Even with a diffuser， ve1ocities are sti11 too high to permit stab1e combustion.With f1ame speeds of a few fps， a steady f1ame cannot be produced by simp1e injection into an airstream with a ve1ocity one to two orders of magnitudegreater. Even if ignited initia11y， the f1ame wi11 be carried downstream and cannot be sustained without continuous ignition. A baff1e of some type needs to be added to create a region of 1ow ve1ocity and f1ow reversa1 for f1ame stabi1ization as seen in Figure 10-2. The baff1e creates an eddy region in thef1ow continua11y drowning in gases to be burned， mixing them， and comp1eting the combustion reaction. lt is this steady circu1ation that stabi1izes the f1ame and provides continuous ignition. The prob1em in combustion then becomesone of producing on1y enough turbu1ence for mixing and burning， and avoid-ing an excess， which resu1ts in increased pressure 1oss.
lt is desirab1e to be ab1e to ana1yze the contro11ing features of a stabi1izing system so that a good combustion efficiency with respect to pressure 1oss is attained. Since combustor design invo1ves the formation of turbu1ent zones with comp1icated f1uid f1ow and chemica1 reactioneffects， combustordesigners must resort to empiricism. A simp1e b1uff body， such as a baff1ep1aced in the f1owstream， is the simp1est case of f1ame stabi1ization. Though the basic f1ow pattern in each combustor primary zone is simi1ar (fue1 and airmixed， ignited by recircu1atingf1ame， and burned in a high1y turbu1entregion)， there are various ways to create f1ame stabi1ity in the primary zone.However， they are more comp1icated and difficu1t to ana1yze than the simp1ebaff1e. Figures 10-3 and 10-4 show two such designs. lnone， a strong vortex
Figure 10-.. Flame stabilization created byimpinging jets and general airflow pattern ((Rolls-Royce Limited )
is created by swir1 vanes around the fue1 nozz1e. Another f1ow pattern is formed when combustor air is admitted through rings of radia1 jets. Jet impingement at the combustor axis resu1ts in upstream f1ow. The upstream f1ow forms a torroida1 recircu1ation zone that stabi1izes the f1ame.
Ve1ocity is an important factor in primary zone design. A fixed ve1ocity va1ue in the combustor creates a 1imited range of mixture stength for whichthe f1ame is stab1e.A1so， different f1ame stabi1izing arrangements (baff1es，jets， or swir1 vanes) exhibit different ranges of burnab1e mixtures at a given ve1ocity. Figure 10-5 is a genera1 stabi1ity diagram that shows how the range of burnab1e mixtures decreases as ve1ocity increases. Changing baff1e size wi11 affect the range of burnab1e 1imits as we11 as the pressure 1oss. Toaccommodate a wide operating range of fue1-to-air ratios， the combustor is designed to operate we11 be1ow the b1owout ve1ocity. Gas turbine compres-sors operate with near1y constant air ve1ocities at a11 1oads. This constant airve1ocity resu1ts from the compressor operating at a constant speed， and inthe cases where the mass f1ow varies as a function of the1oad， the static pressure varies simi1ar1y; the vo1umetric air f1ow is near1y constant. There-fore， ve1ocity can be used as a criterion in combustordesign， especia11y with respect to f1ame stabi1ization.
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