Consistent Theoretical Model of Mean Diameter and Size Distribution by Liquid Sheet Atomization

A consistent theoretical model is proposed and validated for calculating droplet diameters and size distributions. The model is derived based on the energy conservation law including the surface free energy and the Laplace pressure. Under several hypotheses, the law derives an equation indicating that atomization results from kinetic energy loss. Thus, once the amount of loss is determined, the droplet diameter is able to be calculated without the use of experimental parameters. When the effects of ambient gas are negligible, injection velocity profiles of liquid jets are the essential cause of the reduction of kinetic energy. The minimum Sauter mean diameter produced by liquid sheet atomization is inversely proportional to the injection Weber number when the injection velocity profiles are laminar or turbulent. A non-dimensional distribution function is also derived from the mean diameter model and Nukiyama-Tanasawa’s function. The new estimation methods are favorably validated by comparing with corresponding mean diameters and the size distributions, which are experimentally measured under atmospheric pressure.

Staged Premix EV Combustion in Alstom’s GT24 Gas Turbine Engine

Reducing gas turbine emissions and increasing their operational flexibility are key targets in today’s gas turbine market. In order to further reduce emissions and increase the operational flexibility of its GT24, Alstom has introduced an internally staged premix system into the GT24’s EV combustor. This system features a rich premix mode for GT start-up and a lean premix mode for GT loading and baseload operation. The fuel gas is injected through two premix stages, one injecting fuel into the burner air slots and one injecting fuel into the centre of the burner cone. Both premix stages are in continuous operation throughout the entire operating range, i.e. from ignition to baseload, thus eliminating the previously used pilot operation during start-up with its diffusion-type flame and high levels of NOx formation. The staged EV combustion concept is today a standard on the current GT26 and GT24. The EV burners of the GT26 are identical to the GT24 and fully retrofittable into existing GT24 engines. Furthermore, engines operating only on fuel gas (i.e. no fuel oil operation) no longer require a nitrogen purge and blocking air system so that this system can be disconnected from the GT. Only minor changes to the existing GT24 EV combustor and fuel distribution system are required. This paper presents validation results for the staged EV burner obtained in a single burner test rig at full engine pressure, and in a GT24 field engine, which had been upgraded with the staged EV burner technology in order to reduce emissions and extend the combustor’s operational behavior.

Emission Reduction of Fuel Staged Aircraft Engine Combustor Using an Additional Premixed Fuel Nozzle

The Japan Aerospace Exploration Agency (JAXA) is conducting research and development on aircraft engine technologies to reduce environmental impact for the TechCLEAN project. As a part of the project, combustion technologies have been developed with an aggressive target that is an 80% reduction over the NOx threshold of the ICAO CAEP/4 standard. A staged fuel nozzle with a pilot mixer and a main mixer was developed and tested using a single-sector combustor under the target engine’s LTO cycle conditions with a rated output of 40 kN and an overall pressure ratio of 25.8. The test results showed a 77% reduction over the CAEP/4 NOx standard. A reduction in smoke was found under a higher thrust condition than the 30% MTO condition, and a reduction in CO emission was found under a lower thrust condition than the 85% MTO condition. In the present study, an additional fuel burner was designed and tested with the staged fuel nozzle in a single-sector combustor to control emissions. The test results show that the combustor enables an 82% reduction in NOx emissions relative to the ICAO CAEP/4 standard and a drastic reduction in smoke and CO emissions.

Investigation of Precessing-Vortex-Core–Flame Interaction Based on Tomographic Reconstruction Techniques

Unsteady helical flow structures, such as the precessing vortex core (PVC), are often observed in swirling flows with vortex breakdown. Although this type of flow is of high relevance for industrial combustors, the role of these flow instabilities in reacting systems, in particular their effect on flame stabilization and combustion instabilities, remains poorly understood. The three-dimensional structure of the interaction between the helical mode and the flame is difficult to assess with common measurement techniques, such as chemiluminescence imaging, due to the non-axisymmetry of the oscillation pattern. In the present work, a novel method is proposed to determine the full field of the heat release rate perturbation associated with the helical mode. This method requires only line-of-sight integrated information from a single camera. Tomographic reconstruction techniques are used, exploiting the fact that the helical mode is a rotating structure. Reconstruction algorithms are presented that are tailored to the specific spatio-temporal structure of the oscillation pattern, and it is shown that these techniques outperform standard methods. The proposed methodology is applied in a turbulent swirl-stabilized model combustor with significant PVC oscillations. Images from an intensified high-speed camera are used for the reconstruction. The analysis shows that the helical mode perturbs the flame in the inner and the outer shear layers of the annular jet and thereby creates helical traveling waves. The perturbation in the outer shear layer grows significantly in downstream direction and causes strong heat release rate fluctuations when impinging on the combustor wall.

LES-Based Study of the Influence of Thermal Boundary Condition and Combustor Confinement on Premix Flame Transfer Functions

The influence of thermal boundary condition at the combustor wall and combustor confinement on the dynamic flame response of a perfectly premixed axial swirl burner is investigated. Large Eddy Simulations are carried out using the Dynamically Thickened Flame combustion model. Then, system identification methods are used to determine the flame transfer function (FTF) from the computed time series data. Two configurations are compared against a reference case with 90 mm × 90 mm combustor cross section and nonadiabatic walls: 1) combustor cross section similar to the reference case with adiabatic combustor walls, and 2) different confinement (160 mm × 160 mm) with nonadiabatic walls. It is found that combustor confinement and thermal boundary conditions have a noticeable influence on the flame response due to differences in flame shape and flow field. In particular the FTF computed with adiabatic wall boundary condition, which produces a flame with significant heat release in both shear layers, differs significantly from the FTF with nonadiabatic walls, where the flame stabilizes only in the inner shear layer. The observed differences in flow field and flame shape are discussed in relation to the unit impulse response of the flame. The impact of the differences in FTF on stability limits is analyzed with a low-order thermoacoustic model.

Spray Flames Simulation Using a Two Phase Combustion Model With Single Droplet Combustion Mode

There are three combustion regimes of individual droplet combustion behavior: the fully enveloped flame, the partially enveloped flame, and the wake flame. From PLIF measurement results, single droplet combustion phenomenon happens in spray flame, as well as lean type gas turbine combustion chamber sometimes. The drag coefficient, evaporation rate, and combustion rate are different according to the burning modes. At present, in Reynolds Averaged Navier Stokes (RANS) method and Large Eddy Simulation (LES) method, the droplets are treated as point source because the grid scale is bigger than the droplet diameter. A two phase combustion model with the consideration of the individual droplet burning mode is proposed before. In this paper, this model is tested by spray flames here again. Furthermore, this model was used in a concept lean premixed pre-vaporized (LPP) combustion case too. In spray flame, the predicted results are close to the experimental data.

Comparison of Two Methods for Flame Transfer Function Measurements: MOOG Valve or Siren and the Impact of Results on the Instability Analysis in a High Pressure Combustor

Thermoacoustic instabilities may occur in every gas turbine combustor and could be hazardous to the flame stability and the structural integrity. It is important to be able to predict how hazardous the instabilities are: at what frequencies will they occur and will they develop into high amplitude limit cycle oscillations? The former question can be answered with the help of the Flame Transfer Function (FTF). The FTF establishes the coupling between burner passage aerodynamics and combustion dynamics and can be used as an input to an acoustic model to predict the eigenfrequencies and their growth rate. In the present research two methods to measure the FTF are used with different signal excitation instruments: a MOOG Valve and a Siren. Both the methods are based on data from pressure transducers only. The FTF is measured here by determining the combustor pressure response of the flame to fluctuations in the fuel mass flow at the burner exit. A siren unit has been developed and mounted at the upstream end of the fuel supply line of a pressurized combustor and is designed to have a harmonic excitation. The experimental method to measure the FTF by means of factorization in known or measurable sub-functions is briefly explained. Subsequently the Siren method is demonstrated by means of extracting the FTF at elevated pressure and as a function of thermal power. The results are compared with the results obtained in previous work of a MOOG valve excitation unit. The experimental investigation of the FTF is carried out in a high pressure combustor rig named DESIRE which is able to perform thermoacoustic measurements up to 500 kW thermal power at 5 bar absolute pressure. The results are compared and discussed. Subsequently a 1-D acoustic network model is presented which predicts the onset of the limit cycle pressure oscillations in the DESIRE combustor, using the FTF as an input. Thermo viscous damping effects and measured reflection coefficients are also included into the network model to improve the model predictions. Finally, the measured and predicted dynamic behavior of the combustor are compared. The results indicate that the network modeling approach is a promising design tool as it gives good agreement between measured and predicted dynamic behavior of the combustor and instability analysis. Well-defined boundary conditions and thermo viscous damping effects are important for the accuracy of the acoustic network models.

Heating and Efficiency Comparison of a Fischer-Tropsch (FT) Fuel, JP-8+100, and Blends in a Three-Cup Combustor Sector

In order to realize alternative fueling for military and commercial use, the industry has set forth guidelines that must be met by each fuel. These aviation fueling requirements are outlined in MIL-DTL-83133F(2008) or ASTM D 7566-Annex standards and are classified as “drop-in” fuel replacements. This paper provides combustor performance data for synthetic-paraffinic-kerosene- (SPK-) type (Fisher-Tropsch (FT)) fuel and blends with JP-8+100, relative to JP-8+100 as baseline fueling. Data were taken at various nominal inlet conditions: 75 psia (0.52 MPa) at 500 °F (533 K), 125 psia (0.86 MPa) at 625 °F (603 K), 175 psia (1.21 MPa) at 725 °F (658 K), and 225 psia (1.55 MPa) at 790 °F (694 K). Combustor performance analysis assessments were made for the change in flame temperatures, combustor efficiency, wall temperatures, and exhaust plane temperatures at 3%, 4%, and 5% combustor pressure drop (%ΔP) for fuel:air ratios (F/A) ranging from 0.010 to 0.025. Significant general trends show lower liner temperatures and higher flame and combustor outlet temperatures with increases in FT fueling relative to JP-8+100 fueling. The latter affects both turbine efficiency and blade/vane life. In general, 100% SPK-FT fuel and blends with JP-8+100 produce less particulates and less smoke and have lower thermal impact on combustor hardware.

Characterization of the Acoustic Interactions in a Two-Staged Multi-Injection Combustor Fed With Liquid Fuel

Burners operating in lean premixed prevaporized (LPP) regimes are considered as good candidates to reduce pollutant emissions from gas turbines. Lean combustion regimes result in lower burnt gas temperatures and therefore a reduction on the NOx emissions, one of the main pollutant species. However, these burners usually show strong flame dynamics, making them prone to various stabilization problems (combustion instabilities, flashback, flame extinction). To face this issue, multi-injection staged combustion can be envisaged. Staging procedures enable fuel distribution control, while multipoint injections can lead to a fast and efficient mixing. A laboratory-scale staged multipoint combustor is developed in the present study, in the framework of LPP combustion, with an injection device close to the industrial one. Using a staging procedure between the primary pilot stage and the secondary multipoint one, droplet and velocity field distributions can be varied in the spray that is formed at the entrance of the combustion chamber. The resulting spray and the flame are characterized using OH-Planar Laser Induced Fluorescence, High Speed Particle Image Velocimetry and Phase Doppler Anemometry measurements. Three staging values, corresponding to three different flame stabilization processes, are analyzed, while power is kept constant. It is shown that mean values are strongly influenced by the fuel distribution and the flame position. Using adequate post-processing, the interaction between the acoustic field and the droplet behavior is characterized. Spectral analysis reveals a strong acoustic-flame coupling leading to a low frequency oscillation of both the velocity field and the spray droplet distribution. In addition, acoustic measurements in the feeding line show that a strong oscillation of the acoustic field leading to a change in fuel injection, and hence droplet behavior.

Dynamics of Thermoacoustic Oscillations Leading to Lean Flame Blowout

Lean flame blowout induced by thermoacoustic oscillations is a serious problem faced by the power and propulsion industry. We analyze a prototypical thermoacoustic system through systematic bifurcation analysis and find that starting from a steady state, this system exhibits successive bifurcations resulting in complex nonlinear oscillation states, eventually leading to flame blowout. To understand the observed bifurcations, we analyze the oscillation states using nonlinear time series analysis, particularly through the representation of pressure oscillations on a reconstructed phase space. Prior to flame blowout, a bursting phenomenon is observed in pressure oscillations. These burst oscillations are found to exhibit similarities with the phenomenon known as intermittency in the dynamical systems theory. This investigation based on nonlinear analysis of experimentally acquired data from a thermoacoustic system sheds light on how thermoacoustic oscillations lead to flame blowout.