A surface flame holder apparatus is configured to be disposed substantially within the bore of a burner. The flame holder apparatus supports and maintains a flame, resulting from the combustion of a mixture of gaseous fuel and combustion air, on an outer surface of the surface flame holder, the mixture of gaseous fuel and combustion air being established at a location remote from the surface flame holder. The surface flame holder apparatus includes means for establishing a region of recirculation of combustion gases, in the vicinity of the surface flame holder apparatus, for facilitating maintenance of the flame, in the surface of the surface flame holder apparatus. Means are provided for delivering a mixture of gaseous fuel and combustion air to the surface flame holder apparatus.

The pilot flame, which typically, will be kept burning even once the main burner flame is ignited, also resides in the primary flame region. A low-emissions secondary flame region is created by igniting a substantially uniform mixture of fuel, air and flue-gas which is swirling around the primary lean-premixed flame. The fuel-flue-gas mixture is generated by injecting fuel inward in passages which pass through the burner, thereby entraining flue gases. The surface flame holder apparatus is formed, in part, as a substantially cylindrical flame holder member, having a first diameter, operably configured to receive therethrough the mixture of gaseous fuel and combustion air, for ignition thereof for establishment of the flame on the outer surface thereof. The substantially cylindrical flame holder member may be formed, in one case as a cylindrical metal tube having a plurality of perforations therethrough. In another case, the substantially cylindrical flame holder member may be formed as a cylindrical tube formed from a substantially porous flame-resistant material. The pilot burner apparatus also includes an imperforate disc member, having a second diameter, operably arranged on the substantially cylindrical flame holder member, at a position substantially downstream relative to the substantially cylindrical flame holder member.

In this manner, an extremely uniform and well-organized flame is produced, with no significant regions of lean high temperature conditions. The nitrogen oxides emissions are therefore extremely low. Low flame temperatures, caused by extensive recirculation of flue gas into the flame, are responsible for the overall very low emissions. The means for establishing a region of recirculation of combustion gases comprises preferably an annular disc, having a third diameter, and a central aperture therein, in which the central aperture is associated with the means for delivering the mixture of gaseous fuel and combustion air. The substantially cylindrical flame holder member may be arranged on the annular disc, at a position substantially upstream relative to the substantially cylindrical flame holder, so that the substantially cylindrical flame member is between the imperforate disc and the annular disc, in a sandwiched configuration. Preferably, the third diameter of the annular disc is substantially greater than the second diameter of the substantially cylindrical member, so that gases passing the periphery of the annular disc, will be prompted to move in a toroidal path downstream of the periphery of the annular disc, in turn, prompting the gases to circulate in the vicinity of the outer surface of the substantially cylindrical flame holder member. Preferably, the first diameter of the imperforate disc is less than the third diameter of the annular disc.

The lean mixture entering this recirculation region is ignited by the surface flame and then allows the flame to propagate outward to the main circulation region. A large bluff-body type recirculation region is generated downstream of the pilot-flame-holder assembly. These flow field interactions enable this pilot configuration to stably ignite and maintain extremely lean and flue-gas containing main burner flames. The pilot burner apparatus also may include means for maintaining the imperforate disc, the substantially cylindrical flame holder member and the annular disc in the sandwiched configuration, while substantially precluding the exertion of undesired compressive forces on the substantially cylindrical flame holder apparatus, which may be a plurality of spacer members operably disposed between the annular disc and the imperforate disc for maintaining the annular disc and the imperforate disc at a minimum desired spacing from one another. Preferably, the means for delivering a mixture of gaseous fuel and combustion air to the surface flame holder apparatus further comprises a tubular member, operably connecting the annular disc to a source of mixed gaseous fuel and combustion air. The pilot burner apparatus may also include means for igniting the mixture of gaseous fuel and combustion air delivered to the flame holder apparatus.

The transport and diffusion of the gas flow and fuel are calculated by using such methods as calculus of finite differences, finite element analysis, and finite volume analysis and discretizing the fluid equations. The steady-state continuity, momentum, energy, and species equations in the fluid phase and the steady-state energy equation in the solid phase are discretized using a finite-volume method. The flow is laminar. The aspect ratio of the system is so high that any surface-to-surface radiation is most likely emitted and absorbed at nearly the same axial location. Therefore, radiation is omitted from the simulations performed in this work to focus on the effect of diffusive and convective heat transport on flame stability. The boundary conditions used in this model are given as follows. At the inlet a fixed flat velocity profile is assumed. For the species and energy equations, the convective portions of the equations are fixed, and the diffusive portions are calculated implicitly. At the interface between the fluid and the solid, no slip and no normal species diffusive flux boundary conditions are applied. The heat flux at this interface is calculated using Fourier’s law and continuity in temperature and heat flux is ensured. A symmetry boundary condition is applied at the centerline between the two plates. At the exit, the pressure is specified and the remaining variables are calculated assuming far-field conditions, namely zero diffusive flux of species or energy normal to the exit. In the bulk of the wall the energy equation is solved. The exterior convective heat-transfer coefficient is only used for the calculation of the heat flux of the exterior wall edge boundary condition. This heat-transfer coefficient lumps the details of heat loss from the burner and of the process that utilizes the heat generated by the burner. The left and right wall edges are taken to be insulated.

Since the actual flame is extremely thin, it is necessary to generate a calculation grid smaller than the flame band thickness in order to obtain a precise calculation. When such calculation grids are applied to a combustor, the number of calculation grids becomes enormous and the calculation cost becomes very expensive. By considering the flame surface area per unit volume, it is possible to calculate the flame propagation precisely even when the calculation grid is larger than the flame band thickness. Non-uniform node spacing is employed in this work, with more nodes in the reaction zone. The number of nodes varies depending on dimensions. The fluid viscosity, specific heat, and thermal conductivity are calculated by a mass-fraction-weighted average of species properties. The specific heat is calculated using a piecewise polynomial fit of temperature. The fluid density is calculated using the ideal gas law. The conservation equations are solved implicitly with a steady-state segregated solver using an under-relaxation method. The segregated solver first solves the momentum equation, then the continuity equation, and then updates the pressure and mass flow rate. The conservation equations are then checked for convergence. Convergence is determined from the residuals of the conservation equations as well as the difference between subsequent iterations of the solution. The momentum, species, and energy equations are discretized using a second-order upwind approximation. In order to achieve convergence as well as compute extinction points, natural parameter continuation is implemented. The calculation time of each simulation varies, depending on the difficulty of the problem and the initial guess.