Peter Strakey

An Irregularly Portioned Lagrangian Monte Carlo Method for Turbulent Flow Simulation

A novel computational methodology, termed “Irregularly Portioned Lagrangian Monte Carlo” (IPLMC) is developed for large eddy simulation (LES) of turbulent flows. This methodology is intended for use in the filtered density function (FDF) formulation and is particularly suitable for simulation of chemically reacting flows on massively parallel platforms. The IPLMC facilitates efficient simulations, and thus allows reliable prediction of complex turbulent flames. Sample results are presented of LES of both premixed and non-premixed flames via this method, and the results are assessed via comparison with laboratory data.

Assessment of RANS-Based Turbulent Combustion Models for Prediction of Emissions from Lean Premixed Combustion of Methane

Reynolds-Averaged Navier-Stokes (RANS) simulations of Lean Premixed Combustion (LPC) of methane–air in a bluff-body stabilized combustor were performed with several widely used turbulent combustion methodologies in order to assess their prediction capabilities. The methods employed are the Eddy Dissipation Concept (EDC), the Composition Probability Density Function (CPDF) and the Joint Velocity–Frequency-Composition PDF (VFCPDF) models. Where needed, two different models were employed for turbulent transport closure, namely the Renormalization Group (RNG) k-ϵ and Reynolds Stress Transport (RSM) models. The combustion chemistry was represented by two separate augmented reduced mechanisms (ARM9 and ARM19) in order to assess the influence of chemical mechanisms on calculations. Mean temperature and major species predictions of all of the employed methodologies compared well with the experimental data. Intermediate and emission species predictions were sensitive to the resolution of turbulence viscosity, which changes the effective diffusivity of the species. NO emissions predictions were in error by an average ±5 ppm with the EDC models and the CPDF model, with the VFCPDF model showing a somewhat better prediction of NOx. Calculations for some intermediate species (especially H2) deviated qualitatively from the experimental data, which highlights some of the limitations of these methodologies commonly used in detailed prediction of emissions for various fuel blends.

An Irregularly Portioned FDF Simulator

A new computational methodology, termed “irregularly portioned Lagrangian Monte Carlo--finite difference” (IPLMCFD), is developed for large eddy simulation (LES) of turbulent combustion via the filtered density function (FDF). This is a hybrid methodology which couples a Monte Carlo FDF simulator with a structured Eulerian finite difference LES solver. The IPLMCFD is scalable to thousands of processors; thus it is suited for simulation of complex reactive flows. The scalability and consistency of the hybrid solver and the realizability and reliability of the generated results are demonstrated via LES of several turbulent flames under both nonpremixed and premixed conditions.

Unsteady Heat Transfer Analysis to Predict Combustor Wall Temperature in Rotating Detonation Engine

Highly non-uniform thermal transients experienced by the containing walls of a Rotating Detonation Combustor (RDC) are investigated in this paper. Unsteady RDC wall heat transfer is investigated by numerical and analytical work to explain the trends and explore the thermal effects of detonation wave propagation over a finite thickness metal wall. Three-dimensional Computational Fluid Dynamics (CFD) simulations are carried out to provide the thermal boundary conditions for the combustor wall. The analytical work is aimed towards development of a simple one-dimensional transient conduction model incorporating unsteady thermal boundary conditions. The numerical model is a 2-D Thermal-Mechanical transient heat conduction model with appropriate convective boundary condition on the interior wall of the combustor. The unsteady heat flux to the walls is mathematically formulated as function of temperature and time to closely resemble the heat flux profiles obtained from CFD simulations and manually repeated to constitute a periodic profile. Both analytical and numerical model predictions have been compared with published experimental data. The ultimate objective of the present study is to develop a simple and reasonable accurate model for faster prediction of metal temperatures and surface heat flux for long duration operation exposed to periodic high frequency harsh detonation environment.

Development and Operation of a Pressurized Optically-Accessible Research Combustor for Simulation Validation and Fuel Variability Studies

The U.S. Department of Energy Turbines Program has established very stringent NOx emissions goals of less than 3 ppmv for future turbine power generation. These future turbine power plants may operate on hydrogen-rich fuels, such as coal-derived synthesis gas (syngas), or pure hydrogen derived from shifting the syngas. Achieving these goals is expected to require improved combustor concepts which may be dramatically different than current combustor designs. Significant and costly experimental testing is usually required to assess new combustor concepts. Ideally, new concepts could be evaluated with numeric simulations to reduce development time and cost. However, current simulation capabilities are not sufficient to reliably capture the effects of fuel variations on flame extinction, emissions levels, and dynamic stability. Furthermore, very little data with controlled boundary conditions are available to check numeric predictions at actual turbine engine conditions, or simply to assess combustor performance without ambiguous boundary conditions. This paper presents a description of the development and operation of an optically-accessible research combustor, which is designed to provide fundamental combustion data at elevated pressure and inlet air temperature, and with precisely determined thermal, acoustic, and flow boundary conditions. The effects of fuel composition variations are investigated by blending of controlled quantities of hydrogen with natural gas. Recent test results — emissions data, dynamics data, and heat losses for hydrogen addition from 0 to 40% by fuel volume at two combustor pressures — and a description of future testing are also presented. The results show that the addition of hydrogen to natural gas in percentages as low as 5% of total fuel volume can significantly decrease the lean extinction limit, and promote stable operation at lower equivalence ratios while promoting lower NOx emissions. Dynamic pressures were measured, but combustion dynamics were not present due to the combustor configuration. The effect of heat losses on flame temperature and emissions were quantified.© 2005 ASME

NOx Reduction by Air-Side vs. Fuel-Side Dilution in Hydrogen Diffusion Flame Combustors

Lean-Direct-Injection (LDI) combustion is being considered at NETL as a means to attain low NOx emissions in a high-hydrogen gas turbine combustor. Integrated Gasification Combined Cycle (IGCC) plant designs can create a high-hydrogen fuel using a water-gas shift reactor and subsequent CO2 separation. The IGCC’s air separation unit produces a volume of N2 roughly equivalent to the volume of H2 in the gasifier product stream, which can be used to help reduce peak flame temperatures and NOx in the diffusion flame combustor. Placement of this diluent in either the air or fuel streams is a matter of practical importance, and has not been studied to date for LDI combustion. The current work discusses how diluent placement affects diffusion flame temperatures, residence times, and stability limits, and their resulting effects on NOx emissions. From a peak flame temperature perspective, greater NOx reduction should be attainable with fuel dilution rather than air or independent dilution in any diffusion flame combustor with excess combustion air, due to the complete utilization of the diluent as a heat sink at the flame front, although the importance of this mechanism is shown to diminish as flow conditions approach stoichiometric proportions. For simple LDI combustor designs, residence time scaling relationships yield a lower NOx production potential for fuel-side dilution due to its smaller flame size, whereas air-dilution yields a larger air entrainment requirement and a subsequently larger flame, with longer residence times and higher thermal NOx generation. For more complex staged-air LDI combustor designs, dilution of the primary combustion air at fuel-rich conditions can result in full utilization of the diluent for reducing the peak flame temperature, while also controlling flame volume and residence time for NOx reduction purposes. However, differential diffusion of hydrogen out of a diluted hydrogen/nitrogen fuel jet can create regions of higher hydrogen content in the immediate vicinity of the fuel injection point than can be attained with dilution of the air stream, leading to increased flame stability. By this mechanism, fuel-side dilution extends the operating envelope to areas with higher velocities in the experimental configurations tested, where faster mixing rates further reduce flame residence times and NOx emissions. Strategies for accurate CFD modeling of LDI combustors’ stability characteristics are also discussed.Copyright

Numerical Investigation of Rotating Detonation Combustion in Annular Chambers

This article presents two dimensional (2D) and three-dimensional (3D) computational analysis of rotating detonation combustion (RDC) in annular chambers using the commercial computational fluid dynamics (CFD) solver ANSYS-Fluent V13. The applicability of ANSYS-Fluent to predict the predominant phenomena taking place in the combustion chamber of a rotating detonation combustor is assessed. Simulations are performed for stoichiometric Hydrogen-Air combustion using two different chemical mechanisms. First, a widely used one-step reaction mechanism that uses mass fraction of the reactant as a progress variable, then a reduced chemical mechanism for H2-Air combustion including NOx chemistry was employed. Time dependent 2D and 3D simulations are carried out by solving Euler equations for compressible flows coupled with chemical reactions. Fluent user defined functions (UDF) were constructed and integrated into the commercial CFD solver in order to model the micro nozzle and slot injection system for fuel and oxidizer, respectively. Predicted pressure and temperature fields and detonation wave velocities are compared for the two reaction mechanisms. Curvature effects on the properties of transverse detonation waves are studied by comparing the 2D and 3D simulations. The effects of diffusion terms on RDC phenomena are assessed by solving full Navier-Stokes equations and comparing the results with those from Euler equations. Computational results are compared with experimentally measured pressure data obtained from the literature. Results show that the detonation wave velocity is over predicted in all the simulations. However, good agreement between computational and experimental data for the pressure field and transverse detonation wave structure proves adequate capabilities of ANSYS-Fluent to predict the main physical characteristics of RDC operation. Finally, various improvements for RDC modeling are postulated, particularly for better prediction of wave velocity.Copyright

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 sub-filter 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 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 CFD code FLUENT. Validation of the approach is conducted by comparing model results to experimental data collected in a lab-scale burner. The burner is based on an enclosed, scaled-down version of the Low Swirl Injector (LSI) 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 (PLIF) of the hydroxyl molecule was collected for the combusting condition as well as velocity field data using Particle Image Velocimetry (PIV). Thermal imaging of the quartz liner surface temperature was also conducted to validate the thermal wall boundary conditions applied in the LES calculations.Copyright

Spontaneous Raman Scattering Measurements and CFD Simulations of Major Species and Temperature in a Turbulent Dilute Hydrogen Diffusion Flame

Dilute hydrogen diffusion flames represent a possible practical configuration for achieving extremely low emission of Nitric Oxides (NOx), and provide a challenging computational problem due to their complex turbulence-chemistry interactions and molecular transport effects. Experimental measurements of spatially resolved major species concentration and temperature were made in a hydrogen dilute diffusion flame using Spontaneous Raman Scattering, providing insight into the physical mechanisms leading to reductions in NOx and data for computational validation. Results from a computational model using Large Eddy Simulation (LES) were able to correctly predict several important flame features, such as the importance of differential diffusion effects in flame anchoring. Depressed flame temperatures were measured, resulting from the highly strained flame environment, providing a supporting explanation for the low NOx levels reported for this type of flame in the literature. The complex mechanisms governing the behavior of this flame make it an interesting target for computational validation.

Influence of Exhaust Gas Recirculation on Combustion Instabilities in CH4 and H2/CH4 Fuel Mixtures

This paper evaluates the impact of two strategies for reducing CO2 emissions on combustion instabilities in lean-premixed combustion. Exhaust gas recirculation can be utilized to increase the concentration of CO2 in the exhaust stream improving the efficiency in the post-combustion separation plant. This coupled with the use of coal derived syngas or hydrogen augmented natural gas can further decrease CO2 levels released into the environment. However, changes in fuel composition have been shown to alter the dynamic response in lean-premixed combustion systems. In this study, a fully premixed, swirl stabilized, atmospheric burner is operated on various blends of H2/CH4 fuels with N2 and CO2 dilution to simulate EGR. Acoustic pressure and velocity, and global heat release measurements were performed at fixed adiabatic flame temperatures to evaluate the impact of fuel composition and dilution on various mechanisms that drive the instabilities.