High-speed photography and background oriented schlieren techniques for characterizing tulip flame

High-speed photography and background oriented schlieren (BOS) techniques are used to characterize the tulip flame formation in half-open and closed-ends type channels. Flame propagation for a premixed mixture of Liquefied Petroleum Gas (LPG) and air in the equivalence ratio range of 0.8-1.0 are tested. The classical tulip flame is observed only in the closed channel. Refractive index and flow density are calculated numerically using Poisson's equation and Gladstone-Dale relation. Pseudo-schlieren images, shadowgram, and density contours are plotted using the BOS technique, and these reconstructed fields matched well with the direct observations. Instantaneous velocity fields are calculated using consecutive images as the reference image for the BOS correlation method. The results show that the velocity during the early stages of the flame expansion is close to the laminar burning velocity of the premixed LPG-air flame. As the flame started to inverse, the flame tip close to the wall accelerated to approximately 1.16 m/s. But later the fully-formed tulip flame propagates at 0.25-0.3 m/s till the end of the chamber. A reverse flow in the burned gas region is observed after the tulip flame formation. However, BOS could not capture any acoustic changes in the unburned gas except for a high gradient backflow just behind the flame front.(c) 2022 The Combustion Institute. Published by Elsevier Inc. All rights reserved.