Operation theory Turbine

schematic of impulse , reaction turbines, rotor rotating part, , stator stationary part of machine.

a working fluid contains potential energy (pressure head) , kinetic energy (velocity head). fluid may compressible or incompressible. several physical principles employed turbines collect energy:

impulse turbines change direction of flow of high velocity fluid or gas jet. resulting impulse spins turbine , leaves fluid flow diminished kinetic energy. there no pressure change of fluid or gas in turbine blades (the moving blades), in case of steam or gas turbine, pressure drop takes place in stationary blades (the nozzles). before reaching turbine, fluid s pressure head changed velocity head accelerating fluid nozzle. pelton wheels , de laval turbines use process exclusively. impulse turbines not require pressure casement around rotor since fluid jet created nozzle prior reaching blades on rotor. newton s second law describes transfer of energy impulse turbines. impulse turbines efficient use in cases flow low , inlet pressure high.

reaction turbines develop torque reacting gas or fluid s pressure or mass. pressure of gas or fluid changes passes through turbine rotor blades. pressure casement needed contain working fluid acts on turbine stage(s) or turbine must immersed in fluid flow (such wind turbines). casing contains , directs working fluid and, water turbines, maintains suction imparted draft tube. francis turbines , steam turbines use concept. compressible working fluids, multiple turbine stages used harness expanding gas efficiently. newton s third law describes transfer of energy reaction turbines. reaction turbines better suited higher flow velocities or applications fluid head (upstream pressure) low.

in case of steam turbines, such used marine applications or land-based electricity generation, parsons-type reaction turbine require approximately double number of blade rows de laval-type impulse turbine, same degree of thermal energy conversion. whilst makes parsons turbine longer , heavier, overall efficiency of reaction turbine higher equivalent impulse turbine same thermal energy conversion.

in practice, modern turbine designs use both reaction , impulse concepts varying degrees whenever possible. wind turbines use airfoil generate reaction lift moving fluid , impart rotor. wind turbines gain energy impulse of wind, deflecting @ angle. turbines multiple stages may utilize either reaction or impulse blading @ high pressure. steam turbines traditionally more impulse continue move towards reaction designs similar used in gas turbines. @ low pressure operating fluid medium expands in volume small reductions in pressure. under these conditions, blading becomes strictly reaction type design base of blade solely impulse. reason due effect of rotation speed each blade. volume increases, blade height increases, , base of blade spins @ slower speed relative tip. change in speed forces designer change impulse @ base, high reaction-style tip.

classical turbine design methods developed in mid 19th century. vector analysis related fluid flow turbine shape , rotation. graphical calculation methods used @ first. formulae basic dimensions of turbine parts documented , highly efficient machine can reliably designed fluid flow condition. of calculations empirical or rule of thumb formulae, , others based on classical mechanics. engineering calculations, simplifying assumptions made.

turbine inlet guide vanes of turbojet

velocity triangles can used calculate basic performance of turbine stage. gas exits stationary turbine nozzle guide vanes @ absolute velocity va1. rotor rotates @ velocity u. relative rotor, velocity of gas impinges on rotor entrance vr1. gas turned rotor , exits, relative rotor, @ velocity vr2. however, in absolute terms rotor exit velocity va2. velocity triangles constructed using these various velocity vectors. velocity triangles can constructed @ section through blading (for example: hub, tip, midsection , on) shown @ mean stage radius. mean performance stage can calculated velocity triangles, @ radius, using euler equation:

Δ
h
=
u
⋅
Δ

v

w




{\displaystyle \delta h=u\cdot \delta v_{w}}

hence:

Δ
h

t


=



u
⋅
Δ

v

w



t




{\displaystyle {\frac {\delta h}{t}}={\frac {u\cdot \delta v_{w}}{t}}}

where:

Δ
h


{\displaystyle \delta h}

specific enthalpy drop across stage




t


{\displaystyle t}

turbine entry total (or stagnation) temperature




u


{\displaystyle u}

turbine rotor peripheral velocity




Δ

v

w




{\displaystyle \delta v_{w}}

change in whirl velocity

the turbine pressure ratio function of






Δ
h

t




{\displaystyle {\frac {\delta h}{t}}}

, turbine efficiency.

modern turbine design carries calculations further. computational fluid dynamics dispenses many of simplifying assumptions used derive classical formulas , computer software facilitates optimization. these tools have led steady improvements in turbine design on last forty years.

the primary numerical classification of turbine specific speed. number describes speed of turbine @ maximum efficiency respect power , flow rate. specific speed derived independent of turbine size. given fluid flow conditions , desired shaft output speed, specific speed can calculated , appropriate turbine design selected.

the specific speed, along fundamental formulas can used reliably scale existing design of known performance new size corresponding performance.

off-design performance displayed turbine map or characteristic.