Flame Propagation and Tulip Flame Formation in Closed Diverging and Converging Channels

The effect of non-parallel walls on the flame dynamics and the tulip flame formation in a narrow, closed volume channel is investigated experimentally using high-speed visualization techniques and pressure measurements. The angular channel is a $$4^{\circ}$$ planar diverging (D–C) or converging (C–C) channel ignited at one of the ends. The flame propagation is studied for a wide range of equivalence ratios ( $$\phi=0.9-1.5$$ ) of premixed LPG-air mixture in both the channel configurations, and the outcome is extended for a straight channel. The flame speed is higher in the D–C compared to the C–C and highest in the straight channel due to thermal expansion caused by the narrow volume and flame stretch in the varying cross-section. In addition, the pressure changes in the channels are recorded, and the results show that the rate of pressure varies with the flame growth and is not significantly affected by the flame surface area. Analysing the normalized position and time for the flame inversion for different $$\phi$$ and for all the channels, it is found that the flame inversion occurs prominently at halfway of the flame propagation and at about 1/3rd of the total time and it is true for both parallel and non-parallel walled channels at every equivalence ratio. Flame morphology with diverging and converging walls are presented. In particular, the flame deceleration process and tulip flame formation are visualized across different orthogonal cross-sections. Distorted tulip flame and the vortex flows are observed in the burned gas along with asymmetric tulip-shaped flame propagation at different equivalence ratios for the D–C and C–C.