Experimental study on soot suppression efficiency of acetylene diffusion flames in a Rijke tube

Abstract

• The soot suppression effect of acetylene diffusion flames in a Rijke-type combustor is reported for the first time. • Extremely high soot suppression efficiencies are only achieved at the tube’s naturally resonant frequencies (169 Hz and 346 Hz). • As the flow state turns into turbulence, soot is nearly completely suppressed. • With the generation of harmonics, different flame shapes are observed in adjacent periods. • Temperature downstream of the flame is increased from 1773 K to 2250 K. This work reports soot suppression effect of acetylene diffusion flames in a Rijke-type combustor for the first time. The soot generation and suppression from a laminar and turbulent diffusion flame in the Rijke tube by introducing an external acoustic forcing signal are investigated experimentally. A loudspeaker is applied at the bottom of the Rijke tube to induce resonance and control the frequency in the tube. The flame is fluctuated violently at the tube’s naturally resonant frequencies (169 Hz and 346 Hz), and extremely high soot suppression efficiencies are only achieved at these frequencies. Besides, the soot suppression efficiency gradually increases and the flame height gradually decreases with the increase of the acoustic pressure. Different flame shapes are observed in adjacent periods, which is the result of harmonics in the tube. The Reynolds number is calculated based on the velocity of the flame fluctuation. Results show that in laminar flow, the soot suppression efficiency is low. Differently, in turbulent flow, soot suppression efficiency (up to about 100 %) is extremely high. At turbulent condition, the mixing of fuel and air is enhanced, resulting in more soot to be oxidized. The flame temperature fluctuates periodically and the peak flame temperature shows a 96 degrees lag behind the peak acoustic pressure. Under the acoustic resonance, the temperature downstream of the flame gradually increases from 1773 K (without acoustics) to 2250 K.