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Show - g - Even though a direct comparison or predicted heat extraction efficiencies with test data (Jasionowski et al., 1987) is not possible, because the model parameters have not been reported. However, it is encouraging to note that the measured efficiencies between 37 to 53 percent (arter accounting ror the heat extracted by the cooled grate) are consistent with the predicted results given in Fig.5. Note that the presence or a cooled gas distribution grate at the inlet to the process combustor-heater was not modeled. CONCLUSIONS A mathematical model has been developed to predict the thermal perrormance or a surrace combustor-heater. The model was constructed on the basis or first principles and uses empirical inrormation only to the extent that it is absolutely necessary. Once rully verified using experimental data, the model would be used to optimize the design and quantiry the results. The results or computations show that the opacity or the porous bed and the volumetric convective heat transrer coefficient are important model parameters. The opacity or the bed should be large (......,2) and hAe should be sufficiently large (> 1 X lOS W /m3K) ror good efficiency or heat extraction rrom the system. Larger value ot hAe implies smaller diameter particles and smaller permeability. In turn, this means that the pressure drop across the bed will be greater. Hence, the requirement ror a large convection heat transrer coefficient is incompatible with a desire to have a small pressure drop. There are a large number or independent model parameters such as operating conditions, heater geometry, tube diameter and arrangement, particle diameter, as well as the radiative properties or the porous bed such as the extinction coefficient, single scattering albedo and asymmetry ractor that must be studied to provide guidance to the developers and designers or surrace combustor-heaters. The results presented in the paper should be considered preliminary. The idealizations must be relaxed, and realistic values are required ror some model parameters. This inrormation is not available in the literature, and will need to be determined experimentally. Acknowledgements The authors whish to acknowledge Mr. Mark J. Khinkis, Institute or Gas Technology, ror bringing the problem to the their attention and ror numerous discussions. REFERENCES Beckermann, C., Viskanta, R. and S. Ramadhyani, 1988, "Natural Convection in Vertical Enclosures Containing Simultaneously Fluid and Porous Layers," Journal of Fluid Mechanics, Vol. 186, pp. 257-285. |
