Ignition, topology, and growth of turbulent premixed flames in supersonic flows
Ochs, Bradley Alan
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Supersonic combustion ramjets (scramjets) are currently the most efficient combustor technology for air breathing hypersonic flight, however, lack of fundamental understanding and numerous engineering challenges hinder regular deployment of these devices. This work addresses scramjet-relevant knowledge gaps in supersonic turbulent premixed combustion, including laser ignition, numerical modeling, and flame-compressibility interaction. One of the main contributions of this work is introduction of a new turbulent premixed flame arrangement where flame-compressibility interaction can be systematically explored: flame kernels in an expanding flow field. The scramjet flow path is replaced by a simplified channel geometry with a well characterized mean flow acceleration that mimics flow field expansion typically imposed on scramjet combustors to avoid thermal choking. Spherically expanding flames are created via laser ignition and subsequent flame growth and morphology are investigated using combined physical and numerical experiments. Pressure-density misalignment due to flame-compressibility interaction produces vorticity at the flame surface through baroclinic torque, i.e. flame-compressibility interaction acts like a turbulence source. The flame ultimately evolves into a reacting vortex ring that increases the flame speed and enhances reactant consumption. To explore the relative importance of turbulence and compressibility on flame dynamics, the Mach number (M=1.5,1.75,2), equivalence ratio (φ= 1.0,0.9,0.8,0.7), and root-mean-squared turbulent velocity (u'=3.98,4.14,4.45 m/s) are varied systematically. This work also introduces flame kernels in an expanding flow field as a canonical numerical validation test case for flame-compressibility interaction. Inaccuracies in simulation results are easily identified due to high flow velocity and simplicity of the problem. The numerical setup and models are scrutinized to minimize errors. Using the appropriately verified numerical models, simulation results show very reasonable agreement with experimental data. Validated simulations are instrumental in enhancing understanding of the underlying physics of supersonic flame kernels. Laser ignition studies in supersonic flows have historically focused on ignition of non-premixed fuels within cavity flame holders. This work introduces a far simpler and more tractable problem: laser ignition of a fully premixed supersonic gas. Ignition experiments with a range of laser settings are performed to determine supersonic breakdown and ignition probabilities, length of time the ignition event influences flame growth, and Mach number influence on the ignition process. The ignition event has a long-lasting effect on kernel growth, but the influence can be minimized by properly selecting the laser energy. Mach number has a minimal impact on the ignition process, but does affect the initial kernel shape due to flow field variations with Mach number. Kernel growth matches low speed studies closely at early times, but deviates at later times due to vortex ring topology. It is not obvious how the turbulent flame speed will scale for flows with mean compressibility. Therefore, the combined physical and numerical experiments are leveraged to explore this question. The vortex ring causes significant errors in the line of sight-measured burned volume, hence correction factors to convert from line of sight to volumetric measurements are presented. Conditions for displacement and consumption speed equivalency are shown to depend heavily on the particular diagnostic used; which progress variable isocontour is measured and where it is measured within the flame brush must be considered carefully during interpretation of experimental data. Scaling with the RMS turbulent velocity cannot collapse these flame speed data, i.e. previously established flame speed scalings are inappropriate for flames interacting with compressibility. Drawing motivation from vortex ring literature, a new flame speed scaling based on the ring propagation velocity is proposed. The proposed scaling collapses the data and produces a nearly linear scaling regime, which suggests turbulence plays a secondary role to the hydrodynamic instability created by flame-compressibility interaction. In summary, flame kernels are a new and effective canonical configuration for exploring flame-compressibility interactions in supersonic flows.