Nevin Selçuk

Evaluation of discrete ordinates method for radiative transfer in rectangular furnaces

Abstract The discrete ordinates method (DOM) and discrete transfer method (DTM) were evaluated from the viewpoints of both predictive accuracy and computational economy by comparing their predictions with exact solutions available from a box-shaped enclosure problem with steep temperature gradients. Comparative testing shows that the S 4 approximation produces better accuracy in radiative energy source term than in flux density in three orders of magnitude less CPU time than that required by the DTM. The S 4 approximation can therefore be recommended to be used in conjunction with CFD codes.

MOL solution of DOM for transient radiative transfer in 3-D scattering media

Abstract A methodology based on the method of lines solution of discrete ordinates method for solution of the 3-D transient radiative transfer equation is introduced. The method is applied to the prediction of transient and steady state transmittances in a cubical enclosure containing purely scattering medium and validated against Monte Carlo solutions from the literature. The flexibility of the method for implementation of linear spatial differencing schemes, flux limiters and weighted essentially non-oscillatory methods is demonstrated. Van Leer flux limiter is found to provide stable, accurate and efficient solutions.

Investigation of ash deposition in a pilot-scale fluidized bed combustor co-firing biomass with lignite

This study presents the results from investigation of ash deposition characteristics of a high ash and sulfur content lignite co-fired with three types of biomass (olive residue, 49 wt%; hazelnut shell, 42 wt%; and cotton residue, 41 wt%) in 0.3 MW(t) Middle East Technical University (METU) Atmospheric Bubbling Fluidized Bed Combustion (ABFBC) Test Rig. Deposit samples were collected on an air-cooled probe at a temperature of 500 degrees C. Samples were analyzed by SEM/EDX and XRD methods. The results reveal that co-firing lignite with olive residue, hazelnut shell and cotton residue show low deposition rates. High concentrations of silicon, calcium, sulfur, iron, and aluminum were found in deposit samples. No chlorine was detected in deposits. Calcium sulfate and potassium sulfate were detected as major and minor components of the deposits, respectively. High sulfur and alumina-silicate content of lignite resulted in formation of alkali sulfates instead of alkali chlorides. Therefore, fuel blends under consideration can be denoted to have low-fouling propensity.

THE METHOD OF LINES SOLUTION OF THE DISCRETE ORDINATES METHOD FOR RADIATIVE HEAT TRANSFER IN ENCLOSURES

A radiation code based on the method of lines (MOL) solution of the discrete ordinates method (DOM) for transient three-dimensional radiative heat transfer in rectangular enclosures for use in conjunction with a computational fluid dynamics (CFD) code based on the same approach was developed. Assessment of the predictive accuracy of the code by benchmarking its steady-state solutions against exact solutions on one- and three-dimensional test problems shows that the MOL solution of the DOM provides accurate and computationally efficient solutions for radiative heat fluxes and source terms and can be used with confidence in conjunction with CFD codes for transient problems.

THE METHOD OF LINES SOLUTION OF THE DISCRETE ORDINATES METHOD FOR RADIATIVE HEAT TRANSFER IN ENCLOSURES CONTAINING SCATTERING MEDIA

A radiation code based on the method of lines (MOL) solution of the discrete ordinates method (DOM) for three-dimensional radiative heat transfer in rectangular enclosures containing gray, absorbing, emitting, isotropically and anisotropically scattering media was developed. Predictive accuracy of the code was evaluated by applying the code to 1-D and 3-,D problems containing scattering media and benchmarking its predictions against exact solutions, the zone method, and Monte Carlo solutions. Favorable comparisons reveal that MOL solution of the DOM provides accurate solutions for modeling radiative heat transfer in 3-D rectangular enclosures containing purely scattering or absorbing, emitting, and scattering media with isotropic or anisotropic scattering properties.

Performance of method of lines solution of discrete ordinates method in the freeboard of a bubbling fluidized bed combustor

Abstract Predictive accuracy of the method of lines (MOL) solution of discrete ordinates method (DOM) is assessed by applying it to the prediction of incident radiative fluxes on the freeboard walls of a bubbling fluidized bed combustor, and comparing its predictions with those of the zone method and measurements. Freeboard is treated as a three-dimensional rectangular enclosure containing gray, absorbing, emitting and isotropically scattering medium. Radiative properties of the medium are calculated by using Leckners correlations for gas and Mie theory for particles. Data for application and validation are generated from METU 0.3 MW t atmospheric bubbling fluidized bed combustor test rig burning lignite in its own ash. Comparisons reveal that MOL solution of DOM provides accurate and computationally efficient solutions for wall fluxes in the freeboard of fluidized bed combustors containing particle laden combustion gases. Parametric studies are also carried out to analyze the sensitivity of predicted heat flux profiles to the presence of particles, particle load and anisotropic scattering.

The method of lines solution of discrete ordinates method for radiative heat transfer in cylindrical enclosures

Abstract A radiation code based on method of lines solution of discrete ordinates method for radiative heat transfer in axisymmetric cylindrical enclosures containing absorbing–emitting medium was developed and tested for predictive accuracy by applying it to (i) test problems with black and grey walls (ii) a gas turbine combustor simulator enclosing a non-homogeneous absorbing–emitting medium and benchmarking its steady-state predictions against exact solutions and measurements. Comparisons show that it provides accurate solutions for radiative heat fluxes and can be used with confidence in conjunction with CFD codes based on the same approach.

Method-of-lines solution of time-dependent two-dimensional Navier-Stokes equations

SUMMARY A novel approach to the development of a code for the solution of the time-dependent two-dimensional NavierStokes equations is described. The code involves coupling between the method of lines (MOL) for the solution of partial differential equations and a parabolic algorithm which removes the necessity of iterative solution on pressure and solution of a Poisson-type equation for the pressure. The code is applied to a test problem involving the solution of transient laminar flow in a short pipe for an incompressible Newtonian fluid. Comparisons show that the MOL solutions are in good agreement with the previously reported values. The proposed method described in this paper demonstrates the ease with which the NavierStokes equations can be solved in an accurate manner using sophisticated numerical algorithms for the solution of ordinary differential equations (ODEs).

Evaluation of flux models for radiative transfer in rectangular furnaces

Abstract Three flux-type models for three-dimensional radiative heat transfer were applied to the prediction of the radiative flux density and the source term of a box-shaped enclosure problem based on data reported previously on a large-scale experimental furnace with steep temperature gradients. The models, which are a six-term discrete ordinate model and two Schuster-Schwarzschild type six-flux models, were evaluated from the viewpoints of both predictive accuracy and computational economy by comparing their predictions with exact solutions produced previously. The comparison showed that the six-flux model based on angular subdivisions related to the enclosure geometry produces more accurate results and is computationally less expensive than the other two models.

Trace elements partitioning during co-firing biomass with lignite in a pilot-scale fluidized bed combustor

This study describes the partitioning of 18 trace elements (As, Ba, Cd, Co, Cr, Cu, Li, Mn, Mo, Ni, P, Pb, Sb, Se, Sn, Tl, V, Zn) and 9 major and minor elements (Al, Ca, Fe, K, Mg, Na, S, Si, Ti) during co-firing of olive residue, hazelnut shell and cotton residue with high sulfur and ash content lignite in 0.3 MW(t) Middle East Technical University (METU) Atmospheric Bubbling Fluidized Bed Combustor (ABFBC) test rig with limestone addition. Concentrations of trace elements in coal, biomass, limestone, bottom ash, cyclone ash and filter ash were determined by inductively coupled plasma optical emission and mass spectroscopy (ICP-OES and ICP-MS). Partitioning of major and minor elements are influenced by the ash split between the bottom ash and fly ash and that the major proportion of most of the trace elements (As, Ba, Co, Cr, Cu, Li, Mn, Mo, Ni, Pb, Tl, V and Zn) are recovered in fly ash when firing lignite only. Co-firing lignite with biomass enhances partitioning of these elements to fly ash. Co-firing also shifts the partitioning of Cd, P, Sb and Sn from bottom to fly ash.