FM 1-506 Fundamentals of Aircraft Power Plants ... - Survival Books

WWW.SURVIVALEBOOKS.COMFM 1-506CHAPTER 7COOLING SYSTEMThe intense heat generated when fuel and air areburned mandates that some means of cooling beprovided for all internal combustion engines.Reciprocating engines are cooled either by passing airover fins attached to the cylinders or by passing a liquidcoolant through jackets surrounding the cylinders.Cooling is made easier because combustion occurs onlyduring every fourth stroke of a four-stroke-cycle engine.In contrast, the burning process in a gas turbine engine iscontinuous, and nearly all of the cooling air must passthrough the inside of the engine. If enough air wereadmitted to the engine to provide an ideal air-fuel ratioof 151, internal temperatures would increase to morethan 40000°F. In practice, a large amount of air exceedingof the ideal ratio is admitted to the engine. This largesurplus of air cools the hot sections of the engine toacceptable temperatures ranging from 1100° to 1500°F.COOLING OF ENGINE OUTER CASEFigure 7-1 illustrates the approximate engine outercase(skin) temperatures encountered in a properlycooled dual-axial compressor turbojet engine. Becauseof the effect of cooling, the temperatures of the outsideof the case are considerably less than those encounteredinside the engine. The hottest spot occurs opposite theentrance to the first stage of the turbine. Although thegases have begun to cool a little at this point, the conductivityof the metal in the case carries the heat directly tothe outside skin.COOLING OF COMBUSTION CHAMBER ANDGAS PRODUCERThe air passing through the engine the combustionchamberburner cans or liners. The cans are constructedto induce a thin, fast-moving film of air over both the innerand outer surfaces of the can or liner. Can-annular-typeburners are frequently provided with a center tube to leadcooling air into the center of the burner to promote highcombustion efficiency and rapid dilution of the hot combustiongases while minimizing pressure losses. In alltypes of gas turbines, large amounts of relatively cool air7-1