Experimental research of acoustically perturbed Bunsen flames

This dissertation is related to experimental research of the response of Bunsen type laminar premixed methane-air flames to an acoustic excitation of the flow. This study is stimulated by the problem of acoustic instability of burners and combustors used in domestic and industrial heating devices where combustion is organized via laminar Bunsen type flames. The study and accordingly the dissertation can be divided into two parts. The first part is concentrated on a detailed parametric study of the heat release rate response of conical and tent shaped Bunsen flames to the acoustic perturbation of the gas mixture flow velocity. A Transfer Function (TF) concept which relates the response (heat release rate oscillation) with the stimulus (velocity perturbation) is used to characterize the flame’s susceptibility to the excitation. The measurement method is based on the simultaneous registration of the acoustic velocity oscillation via a hot-wire probe installed in the burner in the fresh gas flow just upstream of the flame and heat release rate monitoring via OH* radical chemiluminescence of the flame. The influence of the following parameters of the flame and burner on the flame TF is studied: mean gas velocity and radial flow profile; mixture equivalence ratio; burner tube diameter, material and opening geometry; flame confinement and co-flow. It is shown that the flame TF gain has a complicated jugged form with minima and maxima. The phase of the TF has a "constant time delay" behavior. An extra term of the TF which has weak frequency dependence is found. However, on the basis of the experimental data a universal phenomenological equation describing the flame TF is proposed. Correlations of the TF parameters with the flame/burner parameters are revealed. Plausible physical mechanisms (hypothesis) of the TF formation are formulated and discussed. The second part of the dissertation is related to an experimental examination of the TF origin hypothesis via a study of the flame kinematics and perturbed flow velocity visualization and measurements. Methods used are: Particle Image Velocimetry (PIV), TiO2 smoke tracing, direct flame front imaging with consequent image processing. Accordingly, excited flow patterns inside the flame cone are visualized and studied for different flames and excitation frequencies. Convectively traveling jet perturbation structures of a peristaltic type are found. In order to clarify the role of the flame front motion in the formation of these flow structures a specially organized experiment with a flame anchored on an oscillated flame holder ring is conducted. The perturbed flame front imaging allows studying both the flame front disturbance kinematics and a relation between the flame surface area and the flame heat release rate oscillations. An important role of the flame attachment point’s motion is revealed. A laborious research of the flame base zone provides information about the excited flame anchoring point motion. The description of the experimental results is accompanied by a critical analysis and discussion of existing theoretical models and concepts. As a result the experimental assessment of the theoretical approaches to describe the Bunsen flame with acoustics interaction is provided. The output of dissertation can be summarized and used in the following way: 132 Summary - The experimental data of the flame TF parameterization does serve as an experimental reference both for theoretical and numerical models of the flame response on acoustic excitation. Burners which use a similar type of flame can be examined for the purpose of an acoustic instability analysis using the data measured and presented in this dissertation. - The synthesis of the experimental data in the form of a universal phenomenological relation as well as the correlations found between the TF parameters and the flame/burner characteristics allows to formulate and consequently check the hypothesis about the physical mechanisms of the TF origin. - The excited flow pattern and flame front visualization reveal the role of convective flow structures, the flame anchoring point’s motion and the back-influence of the flame on the upstream flow in the formation of the TF. - On the basis of the knowledge received the explanation of some acoustic instability features in the practical combustors as well as the recommendation to deal with this instability are proposed.