of swirl flames, i.e. high ignition stability and broad operation ranges. The presented experimental work enables to relate the total noise generation of swirl flames to its physical sources:turbulent flow noise, combustion noise, noise caused by a lifted, unstable flame stabilization and the combustion of coherent flow structures, the latter being caused by the burners exit geometry [1, 2]. Different positions of the flame stabilization can cause an increase of combustion noise up to 3 dB. With the results it is possible to prevent additional noise to the unavoidable minimal combustion noise, evoked by stochastic, turbulent fluctuations of the mixture density, i.e. optimization of the ignition stability with a perfectly premixed pilot flame. Further insight was gained about the influence of the gas injection direction on the combustion noise, where some configurations lead to an increase up to 6 dB in comparison to the results of premixed flames [2], which in general are louder than all investigated non-premixed flames.

Combustion Noise from Non-premixed and Lean-premixed Swirl Flames

The development of modern gas turbines and jet engines is focused on the reduction of pollutant emissions and, increasingly, on the reduction of overall noise emission, including combustion noise. This requires the minimization of the noise sources, namely noise from the turbulent flow, combustion noise and noise caused by periodic instabilities and fluctuations of the ignition zone. This has to be achieved under conservation of the benefits of swirl flames, e.g. high ignition stability and broad operation ranges. This paper is focused on the description and characterisation of fluctuations of the ignition zone due to different mixing and stabilization characteristics of noise from swirl flames. Another goal was the description of flame noise caused by the formation and burning of coherent, periodic flow structures, often detected in swirl flows and flames [1, 2, 3, 4]. The physical model offers the possibility to understand the formation mechanism of the coherent flow structures and their effect on the combustion of non-premixed and premixed swirl flames.

The influence of Fuel/Air Mixture Oscillations on the Formation of Self-Sustained Combustion Instabilities in Premixed Combustion Systems

Combustion instabilities are characterized by periodic oscillations of the static pressure in the combustion chamber, in the fuel/air supply lines or in downstream flue gas sections. The energy supply to cover friction and acoustic losses of the oscillating system is provided by the flame in form of periodic heat release rates, which have to fulfill Rayleigh’s criterion for the phase shift between the periodic fluctuations of heat release and pressure. Especially at low frequency high amplitude pressure oscillations, the pollutant emission characteristics are being altered and the fuel burnout is often reduced. Because of the increasing use of premixing type burners, as being promoted by the demand for minimized thermal NOx emissions in industrial combustion facilities, the following investigations have been carried out for a premixed system fired with natural gas. With this system, the influence of periodic pressure oscillations on the fuel gas/air mixture formation was studied, and the effects of periodic changes of the air/fuel ratio on the strength of the pressure excitation and the burnout characteristic of the flame were analysed and discussed.