Abstract
Aim: The paper presents the feasibility of extinguishing a diffusion flame with the aid of acoustic waves. The study includes an assignment of critical frequency values and the acoustic force limit, the point at which a flame is extinguished.
Methodology: In order to achieve the desired aim, it was necessary to construct a test stand, to facilitate the generation of acoustic waves in the direction of a burner. The experimental stand was equipped with a frequency generator and amplifier, speaker with a low frequency band, waveguide and voltage strength and frequency meters. Measurement of sound pressure at the flame location was performed using a Culite probe. The probe was constructed on the principle of a microphone, using a very sensitive membrane, electrode and capacitor. Changing the pressure within the probe resulted in a minimal displacement of the membrane, which in turn caused changes to the charge level of the capacitor. This level was recorded as a change in the voltage flowing through the system. The voltage value was revealed on the screen of the measuring device, which allowed for the calculation of pressure values, taking into account the operating range of the measuring device. Additionally, extinguishing of the flame was observed visually with the aid of Schlieren imagery apparatus. This is considered to be an ideal method for graphical recording of research results associated with the combustion processes where insufficient lighting of a dim environment does not allow for proper imaging, using traditional methods. Whereas, in ambient lit conditions it is not possible to reveal the entire structure of the flame because of strong light emitted by the flame.
Results: Studies reveal that for a given value of flame thermal power it is possible to extinguish a flame with the use of a wide spectrum of critical frequency (35 ÷ 155 Hz) and force limit level below 30 W. Extinguishing ability was estimated by assigning a flame extinguishing effectiveness ratio; for the value of 35 ÷ 45 Hz it was within the range between 35 to 155 Hz. Additionally, the sound pressure level limit value (range of 45 ÷ 55 Pa) was assigned, below which the flame extinguishing activity was virtually unnoticeable.
Conclusions: The described flame extinguishing technique allows for the total extinguishing of flames with the aid of generated sound waves at an appropriate frequency. This is confirmed by results from research experiments and visual observation, using Schlieren imagery apparatus.
Keywords: fire, extinguishing a flame, diffusion flame, acoustic wave
Type of article: original scientific article