Turbine Pilot

More jet engine basics: combustion and exhaust

Last month (see " Turbine Pilot: Jet Engine Basics," September Pilot) we got air into our typical gas turbine engine from the front of the engine and moved it through to the compressor section. So far we have high-pressure, low-velocity air at a temperature of several hundred degrees.

Remember that temperature when we consider this as "cooling air."

Bang: the combustion section

The combustion section — usually called the burner can — consists of an outer casing, an inner liner, a fuel injection system, and a starting ignition system. Although the burner can seems to be the simplest of the components of a gas turbine engine, combustor design has sometimes been referred to as "black art." Engineers are not always sure why one combustor design works and another doesn't when installed on the same engine. Obtaining good combustor performance and service life still takes the bulk of engine research and development time and money, much the same as in the early days of gas turbine engine development.

The job of the combustion section is to take the air delivered by the compressor and inject heat energy to the flowing gases by adding fuel and an ignition source. This addition of heat energy causes expansion and acceleration of gases into the turbine section. When heat is added, the resulting volume of gas is increased, and with the flow area remaining the same, the gases must accelerate.

To function properly a combustion chamber must provide for the proper mixing of the air and fuel, as well as cool the hot combustion products to a temperature that the turbine section and, in fact, the combustion chamber itself can withstand. Since turbine fuel is notoriously hard to ignite and has slow flame propagation times, this job is much more difficult than it may sound.

As we have already seen, 75 to 85 percent of the air entering the inlet of a modern turbofan is routed through the bypass ducting. Of the remaining 15 to 25 percent of the air routed to the jet core, only about 25 percent is used in the actual combustion process. Thus, for every pound of air entering the engine inlet, about one ounce is ultimately mixed with fuel and ignited. The rest of the air is used to cool the burner can itself and the turbine section of the engine.

The air used in the combustion process is termed primary air and it is introduced into the burner can via two routes. About one-half of the primary air enters the burner can axially through swirl vanes in the area around the fuel nozzles. The rest of the primary air is introduced into the burner can around its perimeter through small holes in the first third of the can. Both axial and radial primary air are used in the combustion process. The velocity of the primary air in the flame zone immediately in front of the fuel nozzles must be carefully controlled. Although the velocity of the secondary air through the burner can may reach the speed of several hundred feet per second, the primary air is almost stagnant in order to provide the required mixing time for air and fuel. Because of the slow flame propagation time of jet fuels, if primary air velocity were too high it would blow the flame out of the engine and cause a flameout. Although flameout is an uncommon event in modern engines, it still occurs for a variety of reasons. Turbulent weather, heavy precipitation, high-speed maneuvers, and rapid power reduction at high altitude are some of the more typical reasons for flameout. As can be seen, two of these conditions, rapid power reductions and high-speed maneuvers, are under the direct control of the pilot. The other two are either avoidable or controllable, to some extent.

Combustion occurs in the first third of the combustor liner. In the remaining two-thirds of combustor length the products of combustion are mixed with approximately half of the secondary air to provide a temperature at the turbine inlet that is consistent with long service life and even heat distribution. The remaining secondary air is used as a cooling shroud around the inside and outside of the combustion liner, cooling the surface of the liner and centering the flame to prevent it from contacting the metal surfaces. Any disturbance of this secondary cooling air can result in holes burned in the burner can and sharply decreased component life.

Turbine and exhaust: blow

After the exhaust gases exit the combustion chamber they pass through two or more turbine stages on their way to the engine exhaust. The turbine stage transforms a portion of the kinetic and heat energy in the exhaust gases to mechanical work, enabling the turbine to drive the compressor and the accessory section of the engine. The compressor adds energy to air by increasing its pressure. The turbine extracts this energy by reducing the pressure of the flowing exhaust gases. The first-stage turbine wheel is the N ₂, or high-pressure turbine that powers the high-pressure compressor.

The mass of the airflow does not change in the transfer of energy from the combustion chamber to the turbine wheels, but the energy of the flow is reduced. The pressure of the exhaust gases is converted to velocity at the nozzles formed by the turbine blades and stator vanes, and these gases are directed to the rotor. This slows the gas flow axially (front to rear), but adds shaft power to the rotor system. In other words, tangential velocity is a loss of kinetic energy to the engine, but it is a gain of energy to the turbine. The exhaust gases pass next through the low-pressure (N ₁) turbine stage, which powers the low-pressure compressor and fan stages. These turbine stages work to convert kinetic energy into mechanical energy in the same manner as the high-pressure turbine.

The turbine blades and stator vanes are cooled by compressor discharge air — also called bleed air. There are several cooling arrangements; the two main schemes in use today are internal air- flow cooling and surface film cooling. In the internal airflow cooling arrangement, bleed air flows through hollow blades and vanes and carries the heat away after convection cooling. The surface film cooling also uses bleed air routed to the interior of hollow blades and vanes, but this air then flows from small exit ports in the leading and/or trailing edges to form a heat barrier on the surfaces. The cooling air from both arrangements is discharged into the engine airflow at the cooling location. This cooling of the turbine blades and stator vanes allows these components to function at a temperature 600 to 800 degrees Fahrenheit above the temperature limits for the metals and alloys used for blade and vane construction. Maximum turbine inlet temperatures (TIT) of approximately 3,000 degrees F are common in engines of modern design because of this cooling of blades and vanes.

After extraction of energy by the turbine stages, the exhaust gases are discharged through the exhaust cone, tail cone, and tail pipe. The exhaust cone is sometimes referred to as the exhaust collector, as it collects the exhaust gases expelled from the turbine discharge. The tail cone acts as a diffuser within the exhaust cone and serves to reduce the turbulence downstream of the turbine wheel. The exhaust struts, supporting the tail cone, act to return the airflow to an axial direction. The tail pipe is an airframe part and a convergent duct, which causes the [xhaust gases to accelerate to the design speed necessary to produce the required thrust.

Now that we've followed the air all the way through the engine and have produced some power, we're ready to see how the turbine section also provides power for the accessory section of the engine. Stop back next time for a look at starter-generators and fuel control units.

Linda Pendleton, AOPA 525616 , is an art director and author for King Schools. She has accumulated more than 10,000 hours in her 27 years of flying and has given more than 4,000 hours of jet instruction.