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Show Burners Flares Incinerators Combustion Systems www.cd-adapco.com CFD analysis of a ground flare system: Wind and fence effects on air supply behavior and thermal radiation AFRC Fall 2011 Meeting Mike Henneke (CD-adapco) Miguel Matos (Zeeco) Introduction • Thermal radiation emitted from ground flare installations is a safety consideration • The fence material must be selected to tolerate the high heat fluxes • The fence has an influence on air supply to the burners and can cause air circulation patterns within the fenced area Page 2 Presentation overview • This presentation reviews recent CFD analysis of the flow and combustion in a ground flare installation under a specific wind condition • We also show CFD analysis of a single burner and compare the CFD results to measured heat fluxes Page 3 Single burner analysis - CFD mesh/domain 70 m 50 m 40 m This mesh contains 3.9 million cells Page 4 CFD mesh/domain Page 5 CFD mesh/domain Mesh in vicinity of burner - we do no include the piping to the burner in these simulations Page 6 Modeling details • For the calculations shown, we use the eddy-breakup model with single step methane and propane oxidation. • We simulate the fuels using mixtures of methane and propane - no other hydrocarbons are included • Thermal radiation is simulated using the weighted sum of gray gases model. The discrete ordinates method is used with an S8 discretization. This means that radiative transport includes 80 distinct ordinate directions. • In the single flare simulation, these ordinate directions are apparent on the ground. In the flare field simulation, the incident radiation from all of the firing flares seems to ‘smooth out' these artifacts Page 7 Results • This simulation was set up to be similar to the high-flow test point • The first run (w/ pilot which is not included in CFD analysis) had a fuel temperature of 105F and a fuel pressure of 110 psig • The fuel flow was 645,000 scfh of Tulsa natural gas • The wind speed was 5-8 mph at grade. In our model we use 5mph • The measured radiation level was 420 Btu/hr/ft2 • TNG is 0.58 SG and 910 Btu/scf Page 8 Results - temperature Wind The green dots to the left of the flame are added to the image for reference. They are spaced at 1 meter increments Page 9 Photograph 10 Movie from test 11 Results - Velocity vectors in immediate vicinity of burner Wind Page 12 Results - Reverse streamlines from lower portion of the flame Wind This slide shows ‘reverse streamlines' from the lower portion of the flame. These are shown here primarily for comparison to the full field CFD results presented later. Page 13 Results - Irradiation Wind The ‘flower' pattern is an artifact of the discrete ordinates radiation model. This model generally provides accurate integrated results. The testing results were taken at 100 feet downwind from the flare. We take the average of our results at 100 feet from the flare to make a predicted incident radiation of Page 14 Comparison of model-computed irradiation with measured value • In the model, we compute an average radiation at the 30.48 meter distance of 855 Btu/hr/ft2 (2,697 W/m2) • We then subtract the ‘background level' which includes ambient radiation. In our model the boundaries emit radiation at 311K (80F) so the background radiation is 168 Btu/hr/ft2 (530 W/m2) • The incident radiation due to the flare flame is 687 Btu/hr/ft2 • This prediction is higher than the measured value (420 Btu/hr/ft2). Zeeco further provided a predicted value of 610 Btu/hr/ft2 • This is an area that needs more work. Our models work very well in fired heater applications where radiation is the dominant model of heat transfer. The model does not include atmospheric attenuation (due to humid air) but we expect such attenuation is negligible. Page 15 Full flare field simulation • In these calculations, the in-service burners (spares are not included in the model) are laid out according to the drawings • The fence is included in the analysis • The flow rate of gas through the HP burners is 32,219 lbm/hr with gas MW of 17.27 at a temperature of -44 C • The flow rate of gas through the LP burners is 1,630 lbm/hr with gas MW of 19.44 at a temperature of 39 C • Burner drilling patterns are as per specified by Zeeco Page 16 Geometry In this plot, the LP burners are colored green (72 LP burners) and the HP burners are colored yellow (112 HP burners) Page 17 Mesh • This mesh is 32.4 million cells. With this cell count, the model is coarser than the single burner simulation in the vicinity of the burners. • Domain: • 250 meters in the x-direction (see the triad in the lower left of the image on the previous slide) • 130 meters in elevation (y) • 250 meters in z • The fenced-in area is about 60 meters in the z-direction and 40 meters in the x-direction Page 18 Mesh Page 19 Mesh Closeup view of mesh in vicinity of the burners Page 20 Setup • The model was set up similar to the single-burner simulations • The HP fuel used is 96% methane/4% propane • The LP fuel used is 88% methane/12% propane • The eddy breakup model is used for methane and propane oxidation (single-step) • The discrete ordinates model (S8 discretization) is used for radiation transport with the weighted sum of gray gases model for absorption coefficient of the radiating gases • This model was run on a 64 core cluster for about 6 days and ran 9000 iterations. We started the run with a coarser radiation approximation (S2) to run it faster and then refined the radiation simulation as the calculation proceeded Page 21 Temperature animation (press Shift-F5 to enter presentation mode 5 mph wind direction HP burners LP burners Page 22 Velocity vectors animation 5 mph wind direction Page 23 Velocity vectors animation 5 mph wind direction Page 24 Irradiation on the ground and fence Page 25 Reverse streamlines for full flare field Wind Page 26 Detailed analysis of flow under, over, and through fence Page 27 Conclusions • Single burner simulation and full flare field simulations have been presented • Both HP and LP burners are included in the full field simulation • Irradiation results are shown as well as temperature and velocity vector results Page 28 |
