An investigation of combustion in porous media

An investigation has been carried out to examine the performance of a cylindrical porous medium burner fired with natural gas under both premixed and non-premixed conditions. With a lamellar corrugated structure medium, the burner radiation efficiency reached a maximum of 37%. NOx emissions were reduced to the level below 29 ppm (by volume), and the lean operation equivalence ratio extended to 0.74. The effect of swirl on the burner performance has been investigated through motorizing the burner tube by a variable speed electric motor. The enclosed flame entirely within the porous media blew out because of flame pulsation, while the free flame responded to rotation by extending the firing range. By switching to axial flow operation, the flame was rotated by four vanes attached to the motor shaft upstream of the porous medium. The no-swirl operation of the axial flow burner revealed a lean equivalence ratio of 0.51 and associated low NOx emissions of 1 ppm concentration. The enclosed flame inside the porous medium sustained the rotation and the combusting flow was swirled ahead of the porous zone without interruption. The enhancement in the burner performance relative to the case without swirl was confirmed by an increase in the burner radiation intensity. The CO concentrations were as low as 50 ppm. The swirl influence on a non-premixed flame was examined by confining the burner cylinder by a square duct to permit air flow across the swirled fuel flow at a location downstream from the vane region. The results revealed that under strong swirl strengths, the flame is shortened, and a significant amount of the gas enthalpy is converted into thermal radiation with a relative enhancement up to 5.8 times. The gap between the swirling flow outlet and the porous medium was optimized to allow enough mixing ahead of the porous medium. A numerical model was developed on the basis of one-dimensional flow, single-step reaction, and radiative diffusion approximation. The swirl effect was simulated by varying the volumetric heat transfer coefficient between the gas and solid phases. The results confirmed that the porous medium sustains higher combustible flow rates than those stabilized by free flames. Moreover, the solid temperatures were found to increase with the convective heat transfer coefficient to such an extent that identical two phase temperatures were obtained at sufficiently high heat transfer coefficients.