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Abstract
An extensive set of flame blowout limit curves has been measured for the case of a hydrogen jet flame surrounded by a heated, supersonic, coflowing air stream and some ideas are proposed to explain the observed trends. The stagnation temperature of the Mach 2.2 air stream was varied from 294 K up to the autoignition temperature of 900 K; hydrogen injection velocities were varied up to 1191 m/s. It was found that the flame blowout curves display two distinct stable regions which are bounded by the: (a) far-field blowout limit and (b) near-field blowout limit. Far-field blowout occurs after a flame first lifts off and is associated with a sufficiently large fuel velocity; the shape of the far-field blowout curve can be explained by previous subsonic lifted flame analyses. Near-field blowout is a sudden blowout for which no liftoff occurs; it results from a sufficiently large air velocity. In all cases the flame attachment point is in the shear layer at a distance of about 1 em from the fuel tube lip.
Results show how to select parameters in order to extend the near-field blowout limit curve into the supersonic regime. Supersonic flame stability requires sufficient stagnation temperature and fuel tube lip thickness. The improved stability due to elevated stagnation temperature can be explained by the temperature dependence of the chemical reaction rate. Air density also is shown to be important since it is the momentum of the air stream that determines how the velocity profile and stoichiometric contours overlap. Also quantified were other parameters that are needed to model the flame stabilization process, including the flame liftoff distance and the flame length. Comparisons are made to previous subsonic trends as no other blowout trends involving supersonic coflowing air are available.
