Vortex mixing for supersonic combustion
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Fundamental concepts of mixing and combustion theory are examined in order to define an optimum system. By analogy with the familiar parameter “combustion intensity,” a “mixing intensity” is defined as the total mass of fuel and air mixed per unit time, volume, and density. Mixing intensity is related to the mean kinetic energy of turbulence generated in a mixing system. The efficiency of generation of turbulence is defined by the ratio of the kinetic energy of turbulenee generated to the kinetic energy loss from the mean flow. Bluff body mixers are compared with turbulent jet mixers, and turbulent intensities are determined as a function of fuel/air ratio and jet-to-stream-velocity ratio. For turbulence generated by a baffle system or by jets, the mixing intensity is shown to be proportitional to the rms velocity and the square root of the number of sources. Substantial increases in mixing rates can be obtained by applying a swirling motion to the central fuel jet. Radial and axial pressure gradients are set up which, in the case of strong swirl, result in reverse flow along the axis. Empirical equations for velocity decay, mass entrainment, and angle of jet expansion are given as functions of the degree of swirl. However, further work is needed on the behavior of these parameters in an external flow field. For the combustion chamber of a scramjet, the degree of swirl imparted to the fuel can be altered readily by varying the proportion of fuel introduced axially and tangentially into the swirl chamber. The rate of mixing, angle of jet expansion, and the size of the internal reverse-flow zone can all be controlled by variation of the swirl number. With the ability to initiate and remove the reverse-flow region, an optimized combustion system could operate efficiently from low-subsonic to high-supersonic Mach numbers.
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