Influence of Application of Hottels Zonal Model and Six-Flux Model of Thermal Radiation on Numerical Simulations Results of Pulverized Coal Fired Furnace

Difference of results of numerical simulation of pulverized coal fired furnace when mathematical models contain various radiation models has been described in the paper. Two sets of numerical simulations of pulverized coal fired furnace of 210 MWe power boiler have been performed. One numerical simulation has contained Hottels zonal model, whereas the other numerical simulation has contained six-flux model. Other details of numerical simulations have been identical. The influence of radiation models has been examined through comparison of selected variables (gas-phase temperature, oxygen concentration, and absorbed radiative heat rate of surface zones of rear and right furnace walls), selected global parameters of furnace operation (total absorbed heat rate by all furnace walls and furnace exit gas-phase temperature). Computation time has been compared as well. Spatially distributed variables have been compared through maximal local differences and mean differences. Maximal local difference of gas-phase temperature has been 8.44%. Maximal local difference of absorbed radiative heat rate of the surface zones has been almost 80.0%. Difference of global parameters of furnace operation has been expressed in percents of value obtained by mathematical model containing Hottels zonal model and has not been bigger than 7.0%. Computation time for calculation of 1000 iterations has been approximately the same. Comparison with other radiation models is necessary for assessment of differences

Simulation of free turbulent particle-laden jet using Reynolds-stress gas turbulence model

Free two-phase flows occur in many practical applications, such as sprays or particle drying and combustion. This paper deals with mathematical modelling of a free turbulent two-phase jet. A steady, axisymmetric, dilute, monodisperse, particle-laden, turbulent jet injected into a still environment, has been considered. The model treats the gas-phase from an Eulerian standpoint and the motion of particles from a Lagrangian one. Closure of the system of time averaged transport equations has been accomplished by using a Reynolds-stress turbulence model. The particles-fluid interaction has been considered by the PSI-Cell concept. Both the effect of interphase slip and the effect of particle dispersion have been taken into account. Results of the model have been compared with experimental data for axial and radial profiles of gas-phase mean and turbulent quantities and solid-phase mean velocity. Accuracy of model predictions of particle-laden free jet time averaged characteristics as well as turbulence correlation coefficients have been improved. The modelling of observed turbulence anisotropy levels and correlation coefficients need to be carried out with special care. The model has provided insight into the turbulence structure and aerodynamic characteristics of the particle-laden free jet. A brief sensitivity study has been performed as well, indicating that the specification of inlet boundary conditions exerts pronounced effects on predictions. In this paper, the study refers to the effect of the turbulence kinetic energy dissipation rate, while the other inlet boundary conditions have been applied with respect to the referent measurements. (c) 2006 Elsevier Inc. All rights reserved

A numerical study of pulverized coal ignition by means of plasma torches in air-coal dust mixture ducts of utility boiler furnaces

Paper presents selected results of numerical simulation of processes in air-coal dust mixture duct of pulverized coal utility boiler furnace with plasma-system for pulverized coal ignition and combustion stabilization. Application of the system in utility boiler furnaces promises to achieve important savings compared with the use of heavy oil burners. Plasma torches are built in air-coal dust mixture ducts between coal mills and burners. Calculations have been performed for one of rectangular air-coal dust mixture ducts with two opposite plasma torches, used in 210 MWe utility boiler firing pulverized Serbian lignite. The simulations are based on a three-dimensional mathematical model of mass, momentum and heat transfer in reacting turbulent gas-particle flow, specially developed for the purpose. Characteristics of processes in the duct are analyzed in the paper, with respect to the numerical results. The plasma-system thermal effect is discussed as well, regarding corresponding savings of liquid fuel. It has been emphasized that numerical simulation of the processes can be applied in optimization of pulverized coal ignition and combustion stabilization and enables efficient and cost-effective scaling-up procedure from laboratory to industrial scale. (C) 2007 Elsevier Ltd. All rights reserved

Numerical analysis of discrete phase induced effects on a gas flow in a turbulent two-phase free jet

The paper addresses numerical simulation of turbulent two-phase flow in a long vertical tube and turbulent two-phase free jet formed at the tube outlet, analyzing agreement between the numerical results and the results of corresponding experimental investigation carried out earlier. In the numerical analyses conducted, gas phase was modeled as an air flow (having a mass flow-rate in the range of 1.25-4.00 g/s), while the sand particles of two different sizes (0.25-0.30 and 0.8-1.0 mm) represented a discrete phase (particle to gas mass flow ratio of 0.72-4.08) in the two-phase flow considered. Gas-particle interaction was analyzed based on the gas velocities in the particle-laden two-phase flow and the particle-free gas flow, calculated and measured at various locations along the longitudinal axis and radius of the jet. Mathematical model of continuous phase flow was developed based on the single phase flow models, with certain corrections introduced to account for the effects of particles in the flow. In the simulation model developed, the flow analyzed was modeled as a two-phase mixture, with Eulerian simulation used to account for the gas phase behavior and the Lagrangian simulation modeling the particle movement in the two-phase flow considered. In order to appropriately close the system of time-averaged equations, k-epsilon turbulent model, deemed the most reliable, was used. Phase coupling i.e. fluid-particle interaction was modeled using the PSI-CELL concept. The results obtained via numerical simulation have shown a good agreement with the experimental data acquired. (C) 2011 Elsevier Ltd. All rights reserved

Three-dimensional modeling of utility boiler pulverized coal tangentially fired furnace

This paper presents selected results of numerical simulations of processes in utility boiler pulverized coal tangentially fired dry-bottom furnace. The simulations have been performed by specially developed comprehensive mathematical model. The main features of the model are a three-dimensional geometry, k-epsilon gas turbulence model, Eulerian-Lagrangian approach, particles-to-turbulence interaction, diffusion model of particle dispersion, six-flux method for radiation modeling and pulverized coal combustion model based on the global particle kinetics and experimentally obtained kinetic parameters. Five operation regimes of 210 MW, boiler furnace burning Serbian lignites, with different grinding fineness of coal and coal quality, have been simulated. The model successfully predicts the influence of the parameters on the furnace processes and operation characteristics (like the flue gas temperature and the furnace walls radiation fluxes). The predicted flame temperature and percentage combustibles in bottom ash are in good agreement with the measurements. The developed model can find different applications, both in research and practice. (c) 2006 Elsevier Ltd. All rights reserved

A numerical study of a utility boiler tangentially-fired furnace under different operating conditions

For prediction of complex processes in two-phase turbulent reactive flows within large-scale boiler furnaces firing pulverized coal, a comprehensive 3D differential mathematical model and CFD computer code have been developed in-house. The model incorporates trade-offs between submodels sophistication and computational practicality. An easy-to-use interface for introducing input data and grid generation has been build within the code. The main purpose of the paper is to present numerical predictions of processes in the case-study furnace under different operating conditions, obtained by the developed model. The paper also provides information on characteristics and evaluation of the model, with a grid refinement study and comparisons with comprehensive data. The effects of different operating conditions on the processes in the case-study furnace are correctly predicted, demonstrating the ability of the developed model to perform parametric studies. (c) 2008 Elsevier Ltd. All rights reserved

Experimental and numerical investigation of premixed acetylene flame

In this paper, the mean velocity, turbulence intensity and temperature profiles in different cross-sections of premixed acetylene flame are given. A mathematical model for prediction of velocity, temperature and concentration fields of axisymmetric free premixed turbulent flame is presented in this paper. A second-order closure for turbulent reacting flows is used. Special attentions is paid to model behavior with the respect to the prediction correlation coefficients of turbulent diffusion of the scalar components. Conditional and unconditional statistics of the LDA signals were performed using jet and/or air seed. Compared to commonly used unconditional statistics, conditional statistics of velocity fluctuations can give us more data about intensity of turbulent mixing in the flame. (c) 2006 Elsevier Ltd. All rights reserved

Numerical Prediction of Pulverized Coal Flame in Utility Boiler Furnaces

In optimization of a utility boiler furnace operation, special attention is given to the flame geometry and position. As an illustration of possibilities for application of mathematical prediction and numerical experiment in efficient optimization of the flame, the paper presents selected results of simulations of processes in pulverized coal tangentially Fired furnace of Kostolac-B 350 MW electric boiler unit, To analyze the furnace working under different conditions, a differential 3D mathematical model of two-phase turbulent reactive flow with heat and mass transfer and corresponding computer code have been developed. Using the model and the code, previously carefully verified and validated against field measurements, an extensive numerical study has been performed to investigate the dependence of the furnace flame characteristics oil different operating conditions, including distribution of the coal, air flow rates, and particle size classes over the burner tiers, as well as the quality and grinding fineness of coal and the operation scheme of the coal mills. The numerical predictions of the flame characteristics enable a specific tool for optimization of the boiler unit with respect to efficiency and ecology

Radiative heat exchange inside the pulverized lignite fired furnace for the gray radiative properties with thermal equilibrium between phases

The objective of the research was to find if an agreement of the results of a numerical investigation with experimental data could be achieved considering the two-phase medium in thermal equilibrium. Influence of the gray radiative properties on the radiative heat exchange inside pulverized lignite fired furnaces was investigated using the computational fluid dynamics (CFD) code based on a comprehensive mathematical model of the process. Radiative heat exchange was calculated using Hottels zonal model. Heat transfer rates and wall fluxes increased for small values of the total extinction coefficient, K-t LT 0.2 m(-1); decreased for large values of K-t, K-t GT 2.0 m(-1); and were maximal for moderate values of K-t, 0.2 LT K-t LT 2.0 m(-1). Heat transfer rates and wall fluxes decreased with the increase of the scattering albedo, though the decrease was considerable only for omega GT 0.5. Agreement with the experimental data was obtained for the moderate values of the K-t and for scattering albedo 0.1 LT omega LT 0.5. (C) 2014 Elsevier Masson SAS. All rights reserved

Numerical investigation of processes in the lignite-fired furnace when simple gray gas and weighted sum of gray gases models are used

Comparison of the numerical investigation results was carried out when the simple gray gas (SGG) and weighted sum of gray gases (WSGG) models are used to model the radiative properties of the gas phase inside the lignite fired furnaces. Comprehensive mathematical model of the tangentially fired furnace by pulverized lignite was made. Gas radiative properties were modeled by the SGG and WSGG models. Radiative heat transfer was modeled by the zonal model. Gas-phase variables and absorbed wall fluxes were compared on the basis of the relative differences that were determined for all control volumes and surface zones. Average relative differences of the gas-phase temperatures were about 1.0%. Average relative differences of the absorbed wall fluxes were from 2.0% to 5.0%. Absorbed wall fluxes determined by the SGG model were bigger than those determined by the WSGG model. Differences of the heat transfer rates of the absorbed radiation through the furnace walls were expressed in percents of heat transfer rates determined by the SGG model and were similar to the average relative differences of absorbed wall fluxes. Results justify application of the SGG model in comprehensive mathematical models of lignite-fired furnaces. (C) 2012 Elsevier Ltd. All rights reserved