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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.

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Acoustic resonances in combustion systems like central heating boilers prohibit further technological advances in these systems. The design and construction is obstructed by acoustic problems because they are largely misunderstood, in spite of our increase in knowledge over the last decades. The flame often acts as an active element in the acoustic field, because the flame transfer function of acoustic waves has a large amplitude at low frequencies. Current models of the phase of the flame transfer function of Bunsen-type flames, based on kinematic behavior of the flame dynamics, completely miss the experimentally observed phase, unless the measured flow field is used in the model. In the current paper we analyze numerical results of the flame dynamics, flow field and flame transfer function found with a 2D detailed numerical model of the flow and structure of the flame on a multiple-slit burner. The model is validated with experiments of the flame dynamics (using chemiluminescence), flow dynamics (using PIV) and flame transfer function (using OH luminescence for the heat release fluctuations and heated wire probe for the acoustic distortions) on exactly the same configuration. A very good agreement is found which indicates the importance of predicting all the influences of the flow on the flame and vise-versa.

This paper addresses the task of obtaining the far-field spectrum for a finite structure given the near-field calculated by the aperiodic Fourier modal method in contrast-field formulation (AFMM-CFF). The AFMM-CFF efficiently calculates the solution to Maxwell's equations for a finite structure by truncating the computational domain with perfectly matched layers (PMLs). However, this limits the far-field solution to a narrow strip between the PMLs. The Green's function for layered media is used to extend the solution over the whole super- and substrate. The approach is validated by applying it to the problem of scattering from a cylinder for which the analytical solution is available. Moreover, a numerical study is conducted on the accuracy of the approximate far-field computed with the super-cell Fourier modal method by using the AFMM-CFF with near- to far-field transformation as a reference.

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