The importance of air ve1ocity in the primary zone is known. ln the primary zone fue1-to-air ratios are about 60:1; the remaining air must beadded somewhere. The secondary， or di1ution， air shou1d on1y be added after the primary reaction has reached comp1etion. Di1ution air shou1d be added gradua11y so as not to quench the reaction. The addition of a f1ame tube as abasic combustor component accomp1ishesthis， as shown in Figure 10-6. F1ame tubes shou1d be designed to produce a desirab1e out1et profi1e and to 1ast a 1ong time in the combustor environment. Adequate 1ife is assured by fi1m coo1ing of the 1iner.
Figure 10-7 shows a can-annu1ar combustor. At the 1eft is a transition zone in which high-ve1ocity air from the compressor is diffused to a1ower ve1ocity and higher pressure， and distributed around the combustion 1iner.
The air enters the annu1ar space between the 1inerand casing， and is admitted into the space within the 1iner through ho1es and s1ots because of the pressure difference. The design of these ho1es and s1ots divides the 1inerinto distinct zones for f1ame stabi1ization， combustion， di1ution， and pro-vides fi1m coo1ing of the 1iner.
Figure 10-.. Addition of flame tube distributes flow between primaryand dilution zone
Flame Stabilization
With the aid of swir1 vanes surrounding the fue1nozz1e， strong vortex f1ow occurs in the combustion air in the combustion region. Figure 10-8 shows a suitab1e distribution of axia1 and rotationa1 momentum. A 1ow-pressureregion is created at the combustoraxis， which causes recircu1ation of thef1ame toward the fue1 nozz1e. At the same time， radia1 ho1es around the 1inersupp1y air to the center of the vortex， making the f1ame grow to some extent. Jet ang1es and penetration from the ho1es are such that jet impingement a1ong the combustor axis resu1ts in upstream f1ow. The upstream f1ow formsa torroida1 recircu1ation zone， which stabi1izes the f1ame.
Figure 10-.. Flow pattern byswirl vanes and radial jets
Combustion an. Dilution
With torroida1 air f1ow， combustors wi11 operate without visib1e smoke when proper1y deve1oped for a primary-zone equiva1ence ratio be1ow 1.5. Visib1e smoke is an air-po11ution prob1em.
After combustion， the rich burning mixture 1eaves the combustion zone and f1ows between the rows of air jets entering the 1iner. Each jet entrains airand burning fue1 and carries it toward the combustoraxis， forming torroida1 recircu1ation patterns around each jet that resu1t in intensive turbu1ence and mixing throughout the combustor.
This combustion product is di1uted with air entering through ho1es on the 1iner to make the temperature appropriate for b1ade materia1 and to have enough vo1ume-f1ow in the di1ution zone. Air is jet-penetrated main1y because of converging c1earances and creates high 1oca1 pressure.
Film Cooling o. the Liner
The 1iner experiences a high temperature because of heat radiated by thef1ame and combustion. To improve the 1ife of the 1iner， it is necessary to 1ower the temperature of the 1iner and use a materia1 that has a high resistance to therma1 stress and fatigue. The air fi1m coo1ing method reduces the temperature both inside and outside the surface of the 1iner. This reduc-tion is accomp1ished by fastening a meta1 ring inside the 1iner to 1eave a definite annu1ar c1earance. Air is admitted into this c1earance space through rows of sma11 ho1es in the 1iner and is directed by the meta1 rings as a fi1m of coo1ing air a1ong the 1iner inside. Figure 10-9a shows how the f1ow is induced by the static pressure drop across the 1iner surface. ln high air mass f1owcombustors， this pressure drop may be too sma11 to be effective. lt may be necessary to use the tota1 pressure difference in high air mass f1ow combus-tors. This type of arrangement is shown in Figure 10-9b.
Fuel .tomization an. .gnition
ln most gas turbines， 1iquid fue1 is atomized and injected into the com-bustors in the form of a fine spray. A typica1 1ow-pressure fue1 atomization nozz1e is shown in Figure 10-10. The fue1 spray entrains air because of the momentum and drag of fue1 drop1ets; however， this process produces a 1ow-pressure region inside the spray cone that causes it to converge downstream of the nozz1e. This 1ow-pressure region is counteracted by upstream axia1 f1ow ofcombustion products， preventing convergence in the combustion chamber.
中國航空網 www.k6050.com
航空翻譯 www.aviation.cn
本文鏈接地址：燃氣渦輪工程手冊 Gas Turbine Engineering Handbook 2(43)
