Rainer Lückerath

FLOX® Combustion at High Pressure With Different Fuel Compositions

In Flameless Oxidation (FLOX® ) the combustion is distributed over a large volume by a high internal flue gas recirculation. This technology has been successfully used for many years in technical furnaces under atmospheric conditions with very low NOx emissions. In the work presented here, FLOX® combustion was for the first time investigated at high pressure in order to assess its applicability for gas turbine combustors. A FLOX® burner was equipped with a combustion chamber with quartz windows and installed into a high pressure test rig with optical access. The burner was operated under typical gas turbine conditions at pressure of 20 bar with thermal powers up to 475 kW. Natural gas as well as mixtures of natural gas and H2 were used as fuel. The NOx and CO emissions were recorded for the different operating conditions. OH* chemiluminescence imaging and planar laser-induced fluorescence of OH were applied in order to characterize the flame zone and the relative temperature distributions. The combustion behaviour was investigated as a function of equivalence ratio and fuel composition, and the influence of the gas inlet velocity on mixing and emissions was studied. For various operating conditions the lean extinction limits were determined.Copyright

LASER-BASED INVESTIGATION OF SOOT FORMATION IN LAMINAR PREMIXED FLAMES AT ATMOSPHERIC AND ELEVATED PRESSURES

ABSTRACT An experimental investigation into soot formation in laminar premixed flames at atmospheric and elevated pressures (1–5 bar) has been conducted. The flames were produced in a dual-flame burner enclosed in a pressure housing. Quantitative soot volume fraction measurements were obtained using laser-induced incandescence coupled with a quasi-simultaneous absorption measurement for calibration; the data were corrected for signal trapping using an “onion peeling” algorithm. Temperature measurements were obtained using shifted vibrational coherent anti-Stokes Raman scattering, which yields well-resolved, accurate temperature measurements in sooting and nonsooting environments. Results are presented for stable homogeneous flames using air as oxidizer and ethylene, propylene, and toluene as fuels. The variation of soot volume fraction and temperature with height above burner and as a function of fuel, equivalence ratio, and pressure are presented and discussed. The present soot data are well represented by a first-order growth rate law. The data identify trends and features useful for the validation of numerical models of soot formation.

FLOX® Combustion at High Power Density and High Flame Temperatures

In this contribution, an overview of the progress in the design of an enhanced FLOX ® burner is given. A fuel fiexible burner concept was developed to fulfill the requirements of modern gas turbines: high specific power density, high turbine inlet temperature, and low NO x emissions. The basis for the research work is numerical simulation. With the focus on pollutant emissions, a detailed chemical kinetic mechanism is used in the calculations. A novel mixing control concept, called HiPerMix®, and its application in the FLOX ® burner are presented. In view of the desired operational conditions in a gas turbine combustor, this enhanced FLOX ® burner was manufactured and experimentally investigated at the DLR test facility. In the present work, experimental and computational results are presented for natural gas and natural gas +hydrogen combustion at gas turbine relevant conditions and high adiabatic flame temperatures (up to T ad = 2000 K). The respective power densities are P A = 13.3 MW/m 2 bar (natural gas (NG)) and P A =14.8 MW/m 2 bar (NG + H 2 ), satisfying the demands of a gas turbine combustor. It is demonstrated that the combustion is complete and stable and that the pollutant emissions are very low.

Analysis of the Pollutant Formation in the FLOX® Combustion

FLOX®, or flameless combustion is characterized by ultralow NO x emissions. Therefore the potential for its implementation in gas turbine combustors is investigated in recent research activities. The major concern of the present paper is the numerical simulation of flow and combustion in a FLOX®-combustor [Wunning, J. A., and Wunning, J. G., 1997, Progress in Energy and Combustion Science, 23, pp. 81―94; Patent EP 0463218] at high pressure operating conditions with emphasis on the pollutant formation. FLOX®-combustion is a highly turbulent and high-velocity combustion process, which is strongly dominated by turbulent mixing and chemical nonequilibrium effects. By this means the thermal nitric oxide formation is reduced to a minimum, because even in the nonpremixed case the maximum combustion temperature does not or rather slightly exceeds the adiabatic flame temperature of the global mixture due to almost perfectly mixed reactants prior to combustion. In a turbulent flow, the key aspects of a combustion model are twofold: (i) chemistry and (ii) turbulence/chemistry interaction. In the FLOX®-combustion we find that both physical mechanisms are of equal importance. Throughout our simulations we use the complex finite rate chemistry scheme GRI3,0 for methane and a simple partially stirred reactor (PaSR) model to account for the turbulence effect on the combustion. The computational results agree well with experimental data obtained in DLR test facilities. For a pressure level of 20 bar, a burner load of 417 kW and an air to fuel ratio of λ = 2.16 computational results are presented and compared with experimental data.

Comparison of coherent anti-Stokes Raman-scattering thermometry with thermocouple measurements and model predictions in both natural-gas and coal-dust flames

Mobile coherent anti-Stokes Raman-scattering equipment was applied for single-shot temperature measurements in a pilot-scale furnace with a thermal power of 300 kW, fueled with either natural gas or coal dust. Average temperatures deduced from N(2) coherent anti-Stokes Raman-scattering spectra were compared with thermocouple readings for identical flame conditions. There were evident differences between the results of both techniques, mainly in the case of the natural-gas flame. For the coal-dust flame, a strong influence of an incoherent and a coherent background, which led to remarkable changes in the spectral shape of the N(2)Q-branch spectra, was observed. Therefore an algorithm had to be developed to correct the coal-dust flame spectra before evaluation. The measured temperature profiles at two different planes in the furnace were compared with model calculations.

Flame Characteristics and Emissions in Flameless Combustion Under Gas Turbine Relevant Conditions

Nomenclature d = inner diameter of the FLOX® nozzle, mm Da = Damkohler number dl = position of fuel nozzle with respect to air nozzle exit plane, mm f = focal length, mm k = rate coefficient NOx = oxides of nitrogen (NO and NO2) OH = OH chemiluminescence (electronically excited) Tad = global adiabatic flame temperature, K Tair = air preheat temperature, K v = velocity in air nozzle, ms 1 = air equivalence ratio = equivalence ratio

PLIF Thermometry Based on Measurements of Absolute Concentrations of the OH Radical

Abstract A method for measurements of planar temperature distributions based on planar laser-induced fluorescence (PLIF) of the OH radiacal is described. The technique was developed specifically for the application in lean combustion systems, where OH equilibrium concentrations are largely independent on equivalence ratio and a function of temperature only. It is thus possible to derive a temperature information from measurements of absolute OH concentration, which can be obtained from a combined PLIF/absorption measurement. This paper discusses the basics of the method, and describes validation experiments in high pressure laminar premixed flames which were performed to asses its applicability and accuracy. Therefore, we compared our LIF based results with CARS measurements performed in the same flames. Finally, an example for the application in a lean gas turbine model combustor is discussed.

Low-NOx Combustor Development pursued within the scope of the Engine 3E German national research program in a cooperative effort among engine Manufacturer MTU, University of Karlsruhe and DLR German Aerospace Research Center

Abstract MTU Aero Engines, the University of Karlsruhe and the DLR Aerospace Research Centre co-operated within the scope of the German national aeronautical research program Engine 3E. The program was focused on improving high-bypass turbofan engines. As a part of this program, a low-emission single-annular combustor was developed. The NOx emissions of this combustor are significantly reduced by using the rich-lean combustion concept. The basic idea of this concept is to avoid stoichiometric combustion conditions by splitting the combustion domain into a fuel-rich zone (low-oxygen zone) and a fuel-lean (low temperature zone). The NOx reduction capability of a combustor of this type scales with the homogeneity of the mixture in the rich zone and the time interval needed for the transition from the rich to the lean zone. Based on the insights gained from this cooperative research, an annular combustor was developed and tested at pressures up to 20 bar and inlet temperatures up to 800 K. The tested annular combustor was found to have NOx emissions of about 40% of the ICAO 96 standard. The carbon monoxide and unburned hydrocarbon emissions of the combustor are of about the same levels as present state of the art combustors.

Investigation of Soot Formation and Oxidation in a High-Pressure Gas Turbine Model Combustor by Laser Techniques

Swirl-stabilized, non-premixed C2 H4 /air flames were investigated at pressures up to 9 bar in order to contribute to a better understanding of soot formation and oxidation under gas turbine like conditions. The flames had a thermal power of up to 45 kW and were confined by a squared combustion chamber with quartz windows. Secondary air could be injected into the post-flame zone to study the influence of cooling air on the oxidation of soot. The good optical access to the flame enabled the application of optical and laser measuring techniques. Temperatures were measured by coherent anti-Stokes Raman scattering (CARS) and soot concentrations by 2D laser induced incandescence (LII). The influence of the flow field characteristics, known from previous measurements in similar configurations, on the soot distributions is discussed. Furthermore, the effects of pressure, equivalence ratio and secondary oxidation air on the instantaneous and mean soot distributions were studied. All measurements were performed under well-defined and documented conditions, so that the data sets are suitable for the validation of numerical simulations.© 2007 ASME

High Momentum Jet Flames at Elevated Pressure: B — Detailed Investigation of Flame Stabilization With Simultaneous PIV and OH-LIF

A model FLOX® combustor, featuring a single high momentum premixed jet flame, has been investigated using laser diagnostics in an optically accessible combustion chamber at a pressure of 8 bar. The model combustor was designed as a large single eccentric nozzle main burner (Ø 40 mm) together with an adjoining pilot burner and was operated with natural gas. To gain insight into the flame stabilization mechanisms with and without piloting, simultaneous Particle Image Velocimetry (PIV) and OH Laser Induced Fluorescence (LIF) measurements have been performed at numerous two-dimensional sections of the flame. Additional OH-LIF measurements without PIV-particles were analyzed quantitatively resulting in absolute OH concentrations and temperature fields. The flow field looks rather similar for both the unpiloted and the piloted case, featuring a large recirculation zone next to the high momentum jet. However, flame shape and position change drastically. For the unpiloted case, the flame is lifted, widely distributed and isolated flame kernels are found at the flame root in the vicinity of small scale vortices. For the piloted flame, on the other hand, both pilot and main flame are attached to the burner base plate, and flame stabilization seems to take place on much smaller spatial scales with a connected flame front and no isolated flame kernels. The single shot analysis gives rise to the assumption that for the unpiloted case small scale vortices act like the pilot burner flow in the opposed case and constantly impinge and ignite the high momentum jet at its root.