Influence of Vortical Structure on Flame Acceleration in Hydrogen-Air, Methane-Air, and Hydrogen-Methane/Air Blend

Abstract

Following the ignition of a flame in a duct, the characteristics of the flame-driven flow play an important role in the propagation and acceleration of the flame. When bluff body obstacles are present in the duct, vortical structures are formed in the flow downstream of the obstacle. The subsequent interaction of the flame with these vortices is believed to generate substantial flame deformation and acceleration. In the present study, we investigate the deformation of the flame resulting from its interaction with these vortices. The experiments were performed in stoichiometric hydrogen-air, methane-air, and an equimolar blend of hydrogen and methane at ambient conditions.

Experiments were conducted in a thin, rectangular shock tube, with a rectangular obstacle positioned within it. The shock tube was initially filled with the mixtures at atmospheric pressure. A planar flame was then ignited at the closed end of the shock tube using a long wire ignition technique and propagated toward the open end. The high Reynolds number of the flow ahead of the flame resulted in flow separation at the leading edge of the obstacle, forming a vortical structure. Following the entrainment of the flame into the vortex, the flame surface area increased, resulting in the acceleration of the flame-driven flow. The evolution of the flame was visualized using high-speed direct shadowgraph and Z-type Schlieren. The pressure evolution was measured using pressure transducers placed along the top and bottom walls of the shock tube.

The measured evolution of the flame speed and overpressure inside the shock tube highlights the higher reactivity and flame acceleration of the hydrogen-air flame compared to the flames propagating in methane-air and equimolar hydrogen-methane blend with air. To facilitate a better comparison of the propagating flames, the pressure and velocity measurements obtained from the experiments are expressed in a non-dimensional form. This approach effectively eliminates the dependence on the distinct laminar burning velocities of each mixture. Hydrogen, with its characteristic thin flame structure, is less affected by the turbulent features formed behind the obstacle than mixtures containing methane. These mixtures show a much larger increase in flame surface area and wrinkled structures because their flame times are longer than the characteristic time of the vortical structure. Consequently, they exhibit a higher normalized pressure peak compared to the hydrogen mixture. To compare the normalized overpressure between methane-air and equimolar mixtures, in the equimolar flame, due to the relatively shorter flame time and an increase in the concentration of hydrogen in the positive curvatures of the flame front, the amount of local quenching induced by turbulence is reduced. As a result, the equimolar mixture records a higher normalized pressure than the methane mixture.

In the second set of experiments, two parallel mirrors positioned at a 45-degree angle are installed on the top and bottom walls of the shock tube to visualize the third dimension of the flame propagating along the shock tube. This method is applied for the first time in this study, and the shadowgraph technique is used to capture 3D flame propagation. It is observed that the mixtures with a Lewis number below unity (hydrogen and the equimolar mixtures) show more 3D effects compared to methane. These two flames tend to incline along the width of the shock tube, and this behavior intensified when the flames got entrained into the vortical structures and accelerated.