Non-adiabatic modulation of premixed-flame thermoacoustic frequencies in slender tubes

This paper presents an experimental study of the influence of heat losses on the onset of thermoacoustic instabilities in methane–air premixed flames propagating in a horizontal tube of diameter, $D = 10$ mm. Flames are ignited at the open end of the tube and propagate towards the closed end undergoing strong oscillations of different features owing to the interaction with acoustic waves. The frequency of oscillation and its axial location are controlled through the tube length $L$ and the intensity of heat losses. These parameters are respectively modified in the experiments by a moveable piston and a circulating thermal bath of water prescribing temperature conditions. Main experimental observations show that classical one-dimensional predictions of the oscillation frequency do not accurately describe the phenomena under non-adiabatic real scenarios. In addition to the experimental measurements, a quasi-one-dimensional analysis of the burnt gases is provided, which introduces the effect of heat losses at the wall of the tube on the interplay between the acoustic field and the reaction sheet. As a result, this analysis provides an improved description of the interaction and accurately predicts the excited flame-oscillation harmonics through the eigenvalues of the non-adiabatic acoustics model. Unlike the original one-dimensional analysis, the comparison between the flame oscillation frequency provided by the non-adiabatic extended theory and the frequencies measured in our experiments is in excellent agreement in the whole range of temperatures considered. This confirms the importance of heat losses in the modulation of the instabilities and the transition between different flame oscillation regimes.

Statistical Analysis of Flame Oscillation Characterization of Oxy-Fuel in Heavy Oil Boiler Using OH Planar Laser-Induced Fluorescence

The present work investigated the flame structures and oscillations of oxy-fuel combustions in a heavy oil boiler using OH planar laser-induced fluorescence imaging. Combustion instabilities, such as flame oscillation and combustion fluctuation, can assess the performance of an industrial burner in the boiler. The peak position variation in OH concentration was associated with the change of the reaction zone that corresponded with the fluctuation of the heat-release zone in the combustion chamber, which provides a valuable reference for the design of the combustion chamber. The experimental results suggest that the phenomenon of stratified flame combustion is related to the characteristic of flame oscillation. The substitution of N2 with CO2 will not significantly influence the flame oscillation frequency but increases the number of flame surface. As O2 concentration increased in the O2/CO2 atmosphere, the phenomenon of stratified flame combustion disappeared, and the flame presented an island-like structure. The bimodal oscillation of the combustion center was demonstrated by means of the probability density method; CO2 played a role in the extension of the combustion center. The combustion fluctuation of inner regions was quantitatively described; CO2 could maintain interregional stabilization to some extent. Compared with traditional measurement methods, PLIF technology has great advantages in evaluating burner performance and optimizing the design of the combustion chamber.

Association of flame-oscillation frequency under cross wind diffusion

Turbulent flame-oscillation frequency and combustion efficiency are the important parameters in combustion. It is a useful study to judge combustion efficiency by intuitive flame frequency. Based on a combustion wind tunnel experimental set-up, the variation law of combustion flame-oscillation frequency and combustion efficiency using external wind speeds was studied via the color flames image sequence of oil pan combustion and flue gas composition. The oscillation frequency was calculated with the upper edge of the fire plume which recorded flame image sequence using a high-speed video, combined the MATLAB image processing technique. The combustion efficiency was calculated combined with the C and H element mass of the liquid fuel in the oil pool and CO2 and H2O concentrations in the flue gas. The results show that the oscillation frequency of the flame revealed a tendency to increase followed by a decrease with increasing wind speed, while the overall trend of combustion efficiency increased first and then decreased. The combustion efficiency could be judged by observing the oscillation frequency of the flame image. In addition, the combustion efficiency may ensure in right way when the turbulence oscillation frequency of the oil pan flame ranged between 15 Hz and 18 Hz.