No abstract

No abstract

No abstract

No abstract

Three gray models for the radiative properties of gases were examined for their usage in oxy-combustion simulations of a full scale boiler with flue gas recycle. Fully coupled computational fluid dynamic (CFD) simulations of a full scale boiler were carried out employing the weighted-sum-of-gray-gases model (WSGGM) at air burn, dry-recycle and wet-recycle conditions. The resulting thermal and composition fields were then frozen and the radiative properties of the gaseous media recomputed employing the Exponential Wide Band Model (EWBM) and correlations for total emissivities of gas mixtures. It is shown that when high CO2/H2O ratios were encountered within the boiler such as in dry-recycle scenarios, employing emissivity correlations developed for purely CO2 media within the models can result in incorrect gas properties. The errors associated with this can be significant when there are large pockets within the furnace where the gas radiation dominates the particle radiation.

No abstract

A C o m p a ra tiv e C o m p u ta tio n a l F lu id D y n a m ic s S tudy on F la m e le t-G e n e ra te d M a n ifo ld and S te a d y L a m in a r F la m e le t M o d e lin g fo r T u rb u le n t F la m e s The laminar flamelet model (LFM) (Peters, 1986, "Laminar Diffusion Flamelet Models in Non-Premixed Combustion," Prog. Energy Combust. Sci., 10, pp. 319-339; Peters, "Laminar Flamelet Concepts in Turbulent Combustion," Proc. Combust. Inst., 21, pp. 1231-1250 represents the turbulent flame brush using statistical averaging of laminar flamelets whose structure is not affected by turbulence. The chemical nonequilibrium effects considered in this model are due to local turbulent straining only. In contrast, the flamelet-generated manifold (FGM) (van Oijen and de Goey, 2000, "Modeling of Pre mixed Laminar Flames Using Flamelet-Generated Manifolds," Combust. Sci. Technol., 161, pp. 113-137) model considers that the scalar evolution; the realized trajectories on the thermochemical manifold in a turbulent flame are approximated by the scalar evolu tion similar to that in a laminar flame. This model does not involve any assumption on flame structure. Therefore, it can be successfully used to model ignition, slow chemistry, and quenching effects far away from the equilibrium. In FGM, ID premixed flamelets are solved in reaction-progress space rather than physical space. This helps better solution convergence for the flamelets over the entire mixture fraction range, especially with large kinetic mechanisms at the flammability limits (ansys flu en t 14.5 Theory Guide Help Document, http:llwww.ansys.com). In the present work, a systematic comparative study o f the FGM model with the LFM for four different turbulent diffusion!premixed flames is presented. The first flame considered in this work is methane-air flame with dilution air at the downstream. The second and third flames considered are jet flames in a coaxial flow of hot combustion products from a lean premixed flame called Cabra lifted H2 and CH4 flames (Cabra, et al., 2002, "Simultaneous Laser Raman-Rayleigh-LIF Measurements and Numerical Modeling Results of a Lifted Turbulent H2IN2 Jet Flame in a Vitiated Coflow," Proc. Combust. Inst., 29(2), pp. 1881-1888; Lifted CH4/AirJet Flame in a Viti ated Coflow, http://www.me.berkeley.edu/cal/vcb/data/VCMAData.htmI) where the react ing flow associated with the central jet exhibits similar chemical kinetics, heat transfer, and molecular transport as recirculation burners without the complex recirculating fluid mechanics. The fourth flame considered is a Sandia flame D (Barlow et al., 2005, "Piloted Methane/Air Jet Flames: Scalar Structure and Transport Effects," Combust. Flame, 143, pp. 433-449), a piloted methane-air jet flame. It is observed that the simula tion results predicted by the FGM model are more physical and accurate compared to the LFM in all the flames presented in this work. The autoignition-controlled flame lift off is also captured well in the cases of lifted flames using the FGM model. • Expt --...

In the present work, two equation soot models proposed by Moss-Brookes (MB) and Moss-Brookes-Hall (MBH), available in ANSYS FLUENT14.5, are used to study the soot formation in a turbulent kerosene-air flame. The model constants in the original works of MB and MBH model were primarily tuned for the methane-air or other lower hydrocarbon flames. In this work, the emphasis has been given on the applicability of these models in modeling the soot formation in heavy hydrocarbon fuels. The current work is primarily focused on the parametric study of the various modeling constants for calculating the soot inception and oxidation rates. A parametric study is performed to calculate the soot inception rates by considering different soot precursors like C2H2, C2H4, C6H6 and C6H5. Steady laminar flamelet approach with a detailed chemical reaction mechanism (Jet_SurF_2.0), is used for modeling gas phase combustion. The current numerical predictions are compared with experimental results of Young et al. [1] and earlier published numerical results of Wen et al. [2]. The study is further extended to understand the role of chemical reaction mechanism on soot predictions considering detailed versus reduced (JP10revC) chemical mechanisms.