In this project, an advanced computational software tool was developed for the design of low emission combustion systems required for Vision 21 clean energy plants. Vision 21 combustion systems, such as combustors for gas turbines, combustors for indirect fired cycles, furnaces and sequestrian-ready combustion systems, will require innovative low emission designs and low development costs if Vision 21 goals are to be realized. The simulation tool will greatly reduce the number of experimental tests; this is especially desirable for gas turbine combustor design since the cost of the high pressure testing is extremely costly. In addition, the software will stimulate new ideas, will provide the capability of assessing and adapting low-emission combustors to alternate fuels, and will greatly reduce the development time cycle of combustion systems.The revolutionary combustion simulation software is able to accurately simulate the highly transient nature of gaseous-fueled (e.g. natural gas, low BTU syngas, hydrogen, biogas etc.) turbulent combustion and assess innovative concepts needed for Vision 21 plants. In addition, the software is capable of analyzing liquid-fueled combustion systems since that capability was developed under a concurrent Air Force Small Business Innovative Research (SBIR) program. The complex physics of the reacting flow field are captured using 3D Large Eddy Simulation (LES) methods, in which large scale transient motion is resolved by time-accurate numerics, while the small scale motion is modeled using advanced subgrid turbulence and chemistry closures. In this way, LES combustion simulations can model many physical aspects that, until now, were impossible to predict with 3D steady-state Reynolds Averaged Navier-Stokes (RANS) analysis, i.e. very low NO x emissions, combustion instability (coupling of unsteady heat and acoustics), lean blowout, flashback, autoignition, etc. LES methods are becoming more and more practical by linking together tens to hundreds of PCs and performing parallel computations with fine grids (millions of cells). Such simulations, performed in a few weeks or less, provide a very cost-effective complement to experimental testing. In 5 years, these same calculations can be performed in 24 hours or less due to the expected increase of computing power and improved numerical techniques.This project was a four-year program. During the first year, the project included the development and implementation of improved chemistry (reduced GRI mechanism), subgrid turbulence (localized dynamic), and subgrid combustion-turbulence interaction (Linear Eddy) models into the CFD-ACE+ code. University expertise (Georgia Tech and University of California, Berkeley) was utilized to help develop and implement these advanced submodels into the unstructured, parallel CFD flow solver, CFD-ACE+. Efficient numerical algorithms that rely on in situ look-up tables or artificial neural networks were implemented for chemistry calculations. In the second year, the combustion LES software was ev...
The new combustion Large Eddy Simulation (LES) code developed previously in this project was modified in two ways: 1) an improved inlet boundary condition for turbulence was implemented and 2) a new formulation for the variance (fluctuation) of progress/mixture fraction variables used in the assumed pdf turbulence-combustion interaction subgrid model was implemented. These code modifications were performed under a separate Navy Phase II SBIR and are briefly discussed in this DOE quarterly report for completeness. After these modifications were tested and verified, one SIMVAL case (φ inj =0.7) was run and predictions compared to SIMVAL data. The new LES calculation showed that the flame anchored at the correct location on the premix tube centerbody (in previous cases, the flame was incorrectly anchored just downstream of the swirler). The inlet turbulence effects resulted in more finescale structure being captured by LES, more-in-line with what we expected in LES calculations. The predicted NO x emissions were much higher than measurements, but it was realized that the heat losses throughout the combustion section and exhaust duct were lower than measured. The case will be rerun with the correct heat loss. The predicted pressure dynamics agreed well with the extrapolated measurements, although the case analyzed (φ inj =0.7) was not the best case to assess combustion dynamics. Additional cases are being run at φ inj of 0.6 and 0.65 to assess our ability to correctly predict pressure dynamics. 8321/13
Work in this quarter focused on the continued running of two SIMVAL cases: φ (equivalence ratio) of 1) 0.55 and 2) 0.625. Comparisons were made between RANS and LES predictions for the φ of 0.625 case. The LES calculation showed a different flow pattern in the combustor compared to the RANS calculation, in particular the combustor recirculation flow pattern on the centerline is dramatically different. To demonstrate that the LES solution is accurate (and the RANS is not), non-reacting cases based on the Lilley experiment (Lilley, 1985) were run. Results from the Lilley cases verified that the LES calculations more closely match experimental velocity measurements for highly swirled, turbulent flows with a downstream constriction. In particular, RANS predictions show a strong centerline recirculation zone in the combustor, while LES predictions show positive axial velocity on the centerline, and an annular recirculation zone around the centerline.Animation files were also created this quarter, so as to better demonstrate the LES predictions.iii 8321/16
Previous LES calculations for one SIMVAL case (φ inj = 0.7) have repeatedly shown that the heat transfer/heat losses were not correctly captured, resulting in predicted NO x emissions being much higher than measurements. In a current Navy Phase I SBIR project, we are studying ways to improve the prediction of heat transfer/heat loss in LES calculations. To improve our understanding of heat transfer at walls, a series of thermal channel flow cases were analyzed and compared to DNS predictions. When trying to fully resolve the boundary layer, it was found that the grid size in all directions (i.e. ∆x, ∆y, and ∆z) must be smaller than the size of eddies formed in the boundary layer. Thus, the cell aspect ratio at the wall needs to typically be less that 25 when the boundary layer is being resolved. In previous SIMVAL LES calculations, this cell aspect ratio requirement was not followed, and thus the heat transfer was less than desired. For this quarter, the grid was adjusted, and the SIMVAL case was rerun. The LES results show that vortices are now formed in the exhaust duct near the walls, resulting in more heat loss from the exhaust duct flow and lower NO x predictions (more in line with the measurements).iii 8321/14
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