Radiant Surface Burner performance: A numerical and experimental study

When a gaseous flame is stabilized near the exterior of an inert porous media, the configuration is called a Radiant Surface Burner (RSB). These devices can be used in a variety of applications where radiant heating is advantageous. This work is a numerical and experimental study of the performance and operating characteristics of RSBs.; Fundamental parameters regulating heat release rates, radiant output/efficiency, and exhaust emissions are determined. The numerical model includes multi-step kinetics mechanisms for methane-air combustion. Separate energy equations are used for solid and gas phases allowing for local thermal non-equilibrium. Radiative heat transfer is modeled using the spherical harmonics method with P-3 approximation. The applicability of a few other radiation models such as diffusion, two-flux, and six-flux is also investigated. Radiation from the gaseous combustion products is also accounted for in some representative calculations. It is found that multiple solutions exist for the equation set used in modeling RSBs. The use of the flame speed as an input to the model, instead of the flame location, enables solutions at practical operating conditions.; Experiments are performed on different types of RSBs (fiber-mats and foams) to obtain data on radiative output and exhaust emissions. Numerical predictions are compared to experimental data to confirm the validity of the model. It is found that stable flames could be maintained over a wide range of input heat flux values (200-1200 kW/m2{dollar}sp2).{dollar} Radiant output is maximized at intermediate heat inputs while the radiant efficiency decreases monotonically as input power increases until the porous layer is no longer radiant. NO{dollar}sb{lcub}rm x{rcub}{dollar} emissions are found to increase with input power but remain very low ({dollar}<{dollar}50ppm) throughout the operating range. Model predictions agreed with experimental data within 15% over most of the operating range. Uncertainties in the model are mainly due to the lack of knowledge about the properties of the porous layer, such as the heat transfer coefficient.; The radiative performance of an RSB depends greatly on the properties of the porous material such as optical thickness, porosity, thermal conductivity, scattering albedo, and inter-phase heat transfer coefficient. Optical thickness plays the most dominant role in determining the radiant output from a burner. If optical thickness is low {dollar}(tau<50),{dollar} radiation from a larger portion of the porous material reaches the heat load; thereby, improving the radiant efficiency of the burner. If optical thickness is reduced further {dollar}(tausim 1),{dollar} flames cannot be stabilized near the downstream face of the RSB due to higher radiative preheating. The radiative performance of different burner materials also depends on the combined effect of these properties on the flame location.; The RSB modeling capability is enhanced by including spatial and spectral variation of burner properties. Model predictions show that burner performance can be enhanced greatly by adding a radiator layer downstream of the flame-holder layer. It is shown that thermal and radiative properties of the two layers can be tailored to obtain optimum radiative performance from the burner. Spectral modeling of radiation heat transfer in an RSB demonstrates that changing radiative properties of the porous layer in selective wavelength bands of the electromagnetic spectrum can affect the burner radiative performance significantly. The spectral variation of porous layer properties can be used to design burners for specific heating applications.