Figure 1-... Air distri.ution in a typical com.ustor.
temperature. Normal combustion temperatures range from 3400 0F (1871 0C) to 3500 0F (1927 0C). At this temperature， the volume of nitric oxide in the combustion gas is about 0.01%. If the combustion temperatureislowered， the amount of nitric oxide is substantially reduced.
Typical Combustor .rrangements
There are different methods to arrange combustors on a gas turbine. Designs fall into four categories:
1. Tubular (side combustors)
2. Can-annular
3. Annular
4. External (experimental)
Can-annular and .nnular. In aircraft applications where frontal areais important， either can-annular or annular designs are used to produce favorable radial and circumferential profiles because of the great number of fuel nozzles employed. The annular design is especially popular in newaircraft designs; however， the can-annular design is still used because of the developmental difficulties associated with annular designs. Annular com-bustor popularity increases with higher temperatures or low-Btugases， since the amount of cooling air required is much less than in can-annular designs due to a much smaller surface area. The amount of cooling air required becomes animportant consideration in low-BTU gas applications， since most of the air is used up in the primary zone and little is left for film cooling. Development of a can-annular design requires experiments with only onecan， whereas the annular combustor must be treated as a unit and requires much more hardware and compressor flow. Can-annular com-bustors can be of the straight-through or reverse-flow design. If can-annularcans are used in aircraft， the straight-through design is used， while a reverse-flow design may be used on industrial engines. Annular combustors are almost always straight-through flow designs. Figure 1-25 shows a typicalCan Annular combustor used in Frame type units， with reverse flow. Figure 1-26 is a tubo-annular combustorused in aircraft-type combustors， and Figure 1-27 is a schematic of an annular combustor in an aircraft gas turbine.
Tubular (side combustors). These designs are found on large industrialturbines， especially European designs， and some small vehicular gas turbines.They offer the advantages of simplicity ofdesign， ease of maintenance， and long-life due to low heat release rates. These combustors may be of the
Figure 1-.5. A typical reverse flow can-annular com.ustor.
""straight-through"" or ""reverse-flow"" design. In the reverse-flow design airenters the annulus between the combustor can and itshousing， usually a hot-gas pipe to the turbine. Reverse-flow designs have minimal length. Figure 1-28 shows one such combustor design.
.xternal Combustor (experimental). The heat exchanger used for an external-combustion gas turbine is a direct-fired air heater. The air heater"s goal is to achieve high temperatures with a minimum pressure decrease. It consists of a rectangular box with a narrow convection section at the top. The outer casings of the heater consist of carbon steel lined with lightweight blanket material for insulation and heat re-radiation.
The inside of the heater consists of wicket-type coils (inverted ""U"")supported from a larger-diameter inletpipe， and a return header running along the two lengths of the heater. The heater can have a number of passes for air. The one shown in Figure 1-29 has four passes. Each pass consists of11 wickets， giving a total of 44 wickets. The wickets are made of differentmaterials， since the temperature increases from about300-1，700 0F.Thus， the wickets can range from 304 stainless steel to RA330 at the high-temperature ends. The advantage of the wicket design is that the smooth transition of ""U"" tubes minimizes pressure drops. The U-shaped tubes also allow the wicket to freely expand with thermal stress. This feature eliminates the need for stress relief joints and expansion joints. The wickets are usually
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