The combustion process in aircraft engines and gas turbines is one in which heat is added to the compressed air in the combustor or burner. Thus, the combustion is a direct-fired air heater in which fuel is burned. The combustor is situated between the compressor and turbine, where it accepts air from the compressor and delivers it at an elevated temperature to the turbine. Some engines have a second combustion system that either reheats the flow for the later turbine stages (as described in Chapter 8) or burns more fuel in an afterburner behind the turbines to provide the high exhaust velocity required for the propulsion of supersonic aircraft (as described in Chapters 4 and 5). A part of the supplied energy provides useful work absorbed by the turbine(s) and the remainder goes to waste as heat in the exhaust gas [1]. Combustion in gas turbines is a continuous process that takes place at high pressure in a smaller space and usually at a very high temperature. Fuel is burned in the air supplied by the compressor; an electric spark is required for initiating the combustion process, and thereafter the flame must be self-sustaining.