23rd “Journées d’Etudes” of the Belgian Section of the Combustion Institute, Brussels, 27-28 May 2014 Local Evaluation of the Eddy Dissipation Concept (EDC) Constants for turbulence/chemistry interactions in the context of MILD combustion R. Malik1, F. Contino2, A. Parente1 1. Aero-Thermo-Mecanical Departement, Université Libre de Bruxelles, Brussels, Belgium 2. Departement of Mechanical Engineering, Vrije Universiteit Brussel, Brussels, Belgium 1.1 MILD (Flameless) Combustion 1.2 Eddy Dissipation Concept FLAMELESS [1], or MILD [2] or HITAC[3], combustion Flameless • General model for turbulence-chemistry and detailed kinetics • Main assumption: reactions occur in fine structures and surroundings are inert • Fine structures described as a Perfectly Stirred Reactor (PSR) • High combustion efficiencies. • Low pollutant emissions. FLAMELESS requirements and characteristics: Flame • Fine structures mass fraction: • T above the self-ignition temperature of the fuel. • Strong recirculation of exhaust gases • PSR conditions: limited T increase due to combustion • Thermal NOx formation limited, even at the highest air preheating. • Mean residence time in : Figure 1 - Schematic of a computational cell based on EDC model 2. Methodology Computational domain and grid Modification of the EDC Standard values of and according to Magnussen [4] Burner features • Designed to emulate MILD combustion • Fuel: methane/hydrogen mixture equal in volume (1/1) • Insulated and cooled central fuel jet (i.d.=4.25mm) and annulus (i.d.=82mm) • Internal burner used to provide hot combustion products, that are mixed with air to control O2 level ( dilution with coflow) • Air also used to cool the internal burner Modified (local) values of and proposed by the same authors [5] = 2.1377 Grid = 0.4083 • Symmetric burner  2D axi-symmetric grid • 1 m in axial and 120 mm in radial direction Preliminary results: standard constants vs modified global constants at 3 different Re Figure 2 - Cross-section of the experimental burner of the Adelaide JHC (Jet in Hot Coflow) [6] Figure 3 – Computational domain and grid Summary of physical models (Fluent 14.5) Turbulence model Modified k-ε ( = 1.6 for self-similar round jets [7] ) Radiation Model/Spectral Properties Discrete Ordinate/Weighted Sum of Gray Gases Model Turbulence/Chemistry interactions 1. EDC – standard 2. EDC modified – global constants 3. EDC modified – local constants Kinetic mechanisms Figure 4 – Comparison at Re=5k Figure 5 – Comparison at Re=10k Figure 6 – Comparison at Re=20k Relative error on Tmax [%] Re 5k Re 10k Re 20k z[mm] 1. KEE-58: 17 species and 58 reactions [8] 120 Std Mdf 20.89 4.97 3. Results Std Mdf Std Mdf 27.80 13.60 82.18 66.26 Local Global Comparison between standard and modified local constants  Local evaluation of the EDC is done using an User Defined Function (UDF) for Fluent Comparison between standard and modified global constants Temperature distributions Figure 11 – Re distribution Figure 7 – Comparison between standard, modified global constants Figure 10 – Comparison between standard, modified global and local constants Relative error on Tmax [%] Ctau = 0.4083 Ctau = 1.47 Ctau = 1.47 & Cgamma = 1.90 27.80 13.60 5.60 z[mm] 120 Relative error on Tmax [%] Ctau = 0.4083 z[mm] Figure 8- Standard Figure 9– modified – global 120 27.80 Ctau = 1.47 & Cgamma = 1.90 5.60 UDF 8.02 Figure 13 – modified – local (UDF) Figure 12 – Da distribution Conclusions A novel approach is presented based on functional expressions between the EDC constants and the dimensionless flow parameters (the Reynolds and the Damköhler numbers) taking into account the specific features of the MILD combustion regime. These expression are then applied locally (in each cell) and the approach is validated on the JHC burner.. Results showed that the simultaneous and local modification of the time scale constant , Ctau , and the mass fraction constant, Cgamma, leads to improvements in the model predictions at large axial distances from the burner exit, for both the temperature and the species mass fraction. This analysis is confirmed through the calculation of the relative error in the prediction of the maximum temperature. References [1] Wünning JA, Wünning JG., Prog. Energy Combust. Sci. 1997;23:81-94. [2] Cavaliere A, de Joannon M., Prog. Energy Combust. Sci. 2004;30:329-366. [3] Gupta AK., ASME J. Eng. Gas Turbines Power 2004;126:9-19. [4] Gran IR, Magnussen BF., Comb. Sci. Tech 1996;119:191-217. [5] A. Parente, M.R. Malik, F. Contino, A. Cuoci. Modification of the Eddy Dissipation Concept for turbulence/chemistry interactions in the context of MILD combustion. Proc. Comb. Ins, under revision . 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