Influence of the downstream conditions on the performance of a two-layer porous burner

Abstract

Bilayered porous burners are characterized by being constituted of two porous layers of different porosity. The flame front is stabilized within the higher porosity layer, which is preceded by another layer with pores of smaller diameter. An innovative two-layer porous burner that integrates a newly designed household heating system is numerically studied. The bilayered porous burner studied in this work integrates an innovative household heating system, also composed of a cool flame vaporizer and a multi-jet boiler. As a consequence of this assembly, it operates facing a high temperature wall, which affects the radiative heat losses of the burner. In order to assess the influence of this high temperature wall downstream of the burner outlet section on the performance of the burner, a numerical study of the flow and heat transfer within the combustion chamber was conducted by means of a three-dimensional model of a unit cell representative of the geometry of the two-layered porous burner. The model includes the Navier-Stokes equations, the gas and solid energy balances and the transport equations for chemical species and was computationally implemented using the finite volume/finite differences method. Radiation heat transfer in the solid matrices and local thermal non-equilibrium are accounted for. Additionally, detailed reaction mechanisms are applied to model the combustion process, allowing for the prediction of the pollutants formation. Calculations of the burner over a broad range of operating conditions were conducted, considering either the existence or not of a high temperature wall downstream of the burner outlet. A comparison of the performance of the burner showed that operation facing the high temperature wall: i) promotes the stabilization of the flame front deeper inside the higher porosity ceramic foam, close to the interface of the porous layers for all the range of operating conditions and ii) results in higher peak temperatures and NO and CO emissions.