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Show PARAMETRIC STUDY OF THE THERMAL PERFORMANCE OF A NATURAL GAS-FIRED FURNACE T. H. Song, R. Viskanta School of Mechanical Engineering Purdue University West Lafayette, Indiana, USA Abstract The effects of furnace design and operating parameters on the thermal performance of a natural gas-fired, high temperature, industrial furnaces have been studied by numerically simulating the flow, combustion and radiative as well as convective heat transfer. The k- E model is used to predict turbulent flow, and Bray's model for premixed combustion is employed. Radiation heat transfer is calculated by accounting for the effects of turbulence/radiation interaction and spectral characteristics of the combustion products . The solution of the model equations is obtained using the SIMPLER algorithm. The results of calculations show that the thermal efficiency of furnace is improved by the increase in the adiabatic flame temperature, increase in the sink emissivity, decrease of sink temperature, oblique firing, and improvement in thermal insulation of refractory walls . INTRODUCTION One of the important parameters in assessing the thermal performance of a furnace is the heat flux distribution to its thermal load. Methods based on fundamental principles are now available using numerical techniques that permit predictions to be made. These prediction procedures require empirical inputs to describe turbulent transport, chemical kinetics, thermodynamic, transport and radiative properties of the combustion products. The purpose of this paper is to study the effects of some of the most important furnace design and operating parameters and thereby obtain improved insight of determining the optimum design/operating conditions. To this end, a mathematical model for a high temperature, natural gas-fired, industrial furnace has been constructed. The model has been exercised, and the effects of many design and operating parameters have been determined on the local heat flux distribution at the load and the thermal performance of the furnace. 135 MATHEMATICAL FURNACE MODEL AND THE SOLUTION PROCEDURE A schematic diagram of the furnace is shown in Fig. l. The furnace extends indefinitely in the direction perpendicular to the plane of the figure . The transport processes are steady and two-dimensional. Premixed methane-air enters the combustion space horizontally. The inlet and outlet ends of the furnace are considered to be radiatively adiabatic. The conditions are typical of large industrial furnaces such as are used, for example, in glass melting and metal heating. The momentum, species and energy conservation equati~ns are formulated using k2" E turbulence model , and Bray's combustion model is used for a premixed flame. Radiati0I) heat transfer is modeled using the Pr approximation 4 ' with a modified sum-ofgray- gases model of Hottel . Effects of buoyancy and turbulence/radiation interaction are accounted for in the analysis. The mat~ematical models used are described in detail elsewhere and need not be repeated here. In summary, a total of 28 partial differential and algebraic equations are solved numerically. No-slip boundary conditions were imposed at the walls for all velocity components . An impermeable boundary condition is employed for the mass fraction of fuel. The temperature of the load (sink) at the bottom of the furnace is assumed to be given, and the temperature boundary condition at the remaining walls is obtained by equating the convective and net radiative fluxes to the heat loss through the refractory wall via an overall heat transfer coefficient between the wall surface on the inside of the furnace and the ambient. The inlet and outlet openings of the furnace are assumed to be adiabatic for all transport quantities except mass flow . The transport equations for the physical quantities, either vector or scalar, were solved using a twodimensional transporl equation solver based on the SIMPLER algorithm . Modification of the computer code was made to solve simultaneously the anisotropic |
