Gas turbine/reciprocating engines

inlet ducts

provide the proper amount of high pressure, turbulence-free air to the compressor

single entrance duct

simplest and most effective

divided-entrance duct

reduces friction losses due to length but boundary layer and skin friction distort the air

also cannot be made very large without increasing drag

subsonic and super sonic inlet design

divergent shape

convergent-divergent shape

variable geometry inlet

utilizes mechanical devices such as ramps, wedges, or cones to change shape of the inlet duct as the aircraft speed varies

compressor

supply enough air to satisfy the requirements of the combustion section

three types of compressors

centrifugal

axial

axial-centrifugal

centrifugal flow components and functions

impeller: accelerates the air outward towards the diffuser and increases pressure

diffuser: stationary and convert high velocity air to high pressure

manifold: directs it to the combustion chamber

characteristics of the centrifugal flow compressor

adv: rugged, low cost, good power output over wide range of RPMs, high pressure increases per stage

dis: large frontal area required, impractical for multiple stages

axial flow compressors

air flows in a straight line with two main elements rotor blades and stator vanes

axial flow compressor operation

rotors driven by the turbine increasing velocity and pressure and air goes to the stator vanes which decrease velocity and increase pressure and passes onto the next stage of rotor/stator

dual spool axial flow compressor

compressor is divided into two completely independent rotor spools, each driven by its own turbine and drive shaft (low-pressure and high-pressure compressor)

low-pressure compressor

provide initial P increase to airflow arriving from the inlet

high-pressure compressor

provides further increase to airflow pressure

axial-centrifugal flow compressors

combination of axial and centrifug. large pressure increase yet small enough for helicopters and small aircraft

axial-centrifugal operation

axial section allow for ‘straight through’ ram efficiency and multiple stages for high pressure. the centrifugal portion significantly increases pressure through its one stage

inlet guide vanes

impart a swirling motion to the air entering the compressor in the direction of engine rotation

exit guide vanes aka straightening vanes

located at the discharge end and prepare airflow for the diffuser by straightening the airflow to reduce airflow turbulence as it comes off the rotational movement of the compressor

percentage of primary and secondary air from the compressor

25

75

burner section

provides the means for proper mixing of the fuel and air to assure good combustion

criteria considered when designing the burner section

minimize pressure decrease through the burner

combustion efficiency must remain high

the flame must not blow out

all burning must be complete before the gases enter the turbine section

three general types of combustion chambers

can

annular

can-annular

can combustion chamber

individual combustion cans arranged around the circumference of the burner section

adv and dis adv of can combustion

adv: strength and durability, ease of maintenance, individual cans can be replaced and inspected without disturbing the rest of the engine

dis: poor use of space, greater pressure loss, and uneven heat distribution

annular combustion chamber

fuel is introduced through a series of nozzles where it is mixed and ignited with the incoming air

adv and dis of annular combustion

adv: uniform heat distribution across face of the turbine section, better mixing of the air and fuel

dis: unit cannot be removed without disassembling the engine

can-annular combustion chamber

consists of cans at the front where the fuel and air are mixed and burned. hot gases then pass to the annular area of the chamber where they are mixed together

adv and dis of can-annular combustion chamber

adv: even heat distribution, greater structural stability, lower pressure loss

dis: expensive

turbine

drives the compressor and the accessories and increase airflow velocity

how does acceleration occur in the turbine

stators, rotors, or both

most highly stressed part of the engine

the turbine

creep

elongation of he blades as they are heated

fir tree method

the way rotor blades are attached to the turbine

exhaust

direct the flow of hot gases rearward to cause a high exit velocity to the gases while preventing turbulence

parts of the exhaust

exhaust outer duct, exhaust inner cone, and three or four radial hollow struts

two types of exhaust nozzles and speeds their used for

convergent = subsonic

convergent divergent = supersonic

afterburning / thrust augmentation

method used in turbojets and turbofans to increase the maximum thrust available from an engine by 50% or more

fuel consumption increase using afterburner

300% increase

4 parts of the afterburner

spray bars, flame holders, screech liner, variable exhaust nozzle

spray bars

introduce fuel to the afterburner and located in forward section of the duct

flame holder

region where airflow velocity is reduced and turbulent eddies are formed for proper mixing of air and fuel

screech liner

control the screech to prevent damage from vibrations

variable exhaust nozzle aka “turkey feathers”

can close for basic engine subsonic operation or open to allow gases to expand at the proper rate when the afterburner is being used