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Show EFFECT OF SINK TEMPERATURE - The sink temperature can vary significantly depending on the material, heating function and process time. A number of different temperatures were assumed and the total heat fluxes at the sink predicted are shown in Fig. 6 , and the furnace performance is summarized in Table 6. Both the local and overall heat transfer rate, whether radiative or convective, decrease significanlty as the sink temperature is increased. Convective heat transfer represents a higher fraction of the total heat transfer rate as the sink temperature is lowered. The heat loss decreases as the sink temperature is lowered . Even though the thermal efficiency is smaller for the higher sink temperature, the thermal effectiveness is higher for the higher sink temperature. This is because for the lower sink temperature the burned gas requires longer residence time to cool down as a result of a larger temperature difference between the adiabatic flame temperature and the sink temperature . The ' thermal efficiency decreases rapidly, while the thermal effectiveness increases close to unity as the range of sink temperature increases further . In the range of sink temperatures considered, the thermal efficiency decreases about 2 percent as the sink temperature increases 100 K . EFFECT OF INCOMING TEMPERATURE OF FRESH MIXTURE - The variations in the temperature of the fresh mixture have been considered and are summarized in Table 7. The heat input to the furnace , both sensible and heat of combustion, is maintained at 2.5 MW. As the preheated air temperature is increased, the adiabatic flame temperature is also increased . A s the mass flow rate of gas decreases, the residence time becomes longer. The effect is very significant as can be seen from Table 7. A bout a 2 percent increase in thermal efficiency results from 200 K rise in the inlet air temperature. The convective heat transfer rate and the heat loss from the furnace are increased slightly with the increase of the inlet air temperature. EFFECT OF EXCESS AIR - The air/fuel ratio is maintained as close as possible to the stoichiometric so far as the combustion of fuel is complete . The excess air ratio has a significant effect which can be seen from Fig. 7 . The thermal efficiency increases 0 .36% as the excess air ratio decreases 1 %. Not only the thermal efficiency but also the thermal effectiveness increase because of the longer residence time (or lower mass flow rate) for smaller excess air . CONCLUSIONS In the range of the parameters considered for the reference furnace, the largest improvements in the thermal efficiency are achieved by an increase in gas volume, oblique firing, increase in incoming air temperature, increase of sink emissivity, and decrease in sink temperature . Moderate improvement in 138 thermal efficiency is achieved b* better insulation of the refractory . Enomoto et a1. concluded that the most important factors affecting furnace performance are the thermal load, excess air , process temperature, flamel10ad temperature difference and sink emissivity. These findings have been confirmed by numerical experiments . The effect of firing angle 9is also consistent with the results of Pai et a1.. The parametric results presented in this study may be used for guiding furnace design and operating conditions. REFERENCES 1. Launder, B.E., and Spalding, D.B . , The Numerical Computation of Turbulent Flows, Computer Methods in Applied Mechanics and Engineering 3, 269-289 (1974). 2. Bray, K.N .C ., Turbulent Flows with Premixed Reactants, in Topics in Applied Physics, eds. P .A. Libby and F .A . Williams, Springer-Verlag, New York (1980), pp . 115-183. 3. Menguc, M.P., and Viskanta, R., Radiative Transfer in Three-Dimensional Rectangular Enclosures Containing Inbhomogeneous, A nistropically Scattering Media, J . Quant. Spectrosc. Radiat. Transfer 38,533-549 (1985). 4. Song, T.H . , and Viskanta, R., Interaction of Radiation with Turbulence: Application to Combustion System, J. Thermophysics and Heat Transfer (in press). 5. Song, T.H., Simulation of Flow, Combustion and Heat Transfer in Two-Dimensional Natural GasFired Industrial Furnaces, Ph .D . Thesis, Purdue University, May 1986. 6. Patankar, S. V. , Numerical Heat Transfer and Fluid Flow , McGraw-Hill, New York (1980)'. 7. Dzyuzer, V.Ya., Kokarev, N.r., Kut'in, V.B., Budovkin, V.Yu., Dzaseev, S.S. , and Duda, A.r., Methods of Intensifying Heat Transfer from Flame to Melt, Glass and Ceramics 38, 373-376 (1981). 8. Enomoto, H ., Tsai, Y.W., and Essenhigh, R .H., Heat Transfer in a Continuous Model Furnace: A Comparison of Theory and Experiment, A SME Paper No . 75-HT-5 . 9 . Pai, B.R ., Michelfelder, S ., and Spalding, D.B ., Prediction of Furnace Heat Transfer with a Three-Dimensional Mathematical Model, Int. J. Heat Mass Transfer 23 , 571-580 (1978). 10. Lebedeev , V.r., and Sokolov, V.A., Multiple H eat Exchange in a Model Furnace with Direct Heating, Glass and Ceramics 37,67-69 (1980). |