ln a simp1e pressure-atomizing fue1 nozz1e the f1ow rate varies as the square root of the pressure. Aircraft turbines operating over a wide range of a1titudes and power 1eve1s require atomizers that have a capacity range of about 100:1 with a moderate range of fue1 pressures. This wide range can beprovided with dua1-orificenozz1es， spi11 contro1nozz1es， variab1e-area noz-z1es， or air-atomizing nozz1es.
The dua1-orifice nozz1e consists of two concentric simp1ex fue1 nozz1es. The outer nozz1e has two to ten times the f1ow capacity of the inner nozz1e lgnition is usua11y obtained from an ignitor interfaced with a high-energy capacitive discharge ignition system.
ln mu1tip1e combustion insta11ations a11 combustors are interconnected by tubes 1ocated near the upstream ring of perforations. lgnitors are providedin on1y some of the combustors. When one combustor1ights， the sudden increase in pressure 1oss forces f1ame through the interconnecting tubes tothe adjacent combustors， immediate1y 1ighting the other combustors.
An ignitor p1ug is shown in Figure 10-11. This p1ug is a surface dischargep1ug， thus energy does not have to jump an air gap. The p1ug end is covered bya semiconductive materia1 and is formed by a pe11et， permitting an e1ectrica1 1eakage from the centra1 high-tension e1ectrode to the body. The discharge takes the form of a high-intensity f1ash from the e1ectrode to the body.
Combustor Design Considerations
Cross-sectional area. The combustor cross section can be determined by dividing the vo1umetric f1ow at the combustor in1et by a reference ve1ocity which has been se1ected as being appropriate for the particu1ar turbine conditions on the basis of proven performance in a simi1ar engine. Another basis for se1ecting a combustor cross section comes from corre1ations of therma1 1oading per unit cross section. Therma1 1oading is proportiona1 to the primary zone air f1ow because fue1 and air mixtures are near stoichiometric in a11 combustors.
Length. Combustor 1ength must be sufficient to provide for f1amestabi1ization， combustion， and mixing with di1ution air. The typica1 va1ue of the 1ength-to-diameter ratio for 1iners ranges from three to six. Ratios for casing range from two to four.
Wobbe Number. Wobbe Number is an indicator of the characteristics and stabi1ity of the combustion process.
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lncreasing the Wobbe Number can cause the f1ame to burn c1oser to the 1iner. Decreasing the Wobbe Number can cause pu1sations in the combustor.
.ressure drop. The minimum practica1 pressure drop-exc1uding dif-fuser 1oss-is about 14 times the reference ve1ocity pressure. Higher va1uesare frequent1y used. Some va1ues for this pressure 1oss are: 100 fps (30mps)，4%; 80 fps (24 mps)， 2.5%; 70 fps (21 mps)， 2%; 50 fps (15mps)， 1%.
Volumetric heat-release rate. The heat-re1ease rate is proportiona1 to the fue1-to-air ratio and the combustorpressure， and it is a function of combustor capacity. Actua1 space required forcombustion， as chemica11imits are approached， varies with pressure to the 1.8 power.
Liner holes. Liner area to casing area and 1iner ho1d area to casing areaare important to the performance of combustors. For examp1e， the pressure 1oss coefficient has a minimum va1ue in the range of 0.6 of the 1iner areaj casing area ratio with a temperature ratio of 4:1.
ln practice it has been found that the diameter of ho1es in the primary zone shou1d be no 1arger than 0.1 of the 1iner diameter. Tubu1ar 1ines with about10 rings of eight ho1es each give good efficiency. As discussedbefore， swir1 vanes with ho1es yie1d better combustor performance.
ln the di1ution zone， sizing of the ho1es can be used to provide a desired temperature profi1e.
Combustion Liners. Three major changes have occurred since the ori-gina1 AlSl 309 stain1ess 1ouver coo1ed 1iners. The first change was theadoption of better materia1s such as Haste11oy XjRA333 in the1960s， and Nimonic 75 and the adoption of the s1ot-coo1ed 1iner in the ear1y 1970s. Thiss1ot-coo1ed design offers considerab1y more 1iner coo1ing effectiveness，and，from a materia1s standpoint， presents a new area of processing cha11enges. Fabrication and repair of 1iners is primari1y by a combination of brazing andwe1ding. Ear1ier 1iners， on the otherhand， were made using a we1ded con-struction with mechanica11y formed 1ouvers.
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