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Show Application of CFD to model Air-assisted Industrial Flares under Low- Btu, Low-flow Rate conditions AFRC 2015 INDUSTRIAL COMBUSTION SYMPOSIUM Sept 9-11, 2015 Anchal Jatale ANSYS Inc. 1 © 2014 ANSYS, Inc. August 13, 2015 Stefano Orsino ANSYS Inc. ANSYS Confidential Motivation • Important control practice for destroying unwanted hydrocarbons and other combustible material released in the atmosphere • Complete Combustion is the goal • CH4 etc. are 20-25 times more effective in trapping heat compare to CO2 • Combustion efficiency and destruction efficiency are common measure of effectiveness of flaring systems • Operating conditions have significant effect on flare flame stability, efficiency and emissions • Important to predict these effects to control emissions ( stringent regulation/taxes) 2 © 2014 ANSYS, Inc. August 13, 2015 Most important thing is to ensure a proper mixing of fuel and air ANSYS Confidential Why Little Known about Combustion Efficiency ? • No single or few parameters characterizes the combustion behavior of flare flames. - Complexity of nonlinear mixing, reaction and heat transfer • Motivates the need to accurately measure it from operating flares so that the effect of different designs and operations can be quantified. However, this same complexity, makes such measurements difficult. 3 © 2014 ANSYS, Inc. August 13, 2015 ANSYS Confidential Measurement of Combustion Efficiency • Difficult to measure as needs simultaneous knowledge of the composition and velocity to obtain mass flux surrounding the flare - Measurable combustion species gets diluted in the surrounding atmosphere - Effect of high temperature and radiant heat on test equipment - Irregular nature of flames due to external winds and intrinsic turbulence - Lack of suitable sampling locations due to flare/flame height especially for elevated flares 4 © 2014 ANSYS, Inc. August 13, 2015 ANSYS Confidential CFD Simulations • Industrial flare simulation can provide important information on combustion efficiency, pollutant emissions, Flame structures etc. • It also provides operational parameter sensitivities for design or operation that can not be measured. • It is cost effective and one can test lots of what if scenarios which are not possible to measure in reality. 5 © 2014 ANSYS, Inc. August 13, 2015 ANSYS Confidential CFD Simulations 6 • In the past we modeled and performed CFD on wind tunnel flares or high momentum flares (represent plant startup, shutdown or emergency) • During normal conditions, flares operate in stand-by mode to handle emissions and normal plant vent gases. • Test case: TCEQ 2010 flare study ( low-Btu, low-flow rate flare tests) • Various combinations of fuel BTU and flow rates were tested for air assisted and steam assisted flares and state-of-art measurement techniques were used © 2014 ANSYS, Inc. August 13, 2015 ANSYS Confidential CFD Simulations Challenges • Difficult to compute accurately through traditional computational fluid dynamics (CFD) simulation tools that are based on Reynolds-Averaged Navier-Stokes (RANS) approaches codes - Wide range in length and timescales in the mixing and reacting processes - The large-scale mixing due to vortical coherent structures in these flames is not readily reduced to steady-state CFD calculations with RANS - By time-averaging the equations, unsteady information such as instantaneous mixing and flame shape cannot be captured - Capturing ignition and extinction phenomena 7 © 2014 ANSYS, Inc. August 13, 2015 ANSYS Confidential CFD Tool : Fluent • Various turbulence models which will be able to resolves most of the important characteristics of the flare flames - Reynolds-Averaged Navier-Stokes (RANS) - Large Eddy Simulation (LES) - Detached Eddy Simulations (DES): Unsteady RANS + LES - Scale- Adaptive Simulation (SAS): Improved Unsteady RANS - Embedded Large Eddy Simulation ( ELES) • Various Combustion models are available to capture the chemistry properly ( Non premixed , premixed and partially premixed models) 8 © 2014 ANSYS, Inc. August 13, 2015 ANSYS Confidential Turbulent Models SAS DES ELES 9 © 2014 ANSYS, Inc. August 13, 2015 ANSYS Confidential Fundamental Basis of DES and SAS Models DES and SAS are both Hybrid Models: RANS near Walls and Scale Resolving ability (behaves like LES) away from it. SAS switches between steady and unsteady modes. Eddy Viscosity in Scale Resolving Situation <<< Eddy Viscosity in RANS (Steady Mode). Hence less Damping. More scale production. Basis of DES/SAS: Cut down Turbulent Eddy Viscosity on the Fly to induce turbulent scale formation Eddy Viscosity DES: Reduce k which reduces Eddy Viscosity and triggers scale formation SAS: Increase ω which reduces Eddy Viscosity and triggers scale formation 10 © 2014 ANSYS, Inc. August 13, 2015 ANSYS Confidential Combustion Model: Diffusion Flamelet Generated Manifold (Diffusion FGM) • Fluent's FGM assumes that the thermo-chemical trajectories in a turbulent flame are similar to the trajectories in a laminar flame ‐ No assumption of flame structure ‐ Can model premixed/non premixed flames, flame lift, ignition, quenching and extinction FLAMELET Parameterized by "c" and "f". Reaction Zone: Considered in FGM Model . "c" and "f" Beta distributed. • Diffusion FGM is calculated by generating the Steady Diffusion Laminar Flamelets over a range of strain rates till extinction • Flamelet species field is converted into a Reaction Progress variable ‘c' 11 © 2014 ANSYS, Inc. August 13, 2015 ANSYS Confidential Combustion Model: Diffusion Flamelet Generated Manifold (Diffusion FGM) 12 • The traditional Diffusion FLAMELET based model defaults to an Equilibrium solution. Hence cannot predict super-equilibrium species like CO and OH accurately. • The Diffusion FGM model, solves the 1D system (with full convection & diffusion) in the Progress Variable Space and constructs a speciestemperature manifold based on the FLAMELET solution parameterized by the progress variable. • Because equilibrium assumption is not made, it is expected that species like CO and OH can be predicted more accurately. © 2014 ANSYS, Inc. August 13, 2015 ANSYS Confidential Test Case: Air assisted TCEQ flare tests Case # Jet (m/s) Pilot (m/s) Cross Wind (m/s) Air Assist (m/s) Air Ast./Cr. wind 1 0.22 0.19 7.12 1.77 0.249 2 0.59 0.19 4.61 3.00 0.651 3 0.59 0.19 5.31 7.36 1.38 4 0.23 0.19 1.12 5.10 4.55 Case # Propylene Natural Gas Nitrogen Excess Air factor 1 117.8 11.86 221.21 5.91 181.23 18.77 702.55 6.51 181.23 18.76 704.18 15.94 71.26 7.55 271.37 28.01 2 3 4 Fuel Composi tion (lb/hr) Excess Air factor= ratio of actual amount of air-assist and the stoichiometric amount of air 13 © 2014 ANSYS, Inc. August 13, 2015 ANSYS Confidential Test Case: Air assisted TCEQ flare Computational Domain crosswind 10 m Air Assist Vent Gas Pilot 14 © 2014 ANSYS, Inc. August 13, 2015 ANSYS Confidential Mesh and Modeling Approach 15 • ~3 million mesh with refinement concentrated in area of interest, was used • Two combustion models were used : steady diffusion flamelet and Diffusion FGM • SAS was used for modeling turbulence • Effects of crosswind, air assist velocities and combustion model were studied on flame structure and combustion efficiencies © 2014 ANSYS, Inc. August 13, 2015 ANSYS Confidential Flame Shape for different crosswind and air assist conditions Case 1 Case 2 Case 4 Case 3 16 © 2014 ANSYS, Inc. August 13, 2015 ANSYS Confidential Effect of Combustion Model on Flame Structure SDFL vs FGM Case 1 Case 2 FGM model shows the presence of incomplete combustion and lower max temperature than SDFL. FGM can capture ignition and extinction phenomena properly. 17 © 2014 ANSYS, Inc. August 13, 2015 ANSYS Confidential Effect of Combustion Model on Flame Structure SDFL vs FGM Case 3 Case 4 FGM model shows the presence of incomplete combustion and lower max temperature than SDFL. FGM can capture ignition and extinction phenomena properly. 18 © 2014 ANSYS, Inc. August 13, 2015 ANSYS Confidential FGM results for Case 2@XZ plane in the domain Combustion Efficiency 19 © 2014 ANSYS, Inc. August 13, 2015 CO mass fraction ANSYS Confidential Combustion efficiency @ XZ plane in domain. Case 1 20 © 2014 ANSYS, Inc. August 13, 2015 Case 3 ANSYS Confidential Case 4 Combustion Efficiencies SDFL suitable for modeling combustion with relatively fast chemistry, hence non equilibrium effects are not captured and we see a complete combustion all the time 21 © 2014 ANSYS, Inc. August 13, 2015 ANSYS Confidential Benefits of using SAS and FGM 22 • Better Prediction of Combustion efficiency and species like CO and NOx concentration • SAS give much better structures and flow patterns compared to unsteady RANS • Though the results are not as good as LES but SAS saves lots of computational time. For current cases this difference was almost 50% © 2014 ANSYS, Inc. August 13, 2015 ANSYS Confidential Summary 23 • Diffusion Flamelet Generated Manifold combustion model along with SAS turbulent model was predicting CE better than the steady diffusion flamelet models • The effect of crosswind velocity and air assist velocity on efficiency and flame structure was investigated © 2014 ANSYS, Inc. August 13, 2015 ANSYS Confidential 24 © 2014 ANSYS, Inc. August 13, 2015 ANSYS Confidential Appendix 25 © 2014 ANSYS, Inc. August 13, 2015 ANSYS Confidential Diffusion FGM example • LES of Sandia flame D 26 © 2014 ANSYS, Inc. August 13, 2015 ANSYS Confidential Cabra CH4 Lifted Flame • PFGM Captures the flame lift-off - Measured lift-off distance = 35Dn - Predicted lift-off distance = 33Dn PFGM Details are available at: http://www.me.berkeley.edu/c al/vcb/data/VCMAData.html SLFM Flame lift-off Proceedings of ASME 2013 Gas Turbine India Conference December 5-6, 2013, Bangalore, Karnataka, India 27 © 2014 ANSYS, Inc. August 13, 2015 ANSYS Confidential |
