Multidimensional modelling and calculation of combustion in porous media

AUTOR(ES)
DATA DE PUBLICAÇÃO

2005

RESUMO

In contrast with household heating systems based on open combustion flames, burners based on premixed combustion in inert porous media are characterized by higher thermal efficiency and high turndown ratio allied with compacity of the units. Moreover, porous burners, where a flame is anchored inside a porous matrix, are associated with low pollutants formation, namely CO and NOx emissions, and with the possibility to burn leaner mixtures and fuels with lower heating values. Numerical modelling plays an important role in the development of porous burners, providing guidance for future designs. In the present dissertation, numerical models developed to support the study the operation of an innovative porous burner are described and discussed. The studied burner consists of a thin layer of a 90% porosity foam of silicon carbide preceded by a finely perforated plate made of an insulating material and with an equivalent porosity of 4%. It is being built to operate fuelled by a mixture of air and a vaporized biofuel to be supplied by a newly developed cool flame vaporizer. From these constructional features, the need arose to study some of the processes taking place owing to the presence of a highly porous matrix in a flow. Initially, the dispersion of a scalar in the flow within ceramic foams is considered. This study makes use of one- and two-dimensional numerical models and shows that the model by Koch and Brady (1985) for dispersion of a scalar in packed beds of spheres is able to reproduce the dispersion of a tracer in a highly porous ceramic foam when the influence of the shortness of the porous sample is taken into account and the pore diameter is used as characteristic length. Based on these results, an up-scaling procedure is applied to model the effective conductivity of the perforated plate as a porous medium with very low porosity. Additionally, an investigation conducted on the stability of a cold flow at the inlet interface of a ceramic foam inserted in a pipe is presented with a comparison of two of the available models for turbulent flow in porous media. Making use of two-dimensional models of flow within a porous slab inserted in a pipe and downstream of a sudden expansion, it is shown that while the model by Antohe and Lage (1997) leads to relaminarization of the flow in a short distance inside the foam, the model by Pedras and de Lemos (2001) predicts that turbulence is either damped or enhanced inside the porous matrix, depending on the turbulence condition at the inlet of the porous insert. Completing the analysis of flow and flame stability at the interface regions of a porous burner, the stabilization of the flame at the interface between a perforated plate and a highly porous ceramic foam is numerically studied by means of a three-dimensional model. It is shown that the diameter of the holes of the perforated plate are small enough as to prevent flashback to occur. To overcome the problems introduced in the modelling of a burner composed by two layers of very different porosities, a three-dimensional model of the combustion chamber was developed. The physical-mathematical model comprises the three-dimensional balances of mass, momentum, and energy for both the solid and the fluid phase, along with the transport equations for the chemical species considered in the mixture. Radiation heat transfer within the foam is accounted for and detailed chemical kinetic models for the oxidation of methane and heptane are considered, allowing for the prediction of the pollutants emissions. This model was implemented numerically by means of computational fluid dynamics techniques. Predictions of the burner operating fuelled with methane over a range of operating conditions are compared with experimental results obtained with a burner prototype to evaluate the model. The performance of the burner is numerically studied using heptane as model fuel. The influence of the operating conditions on the stabilization and structure of the flame front, temperature and concentration profiles, energy balance and pollutants emissions is discussed. Among other results, it is shown that the flame front stabilizes deep within the SiC foam layer, close to the interface with the perforated plate for the design range of operating conditions, and that the predicted CO and NOx emissions are within the limits observed for heating devices of the same kind fuelled with other liquid fuels.

ASSUNTO(S)

fenomenos de transporte meios porosos combustion in porous media mecânica dos fluidos computacional combustão

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