Peter A. Strakey

Development and Validation of a Thickened Flame Modeling Approach for Large Eddy Simulation of Premixed Combustion

The development of a dynamic thickened flame (TF) turbulence-chemistry interaction model is presented based on a novel approach to determine the subfilter flame wrinkling efficiency. The basic premise of the TF model is to artificially decrease the reaction rates and increase the species and thermal diffusivities by the same amount, which thickens the flame to a scale that can be resolved on the large eddy simulation (LES) grid while still recovering the laminar flame speed. The TF modeling approach adopted here uses local reaction rates and gradients of product species to thicken the flame to a scale large enough to be resolved by the LES grid. The thickening factor, which is a function of the local grid size and laminar flame thickness, is only applied in the flame region and is commonly referred to as dynamic thickening. Spatial filtering of the velocity field is used to determine the efficiency function by accounting for turbulent kinetic energy between the grid-scale and the thickened flame scale. The TF model was implemented into the commercial computational fluid dynamics code FLUENT. Validation in the approach is conducted by comparing model results to experimental data collected in a laboratory-scale burner. The burner is based on an enclosed scaled-down version of the low swirl injector developed at Lawrence Berkeley National Laboratory. A perfectly premixed lean methane-air flame was studied, as well as the cold-flow characteristics of the combustor. Planar laser induced fluorescence of the hydroxyl molecule was collected for the combusting condition, as well as the velocity field data using particle image velocimetry. Thermal imaging of the quartz liner surface temperature was also conducted to validate the thermal wall boundary conditions applied in the LES calculations.

STABILITY CHARACTERISTICS OF TURBULENT HYDROGEN DILUTE DIFFUSION FLAMES

Diffusion flame combustion of high-hydrogen fuels in land-based gas turbine combustors may include dilution of the fuel with inert gases and high velocity fuel injection to reduce NOx emissions. Stability regimes of such combustors are investigated in this study by examining turbulent dilute diffusion flames of hydrogen/nitrogen mixtures, issuing into a quiescent environment from a thin-lipped tube. This study has revealed two distinctly different types of lifted flames: lifted, laminar-base flames, for which liftoff heights vary from 1 to 3 jet diameters above the jet exit and are controlled by differential diffusion, and lifted, turbulent-base flames that stabilize much further downstream and are dominated by turbulent processes. In addition, stability limits governing the detachment or reattachment of the flame to the lip of the burner are examined, as well as the limits governing transitions between the two types of lifted flames and transition from these lifted flames to blowout.

Characterization of a Nitrogen Diluted Hydrogen Diffusion Flame for Model Validation

Dilute hydrogen diffusion flames have been considered as a gas turbine combustion strategy that provides relatively low levels of NOx emissions for application in integrated gasification combined cycle power generation. These flames also represent a challenging environment for computational modeling efforts due to the complexity of molecular transport effects, turbulence-chemistry interaction, and near extinction flame conditions. In order to provide data for validation of computational modeling efforts, measurements of major species concentration and flame temperature were made in such a flame using spontaneous Raman scattering. Experimental results demonstrate the importance of differential species diffusion, which occurs due to the disparity between diffusion characteristics of hydrogen and nitrogen. Additionally, the flame temperatures observed were quite low relative to the equilibrium flame temperature, due to flame strain. This confirms the fact that suppression of the thermal mechanism of NOx formation plays a significant role in reducing NOx emissions from this type of flame.

Development of a Three-dimensional Transient Wall Heat Transfer Model of a Rotating Detonation Combustor

Numerical simulation of transient heat transfer characteristics of a Rotating Detonation Combustor (RDC) is presented in this paper. A three-dimensional transient conduction model was developed to study the effect of large variation, periodic gas temperature exposed to the inner walls of the rotating detonation combustor outer body. The objective of the simulation is to predict heat flux transients and the interior wall surface temperatures from start up to 10s operation. The time varying, three-dimensional periodic convective boundary condition used for the heat transfer simulation is representative of the detonation wave propagation and other physical characteristics of RDC operating around 3000Hz and is derived from a separate computational fluid dynamics (CFD) simulation. The complex flow distribution downstream of the detonation/fill region results in a wall temperature and fluid dynamics that varies temporally and spatially in all directions. Simulation results were compared with experimental temperature data from literature on the outer body of an uncooled RDC. Combustor wall temperature variation in the axial direction indicates effect of non-uniformity on gas temperature distribution in the combustor for a non-premixed geometry. The simulation provides an estimate of transient heat load and hot spot locations that are critical to design efficient combustor cooling strategies.

Testing of a Hydrogen Diffusion Flame Array Injector at Gas Turbine Conditions

High-hydrogen gas turbines enable integration of carbon sequestration into coal-gasifying power plants, though NOx emissions are often high. This work explores nitrogen dilution of hydrogen diffusion flames to reduce thermal NOx emissions and avoid problems with premixing hydrogen at gas turbine pressures and temperatures. The burner design includes an array of high-velocity coaxial fuel and air injectors, which balances stability and ignition performance, combustor pressure drop, and flame residence time. Testing of this array injector at representative gas turbine conditions (16 atm and 1750 K firing temperature) yields 4.4 ppmv NOx at 15% O2 equivalent. NOx emissions are proportional to flame residence times, though these deviate from expected scaling due to active combustor cooling and merged flame behavior. The results demonstrate that nitrogen dilution in combination with high velocities can provide low NOx hydrogen combustion at gas turbine conditions, with significant potential for further NOx reductions via suggested design changes.

Testing of a Hydrogen Dilute Diffusion Array Injector at Gas Turbine Conditions

The U.S. Department of Energy’s Turbines Program is developing advanced technology for high-hydrogen gas turbines to enable integration of carbon sequestration technology into coal-gasifying power plants. Program goals include aggressive reductions in gas turbine NOx emissions: less than 2 ppmv NOx at 15% oxygen and 1750 K firing temperature. The approach explored in this work involves nitrogen dilution of hydrogen diffusion flames, which avoids problems with premixing hydrogen at gas turbine pressures and temperatures. Thermal NOx emissions are partially reduced through peak flame temperature control provided by nitrogen dilution, while further reductions are attained by minimizing flame size and residence time. The injector design includes high-velocity coaxial air injection from lobes surrounding the central fuel tube in each of the 48 array units. This configuration strikes a balance between stability and ignition performance, combustor pressure drop, and flame residence time. Array injector test conditions in the optically accessible Low Emissions Combustor Test & Research (LECTR) facility include air preheat temperatures of 500 K, combustor pressures of 4, 8 and 16 atm, equivalence ratios of 0.3 to 0.7, and three hydrogen/nitrogen fuel blend ratios. Test results show that NOx emissions increase with pressure and decrease with increasing fuel and air jet velocities, as expected. The magnitude of these emissions changes deviate from expected NOx scaling relationships, however, due to active combustor cooling and array spacing effects. At 16 atm and 1750 K firing temperature, the lowest NOx emissions obtained is 4.4 ppmv at 15% O2 equivalent (3.0 ppmv if diluent nitrogen is not considered), with a corresponding pressure drop of 7.7%. While these results demonstrate that nitrogen dilution in combination with high strain rates provides a reliable solution to low NOx hydrogen combustion at gas turbine conditions, the injector’s performance can still be improved significantly through suggested design changes.Copyright

Experimental and Numerical Study of Flashback in the SimVal Combustion Chamber

The effects of hydrogen addition on a lean-premixed swirl-stabilized combustor operating on natural gas and air were studied. Measurements of equivalence ratio and hydrogen concentration at flame flashback have been made at pressures ranging from 1 to 8 atmospheres, hydrogen concentration in the fuel of 60 to 100% and inlet velocities of 10, 20, 40 and 80 m/s. Increasing the hydrogen concentration in the fuel was found to significantly lower the equivalence ratio at flashback. This was believed to be the result of the much higher flame speed for hydrogen compared to methane. Increasing pressure was found to also decrease the equivalence ratio at flashback, while increasing the inlet velocity was found to increase the equivalence ratio at flashback. Two of these experiments were reproduced numerically using the FLUENTTM software. Numerical data were found to be in good agreement with experimental data at atmospheric pressure. The flashback process was investigated using the numerical data.