Analysis of the final stage of flame acceleration and the onset of detonation in a cylindrical tube using high-speed stereoscopic imaging

The three-dimensional structure of an accelerating flame front makes it very difficult to visualize. Most experimental studies that provided a visualization of flame acceleration and deflagration-to-detonation transition (DDT) events were done in rectangular tubes using the shadow technique. Qualitative information obtained by these optical methods has been two-dimensional and line-of-sight integrated, so local peculiarities and trends of the phenomena may not be spatially resolved in the third dimension. Here, we present a high-speed imaging analysis (self-luminous stereoscopic photography) of the final stage of fast flame propagation, the formation of new autoignition kernels and the onset of localized explosions in a long smooth transparent cylindrical tube. We investigated the evolution of the accelerating flame shape along the tube and found that before transition to detonation the flame shape is quite stable and looks like a paper cone or waffle cone, instead of the usual shape that is referred to as a tulip flame. Simultaneous images captured by two high-speed cameras positioned 90 degrees apart allowed us to determine the location of the auto-ignition kernels and the explosion points in reaction gas volume behind the leading shock wave. Both the horizontal position with respect to the leading edge of the flame and the position across the tube cross-section were determined. We observed the four typical scenarios for the onset of detonation in front of the accelerating flame where explosion occurs in the boundary layer to be: (1) between the secondary reaction fronts that emerge from the auto-ignition kernels; (2) between the main flame front and the secondary reaction fronts that emerge from the auto-ignition kernels; (3) at the leading tip of the secondary reaction fronts; (4) at the tip of the main turbulent flame front. We determined the local gas parameters between the flame front and the leading shock wave directly before the onset of detonation. The flow stagnation in the boundary layer near the tube wall leads to a significant local temperature increase and strongly reduces the ignition delay time of the mixture. Simple kinetic calculations show that for our test conditions, a temperature rise of approximately 275 K results in an induction time reduction by 15 times. (C) 2020 The Combustion Institute. Published by Elsevier Inc. All rights reserved.