Study Of Buoyant Diffusion Flame Instability And Oscillation Frequency In Cross-Wind

Fire or flame in cross-wind occurs and is widely used in outdoor environment, industrial furnace, boiler, chimney, tunnels and engine combustion chamber and so on. However, the flame instability generated in those scenarios would enhance the fire spreading, destroy industrial equipment structure and produce noise. As an essential characteristics of fire, on one hand, the phenomenon of flame oscillation would cause harms; on the other hand, it plays a role in distinguishing the fire and the non-fire which is applied to the field of video detection. The frequency domain of fire flame varies widely with some harmless light like lamplight and sunlight, and the special frequency domain can be captured by video detection to achieve early fire detection and alarm. However, the fire frequency domain in cross-wind is different with that in still air, which would affect the accuracy of the fire video detection. Therefore, the study of flame oscillation phenomenon in cross-wind can not only help to avoid harms caused by it, but also provide a theoretical basis for parameter design of fire video detection in cross-wind, which shows a great significance.In the context of knowing the harms caused by flame oscillation in cross-wind and the application value for fire video detection, the theoretical prediction model of flame oscillation frequency in cross-wind for propane buoyant diffusion flame was established in this paper. The experiment platform of buoyancy diffusion combustion was designed for studying the effects of cross-wind and burner diameter on the flame oscillation frequency and their interrelation, and the flame oscillation mechanism was revealed.In terms of theoretical research, the drag coefficient CW,D which represents the momentum flux ratio, and the decay coefficient KE which represents the ratio of the average flame axial velocity and the initial velocity in cross-wind are introduced in this paper. The effects of cross-wind, buoyancy and entrainment on flame oscillation frequency are considered along with the introduction of the drag coefficient and decay coefficient. Two kinds of frequency expressions are put forward correspondingly by two forms of Richardson number in which the characteristic length is replaced by burner diameter (Ri(D)) and the flame length (Ri(L1)), respectively.In terms of experimental study, the conditions of still air and four cross-wind velocities of 1.2,1.7,2.2,2.6 m/s, five burner diameters of 0.05,0.075,0.106,0.13, 0.15 m and four volume flow rates were designed in the experiments. The flame temperature, flame image, volume flow rate of propane and flame radiation flux can be measured and controlled synchronously, furthermore, the cross-wind velocities were monitored during the experiments. Through data analysis and processing, the characteristic parameters of flame image, flame temperature, flame length, flame width, flame tilt angle and the flame oscillation frequency were gained, which provided experimental verification for the theoretical predictions of buoyant diffusion flame oscillation frequency in cross-wind.Combining theoretical models and experimental results, the analyses of the experimental results are conducted from the flame morphology, flame temperature, flame length and flame frequency. First, on the base of flame oscillation mechanism in still air, it is shown from the findings of flame morphology that the oscillation of buoyant diffusion flame in cross-wind is induced by Rayleigh-Taylor instability and Kelvin-Helmholtz instability in common, which shows smaller oscillation amplitude and more complex nature than that in still air. Subsequent analysis reaches that the flame radiation and flame temperature both decrease first then increase with increasing cross-wind velocity. The cross-wind has little effect on the flame length during the range of cross-wind velocity in this paper.Then, the consistent analysis and difference comparison for theoretical predictions and experimental results of flame oscillation frequencies which correspondingly put forward by two forms of Richardson number are carried out. The results show that the flame oscillation frequencies decrease with increasing 1/R (the reciprocal of momentum flux ratio) for both forms, however, the flame oscillation frequency expression gained by Ri(D) shows a better agreement between experimental and theoretical, which is more reasonable. Finally, by the dimensionless analysis of flame oscillation frequency, it is obtained that the Strouhal number has a good linear relationship with the Froude number.