Kram Koen Schreel

Response of burner-stabilized flat flames to acoustic perturbations

The response of burner-stabilized flat flames to acoustic velocity perturbations is studied numerically and analytically. The numerical setup involves the set of one-dimensional transport equations for the low-Mach number reacting flow using a simple and a more complex reaction mechanism. The physical background of the phenomena observed numerically is explained by a simple analytical model. The model uncouples the unsteady transport equations into two parts: the first part describes the flame motion through the G-equation and the second flamelet part describes the inner flame structure and mass burning rate of the flame. The G-equation can be solved exactly in the case of a quasi-steady flame structure. The mass burning rate is assumed to be directly related to the flame temperature. Relations for the fluctuating heat release and heat loss to the burner are derived, from which the coupling between the velocity fluctuations at both sides of the flame is found. Comparison of the numerical and analytical results with earlier work of McIntosh and with primary experimental results on a lean methane/air flame shows the validity of the models. The origin of the differences encountered is discussed. The resulting transfer function for the velocity perturbation can be applied to the acoustic stability analysis of combustion systems. The most interesting application is the acoustic behaviour of central heating boilers.

The acoustic response of burner-stabilized premixed flat flames

The behavior of acoustically driven flat flames has been analyzed experimentally. In this study, pressure transducers and laser Doppler velocimetry are used to characterize the acoustical waves upstream and downstream of a flat flame stabilized on a flame holder. Two different flame holders have been used, a perforated brass plate and a ceramic foam, exhibiting very different surface temperatures. From these experiments, the acoustical transfer function can be derived. This transfer function shows a resonance-like behavior, of which the shape and peak frequency is governed mainly by the surface temperature of the burner and the velocity of the unburned mixture. The brass burner exhibits a resonance frequency around 140 Hz, where the resonance of the ceramic burner seems to have shifted to much higher frequencies and is much more damped. All results can be understood very well with an analytical model in terms of Zeldovich number, standoff distance, and heat conductivity. Apart from the analytical model for the brass flame holder, numerical simulations with detailed chemistry have also been performed. Again, the correspondence is good. The most interesting application is the acoustic behavior of central heating systems, in which these burners are frequently used. For the purpose of modeling the acoustical behavior of complete boiler systems, the analytical model can be used with minor adjustments to the Zeldovich number and heat conductivity, yielding a fairly accurate semiempirical model describing the transfer function.

Solution of the Integrated Radiative Transfer Equation for Gray and Nongray Media

In this article a spectrally resolved solution of the integrated radiative transfer equation for the discrete transfer method with linear temperature interpolation is presented. Combined with the discrete ordinate method, the radiative heat fluxes are determined for use in the finite-volume energy equation. The spectral dependence of the absorption coefficient is approximated as a series of bands of constant value and scattering is neglected. The method is shown to be computationally efficient when applied to a course mesh typical for conduction problems and at the same time to be accurate for both optically thin and thick media, including semitransparent media.