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Show 1 © 2014 ANSYS, Inc. August 22, 2014 ANSYS Confidential Modeling CO and Estimating Combustion Efficiency with Flamelet-Generated Manifolds in Flares Anchal Jatale ANSYS Inc. AFRC 2014 INDUSTRIAL COMBUSTION SYMPOSIUM Sept 7-10, 2014 Stefano Orsino ANSYS Inc. Philip Smith University of Utah 2 © 2014 ANSYS, Inc. August 22, 2014 ANSYS Confidential • 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 is a most common measure of effectiveness of flaring systems • Operating conditions have significant effect on flare flame stability, efficiency and emissions • Important to predict this effect to control emissions ( stringent regulation/taxes) Motivation Most important thing to ensure a proper mixing of fuel and air 3 © 2014 ANSYS, Inc. August 22, 2014 ANSYS Confidential • 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. Why little known about Combustion Efficiency ? 4 © 2014 ANSYS, Inc. August 22, 2014 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 5 © 2014 ANSYS, Inc. August 22, 2014 ANSYS Confidential CFD Simulations • Industrial flare simulation can provide important information on combustion efficiency and pollutant emissions, • It also provides operational parameter sensitivities for design or operation that can not be measured. • It is cost effective and one can test loads of what if scenarios which are not possible to measure in reality. 6 © 2014 ANSYS, Inc. August 22, 2014 ANSYS Confidential • 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 CFD Simulations 7 © 2014 ANSYS, Inc. August 22, 2014 ANSYS Confidential • Various turbulence models which will be able to resolves all the important characteristics of the flare flames - 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) Fluent as CFD tool 8 © 2014 ANSYS, Inc. August 22, 2014 ANSYS Confidential • Fluent's premixed 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 flames, flame lift, ignition, quenching and extinction • Diffusion FGM model is present - For predominantly diffusion turbulent flames - Calculate detailed chemistry a-priori in opposed flow laminar diffusion flames and parameterize by f and c Combustion Model: Diffusion Flamelet Generated Manifold (Diffusion FGM) 9 © 2014 ANSYS, Inc. August 22, 2014 ANSYS Confidential Combustion Model: Diffusion Flamelet Generated Manifold (Diffusion FGM) • The traditional Diffusion FLAMELET based Partially Premixed Model defaults to an Equilibrium solution for a pure or pre-dominantly premixed case. Hence cannot predict super-equilibrium species like CO and OH accurately. • The Diffusion FGM model, solves the 1D premixed system (with full convection & diffusion) in the Progress Variable Space and constructs a species-temperature manifold based on the Premixed 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. 10 © 2014 ANSYS, Inc. August 22, 2014 ANSYS Confidential Diffusion FGM example • LES of Sandia flame D 11 © 2014 ANSYS, Inc. August 22, 2014 ANSYS Confidential Cabra CH4 Lifted Flame Details are available at: http://www.me.berkeley.edu/c al/vcb/data/VCMAData.html Proceedings of ASME 2013 Gas Turbine India Conference December 5-6, 2013, Bangalore, Karnataka, India SLFM PFGM Flame lift-off • PFGM Captures the flame lift-off - Measured lift-off distance = 35Dn - Predicted lift-off distance = 33Dn 12 © 2014 ANSYS, Inc. August 22, 2014 ANSYS Confidential Case Study: Wind Tunnel Flare Cross Wind Flue Gas 13 © 2014 ANSYS, Inc. August 22, 2014 ANSYS Confidential Mesh ~8.5 million cells 14 © 2014 ANSYS, Inc. August 22, 2014 ANSYS Confidential Turbulence Model: LES Subgrid-scale Model: WALE Combustion Model: Partially Premixed diffusion FGM Turbulence-Chemistry Interaction: Finite-Rate Variance Method: Algebraic Models used 15 © 2014 ANSYS, Inc. August 22, 2014 ANSYS Confidential Fuel Stream @300 K Flow Rate: 0.00556 Kg/s Mole % : CH4 - 95.35 CO2 - 0. 62 C2H6 - 2.1 C3H8 - 0.13 N2 - 1.8 Oxidizer @ 287 K Mole % : N2- 78.9 O2- 21.1 Inlet and Boundary Conditions 16 © 2014 ANSYS, Inc. August 22, 2014 ANSYS Confidential • First Establish mixture fraction variance field by running the case for 10 time steps ( time-step size=0.05 s) • Whole domain was patched with progress variable =1 • The simulation was ran till 2 second of run time. This give enough time to get a developed flow • Time step size was reduced to 0.0005 sec and simulation was run for 2 more second of run time. • Data was sampled from t=4 sec to t= 8 sec Simulation Strategy 17 © 2014 ANSYS, Inc. August 22, 2014 ANSYS Confidential Flare at different crosswind velocity 6 m/s 12 m/s Buoyancy effects after certain distance at lower crosswind velocity 18 © 2014 ANSYS, Inc. August 22, 2014 ANSYS Confidential Case Study: Wind Tunnel Flare A part of the Temperature profile at lower crosswind in the wind tunnel 19 © 2014 ANSYS, Inc. August 22, 2014 ANSYS Confidential CFD Results Vs Experimental Values 20 © 2014 ANSYS, Inc. August 22, 2014 ANSYS Confidential Combustion Efficiency With increase in crosswind velocity efficiency goes down 21 © 2014 ANSYS, Inc. August 22, 2014 ANSYS Confidential Effect of Turbulence Models on Combustion Efficiency Unsteady RANS predict almost complete combustion 22 © 2014 ANSYS, Inc. August 22, 2014 ANSYS Confidential Flare structure wrt to turbulence model LES U-RANS SAS 23 © 2014 ANSYS, Inc. August 22, 2014 ANSYS Confidential CO Mass Percentage 24 © 2014 ANSYS, Inc. August 22, 2014 ANSYS Confidential Effect of turbulence model on other species CH4 CO2 O2 25 © 2014 ANSYS, Inc. August 22, 2014 ANSYS Confidential Diffusion Flamelet Generated Manifold combustion model along with LES turbulent model provides a good combination to tackle the difficult combustion systems like flares Summary 26 © 2014 ANSYS, Inc. August 22, 2014 ANSYS Confidential |