Peter Kutne

PHASE RESOLVED ANALYSIS OF FLAME STRUCTURE IN LEAN PREMIXED SWIRL FLAMES OF A FUEL STAGED GAS TURBINE MODEL COMBUSTOR

A scaled model of a gas turbine (GT) burner with coaxially mounted swirlers has been used to study the effects of fuel staging on the behavior of lean premixed methane air flames. Lean flames are known to be susceptible to instabilities that can lead to unsteady operation, flame extinction, and thermo-acoustic oscillations. High speed (10 kHz) laser and optical diagnostic techniques have been used to investigate the fuel staging effect on the mechanisms involved in such instabilities. Methane air flames at atmospheric pressure have been investigated at a constant thermal power of 58 kW. The global equivalence ratio was kept constant, while the fuel staging was varied. The bulk flow velocity at the exit plane was kept constant at 20 m/s. Simultaneous high speed OH PLIF, OH* CL, and acoustic measurements were performed at kHz repetition rate to characterize the flames and determine the operability limits of the combustor. The characterization measurements reveal significant changes in flame shape for various staging ratios as well as onset of self-excited thermo-acoustics in flames with more than 55% fuel injection in the outer swirler. The phase resolved analysis of the OH* CL revealed pulsation in the heat release due to acoustics in flames with higher percentage of fuel in the outer swirler. Comparison of the pressure oscillation in the combustion chamber with the heat release yielded a clear picture regarding the feedback mechanism that sustains the self-excited thermo-acoustic pulsations. The variation of local equivalence ratio of the mixture seems to be the driving force that initiates the onset of acoustics pulsations.

Nonintrusive Flowfield, Temperature and Species Measurements on a Generic Aeroengine Combustor at Elevated Pressures

A generic combustor was built, that gives wide optical access at higher pressure and shares typical features with aero engine combustors. A comprehensive data set for validation of RANS and LES codes was generated at isothermal as well as combusting conditions at 2 and 10 bars with 650 K preheat using natural gas as fuel. The velocity field was measured using LDA (Laser Doppler Anemometry) and DGV (Doppler Global Velocimetry) as well as PIV (Particle Image Velocimetry). Temperature data were acquired using CARS (Coherent Anti stokes Raman Scattering) and SRS (Spontaneous Raman Scattering). Major species concentrations as well as the mixture fraction in the primary zone of the combustor were also measured using SRS. Mean and RMS values of the temperature measured by CARS in the secondary zone illustrate the influence of the jet impingement on the unsteady mixing of the jets with the swirling primary air. [Keywords: non intrusive measurements, aero engine combustion, elevated pressure]

Investigation of Flame Stabilization in a High-Pressure Multi-Jet Combustor by Laser Measurement Techniques

In this contribution, comprehensive optical and laser based measurements in a generic multi-jet combustor at gas turbine relevant conditions are presented. The flame position and shape, flow field, temperatures and species concentrations of turbulent premixed natural gas and hydrogen flames were investigated in a high-pressure test rig with optical access.The needs of modern highly efficient gas turbine combustion systems, i.e., fuel flexibility, load flexibility with increased part load capability, and high turbine inlet temperatures, have to be addressed by novel or improved burner concepts. One promising design is the enhanced FLOX® burner, which can achieve low pollutant emissions in a very wide range of operating conditions. In principle, this kind of gas turbine combustor consists of several nozzles without swirl, which discharge axial high momentum jets through orifices arranged on a circle. The geometry provides a pronounced inner recirculation zone in the combustion chamber. Flame stabilization takes place in a shear layer around the jet flow, where fresh gas is mixed with hot exhaust gas. Flashback resistance is obtained through the absence of low velocity zones, which favors this concept for multi-fuel applications, e.g. fuels with medium to high hydrogen content.The understanding of flame stabilization mechanisms of jet flames for different fuels is the key to identify and control the main parameters in the design process of combustors based on an enhanced FLOX® burner concept. Both experimental analysis and numerical simulations can contribute and complement each other in this task. They need a detailed and relevant data base, with well-known boundary conditions. For this purpose, a high-pressure burner assembly was designed with a generic 3-nozzle combustor in a rectangular combustion chamber with optical access. The nozzles are linearly arranged in z direction to allow for jet-jet interaction of the middle jet. This line is off-centered in y direction to develop a distinct recirculation zone. This arrangement approximates a sector of a full FLOX® gas turbine burner. The experiments were conducted at a pressure of 8 bar with preheated and premixed natural gas/air and hydrogen/air flows and jet velocities of 120 m/s.For the visualization of the flame, OH* chemiluminescence imaging was performed. 1D laser Raman scattering was applied and evaluated on an average and single shot basis in order to simultaneously and quantitatively determine the major species concentrations, the mixture fraction and the temperature. Flow velocities were measured using particle image velocimetry at different section planes through the combustion chamber.Copyright

Numerical and Experimental Investigation of a Semi-Technical Scale Burner Employing Model Synthetic Fuels

In this paper the development and the application of a numerical code suited for the simulation of gas-turbine combustion chambers is presented. In order to obtain an accurate and flexible framework, a finite-rate chemistry model is implemented, and transport equations for all species and enthalpy are solved. An assumed PDF approach takes effects of temperature and species turbulent fluctuations on the chemistry source term into account. In order to increase code stability and to overcome numerical stiffness due to the large-varying chemical kinetics timescales, an implicit and fully-coupled treatment of the species transport equations is chosen. Low-Mach number flow equations and k-e turbulence model complete the framework, and make the code able to describe the most important physical phenomena which take place in gas-turbine combustion chambers. In order to validate the numerical simulations, experimental measurements are carried out on a generic non-premixed swirl-flame combustor, fuelled with syngas-air mixtures and studied using optical diagnostic techniques. The combustor is operated at atmospheric and high-pressure conditions with simulated syngas mixtures consisting of H2, N2, CH4, CO. The combustor is housed in an optically-accessible combustion chamber to facilitate the application of chemiluminescence imaging of OH* and planar laser-induced fluorescence (PLIF) of the OH-radical. To investigate the velocity field, particle image velocimetry (PIV) is used. The OH* chemiluminescence imaging is used to visualise the shape of the flame zone and the region of heat release. The OH-PLIF is used to identify reaction zones and regions of burnt gas. The fuel composition is modelled after a hydrogen-rich synthesis gas, which can result after gasification of lignite followed by a CO shift reaction and a sequestration of CO2. Actual gas compositions and boundary conditions are chosen so that it is possible to outline differences and similarities among fuels, and at the same time conclusions about flame stability and combustion efficiency can be drawn. A comparison between experimental and numerical data is presented, and main strengths and deficiencies of the numerical modelling are discussed.Copyright

Numerical and experimental investigation of syngas combustion in a semi-technical scale burner

A semi-technical scale burner burning a syngas mixture is presented and deeply investigated by means numerical tools and experimental campaigns. The combustor is operated at atmospheric and high-pressure conditions with simulated syngas mixtures consisting of H2, N2, CH4, CO. Actual gas compositions and operating conditions are chosen so that a sensitivity analysis with respect to fuel composition and combustion stability can be carried out. To investigate the velocity field, particle image velocimetry (PIV) is used. The DLR CFD code THETA is used to simulate the turbulent reacting flow. A finite-rate chemistry approach is used to retain the whole spectrum of the chemistry timescales and to allow the investigation of single reactions on the combustion efficiency. Turbulent reaction rate terms are closed by employing an assumed-PDF approach, which is able to describe the impact of the temperature and species fluctuation on the chemistry by transporting two additional quantities (temperature variance and sum of species variances). Reference calculations will be presented and validated against the available PIV measurements. The influence of the combustion model and chemical kinetics mechanism used on the chamber performance is investigated and a detailed comparison is presented.

Measurement of the Equivalence Ratio in an Atmospheric Gasifier Using Laser Induced Breakdown Spectroscopy

Laser induced breakdown spectroscopy was used to measure the local equivalence ratio in an atmospheric gasifier operated with a mixture of ethylene glycol and char particles as model fuel.

Development of an Infrared Absorption Spectrometer for Temperature and Species Concentration Measurements in a Gasifier

An absorption spectrometer utilizing a tunable distributed feedback diode laser at 2.3µm and an interband cascade laser at 3.1µm has been developed to measure temperature and concentrations of CO, CH4, C2H2 and H2O under gasification conditions. The setup has been tested and then applied for measurements at an atmospheric entrained flow gasifier.

Laser induced breakdown spectroscopy for gas phase measurements of alkaline species

An experimental setup to produce a preheated air flow, seeded with reproducible amounts of earth alkaline or alkaline species is described. Results from the first measurements with Laser induced breakdown spectroscopy are presented.