BLOWOUT STABILITY LIMITS OF A HYDROGEN JET FLAME IN A SUPERSONIC, HEATED, COFLOWING AIR STREAM

An extensive set of flame blowout limit curves has been measured for t he case of a hydrogen jet flame surrounded by a heated, supersonic, co flowing air stream and some ideas are proposed to explain the observed trends. The stagnation temperature of the Mach 2.2 air stream was var ied from 294 K up to the autoignition temperature of 900 K; hydrogen i njection velocities were varied up to 1191 m/s. It was found that the flame blowout curves display two distinct stable regions which are bou nded by the: (a) far-field blowout limit and (b) near-field blowout li mit. Far-field blowout occurs after a flame first lifts off and is ass ociated 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 cas es the flame attachment point is in the shear layer at a distance of a bout 1 cm from the fuel tube lip. Results show how to select parameter s in order to extend the near-field blowout limit curve into the super sonic regime. Supersonic flame stability requires sufficient stagnatio n 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 t o be important since it is the momentum of the air stream that determi nes how the velocity profile and stoichiometric contours overlap. Also quantified were other parameters that are needed to model the flame s tabilization process, including the flame liftoff distance and the fla me length. Comparisons are made to previous subsonic trends as no othe r blowout trends involving supersonic coflowing air are available.