R Ronald Rook

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 flat flames: a two-dimensional numerical analysis

The response of burner-stabilized flat flames to acoustic perturbations is studied numerically. So far, one-dimensional models have been used to study this system. However, in most practical surface burners, the scale of the perforations in the burner plate is of the order of the flame thickness. This is expected to disturb the one-dimensional behavior and the main effects are studied using a 2-D numerical model. The numerical setup involves the set of 2-D transport equations for low-Mach number reacting flows using a simple reaction mechanism. After a short review of the physical phenomena observed so far in one-dimensional flames, the effect of a finite width of the perforations in a perfectly cooled burner is investigated for steady and unsteady flames in a 2-D micro-slit configuration. The influence on the acoustic response is also studied and the results are compared with one-dimensional calculations using volume-averaged models for the heat and mass transfer in the perforated burner plate to incorporate the 2-D effects. This last study shows that effective one-dimensional models are not able to incorporate all 2-D effects due to the presence of the burner plate in the flow. The acoustic transfer matrix can be applied to the stability analysis of acoustic systems with a porous or perforated surface burner. The most interesting application is the acoustic behavior of central heating systems.

Experimental and Numerical Investigation of the Acoustic Response of Multi-slit Bunsen Burners

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. 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.

Near-to far-field transformation in the aperiodic Fourier modal method

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 Maxwells 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 Greens 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.