An Investigation of Blast Waves Generated by Constant Velocity Flames
Luckritz, Robert Thomas
Marchello, Joseph M.
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The relevant flow field parameters associated with the generation and propagation of blast waves from constant velocity flames were systematically studied through numerical integrations of the non-steady equations for mass, momentum, and energy. The flow was assumed to be that of an adiabatic inviscid fluid obeying the ideal gas law and the flame was simulated by a working fluid heat addition model. The flame velocity was varied from infinitely fast (bursting sphere) through velocities characterized by the nearly constant pressure deflagration associated with low Mach number laminar flames. The properties noted included peak pressure, positive impulse, energy distribution, and the blast wave flow field. Results were computed for the case of a methane-air mixture assuming an energy density, q = 8.0, an ambient specific heat ratio, Yo = 1.4 and a specific heat ratio behind the flame, Y4 = 1.2. In the source volume, as the flame velocity decreased to Mach 4.0 the overpressure increased. For flame velocities below Mach 4.0 the overpressure decreased, and approach the acoustic solution originally developed by Taylor. In the far field the overpressure curves for supersonic flame velocities coalesced to a common curve at approximately 70% of Baker's pentolite correlation. Far field overpressures for subsonic flame velocities decreased as the flame velocity decreased. For the flame velocities investigated the near field impulse was greater than the impulse from Baker's pentolite correlation. In the far field the flame generated impulse decreased to 60 to 75% of the pentolite impulse. In cases where the flow was expected to reduce to a self-similar solution and/or show Rayleigh line behavior it did. The calculations showed that the flow field behaved normally where expected, and for flow velocities where steady state behavior is not expected, non-steady behavior was observed.