EMERGING TECHNOLOGY × Brigham Young University × Fall 1995
One of ACERC's primary objectives is to develop a computational tool for predicting lean, premixed combustion in gas turbines, with particular emphasis on predicting emissions of CO and ultra-low NOx. LEPRECON is a critical element of this capability. The purpose of LEPRECON is to perform finite-rate chemistry calculations, with chemistry/turbulence interactions. Given the mean velocity of a turbulent flowfield and its local turbulence properties, density and other flowfield thermochemical properties of interest are predicted.
LEPRECON is under development and is expected to be completed in its first version by about the Fall of 1997. It uses the velocity-scalar, probability density function (PDF) transport method (Correa and Pope, 1992; Pope, 1985), and is being developed from the foundation of PDF2DS, a commercially-available two-dimensional, non-reacting, velocity-scalar PDF code. It will use multi-step, reduced chemistry with table lookup and/or an intrinsic low-dimensional manifold method for calculating reaction rates. Current plans are to implement LEPRECON in two-dimensional form in 96-PCGC-3 for axisymmetric calculations and in 97-COSMO/GT for practical gas turbine simulations.
VELOCITY-SCALAR PDF METHOD
PDF methods for turbulent flows have been described by Pope (1985). The velocity-scalar PDF is the joint PDF of velocity and any number of scalars, such as a mixing scalar (e.g., mixture fraction) and reaction scalars. A transport equation for the joint PDF can be derived from the governing equations for turbulent reacting flow. Its major advantage is that reaction effects can be included exactly, i.e., no time-averaging is required as is the case in the standard approach of Reynolds decomposition and time-averaging of the species continuity equations. Therefore, arbitrarily complex chemistry can be included in the turbulent flowfield solution without approximation, limited only by computer resources. The Monte Carlo PDF solution method is preferred to the finite difference approach, because the computational work scales linearly with the number of variables (i.e., dimensionality of the PDF) rather than exponentially.
The PDF method is equally applicable to premixed and non-premixed combustion. As input, it requires the local turbulence length scale and is, therefore, not a standalone, multidimensional flow model for turbulent combustion. In practice, it is often coupled with a separate flow code. The flow code provides the velocity field with turbulence properties, and the PDF code calculates the density field and other thermochemical properties of interest. The PDF code also calculates the velocity field, which is kept consistent with the velocity field and turbulence properties calculated by the flow code. The two codes are converged sequentially, in a loop, with the PDF code providing the density field to the flow code. This so-called "hybrid approach" to predicting turbulent, reacting flowfields by incorporating Monte Carlo PDF calculations in a finite-volume mean flow solver is described by Correa and Pope (1992).
From a practical standpoint, the PDF method is currently limited to approximately five scalars. Reduced chemistry and table-lookup are used for efficiency. LEPRECON will focus on lean, premixed combustion of natural gas in gas turbines. Appropriate mechanisms (e.g., Seshadri and Peters, 1990) will be used. Intrinsic, low-dimensional manifold (ILDM) methods (e.g., Maas and Pope, 1994) are also being considered.
Lean, premixed gas tubines are targeted for single-digit NOx. Thermal NOx is effectively eliminated. Nitrous oxide NOx and (possibly) prompt NOx are the two potential contributers at low-NOx conditions, and LEPRECON is expected to include both of these mechanisms.
LEPRECON is a submodel designed to be incorporated in other turbulent flow codes. It will be available in COSMO/GT, and is not anticipated being available separately. It represents the state-of-the art in turbulent combustion modeling, and its use is recommended for those technical specialists with expertise in that area.
PRE- AND POST-PROCESSING
No requirement for pre-processing is anticipated. Post-processing capabilities for viewing model results will be provided with COSMO/GT.
Every effort is being made to maintain ANSI FORTRAN 77 coding standards. As such, the code will be portable to several different platforms.
AVAILABILITY AND USER SUPPORT
General release of LEPRECON incorporated in COSMO/GT is planned for about December, 1997.
A user's manual explaining code theory and use will be available. Training workshops will also be conducted.
For more information about code availability and licensing fees, contact:
PDF2DS has been incorporated as a submodel in PCGC-3 and COSMO and non-reacting flow calculations have been performed. Lookup tables for methane combustion chemistry have been developed and are being incorporated and tested. The submodel will be tested for premixed combustion, extended to include swirling flow, validated with data from the Brigham Young University (BYU) laboratory-scale gs turbine combustor (LSGTC) described in a separate document, and extended to three-dimensional flow. A working version for premixed combustion is expected by the end of 1995.
This work is being sponsored by the U.S. Department of Energy/ Morgantown Energy Technology Center under the Advanced Gas Turbines Systems Research (AGTSR) program (Subcontract No. 93-01-SR014) administered through the South Carolina Energy Research and Development Center and by subcontract with Advanced Fuel Research, Inc. (Contract No. DE-AC21-93MC30040).
Contributors to the submodel include Dr. B. Scott Brewster, Steve Cannon (Ph.D. candidate), and Dr. Fanli Meng. Dr. L. Douglas Smoot is the project director.