Thermo-acoustics of Bunsen type premixed flames

Abstract

This thesis is stimulated by the phenomenon of undesired thermo-acoustic fluctuations in small scale combustion devices where multiple Bunsen type flames are organized on a perforated burner deck. Methods to predict the instability of a combustion system exist, however it requires the information of the system linear acoustics and the flame response to the acoustic fluctuation. The present experimental work mainly focuses on the latter. In addition, system stability prediction and burner design strategy with desired TF for stable operation are also addressed. There exist many methods to characterize the flame response to the acoustic fluctuations. The flame transfer function (TF), which is a widely accepted method, is used to characterize the flame response to the acoustic fluctuations. Various parameters affecting the TF of the multiple conical flames were studied. These parameters are: the flow velocity, the equivalence ratio, the perforation hole diameter and the pitch of the perforation. The velocity and the heat release rate fluctuations are measured with a hot wire probe and a photomultiplier tube respectively. The flame response is quantified in the linear regime by providing the amplitude of acoustic fluctuations in a controlled way. It is shown that the flame TF gain has a complicated jugged form with several recognizable minima and maxima and the phase of the TF has a constant time delay behavior for all the measured TF’s. Qualitative and the quantitative similarities between the TF in different combustion regimes and for multiple flames can be expressed with a single phenomenological expression with four fitting parameters. The correlations between the TF behavior and the burner/flame parameters are revealed. Plausible hypothesis of the physical mechanisms of the TF formation are formulated and discussed. For the multiple conical flames, a method of de-composing the TF to/from the TF’s of its elementary flames is formulated. A good agreement between the experimentally measured and the composed TF of a composite burner is found. On this basis, a burner deck can be constructed with desired acoustic characteristics. In addition to the TF of the multiple conical flames, the acoustic response of the single conical flame is studied in laminar, transient and turbulent combustion regimes. The qualitative and quantitative differences/similarities between the TF’s for the measured combustion regimes are reported. A method to obtain the acoustic system time lag from the burner/flame geometry is proposed. A simplified combustion system is modeled with the 1D network modeling approach. The TF obtained previously is used as input to calculate the thermo-acoustic stability of the combustion system. The predicted and experimentally obtained unstable modes are compared. It was found that the network model slightly over-predicts the self-oscillating combustion regime. In conclusion, this work provides techniques for further development of appliances where the multiple conical flames on surface stabilized burners are used.