Axial Flow Compressors

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Axial Flow Compressors
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Axial Flow Compressors

• Elementary theory

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Axial Flow Compressors
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Axial Flow Compressors
Comparison of typical forms of turbine and
compressor rotor blades
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Axial Flow Compressors
Axial Flow Compressors Stage SR S stator
(stationary blade) R rotor (rotating
blade) First row of the stationary blades is
called guide vanes

• Basic operation
• Axial flow compressors
• series of stages
• each stage has a row of rotor blades followed by
  a row of stator blades.
• fluid is accelerated by rotor blades.

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Axial Flow Compressors

• In stator, fluid is then decelerated causing
  change in the kinetic energy to static pressure.
• Due to adverse pressure gradient, the pressure
  rise for each stage is small. Therefore, it is
  known that a single turbine stage can drive a
  large number of compressor stages.
• Inlet guide vanes are used to guide the flow into
  the first stage.

Elementary Theory Assume mid plane is constant
r1r2, u1u2 assume Caconst, in the direction
of u.
, in the direction of u.
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Axial Flow Compressors
Inside the rotor, all power is consumed. Stator
only changes ?K.E.??P static, To2To3 Increase in
stagnation pressure is done in the rotor.
Stagnation pressure drops due to friction loss
in the stator C1 velocity of air approaching
the rotor.
angle of approach of rotor. u blade speed. V1
the velocity relative t the rotor at inlet at an
angle ?1 from the axial direction. V2 relative
velocity at exit rotor at angle ?2 determined
from the rotor blade outlet angle. ?2 angle of
exit of rotor. Ca axial velocity.
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Axial Flow Compressors
Two dimensional analysis Only axial ( Ca) and
tangential (Cw). no radial component
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Axial Flow Compressors
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Axial Flow Compressors
from velocity triangles assuming
the power input to stage
where
or in terms of the axial velocity From equation
(a)
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Axial Flow Compressors
Energy balance
pressure ratio at a stage
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Axial Flow Compressors
Degree of reaction
is the ratio of static enthalpy in rotor to
static enthalpy rise in stage
For incompressible isentropic flow Tdsdh-vdp
dhvdpdp/? Tds0 ?h?p/? ( constant ?) Thus
enthalpy rise could be replaced by static
pressure rise ( in the definition of ?)
but generally choose ?0.5 at mid-plane of the
stage.
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Axial Flow Compressors
?0 all pressure rise only in stator ?1 all
pressure rise in only in rotor ?0.5 half of
pressure rise only in rotor and half is in
stator. ( recommend design)
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Axial Flow Compressors
special condition
?0 ( impulse type rotor) from equation 3
?1-?2 , velocities skewed left, h1h2,
T1T2 ?1.0 (impulse type stator from equation
1) ?1-Ca(tan?1tan?2)/2u, ?2?1 velocities
skewed right, C1C2, h2h3?T2T3
?0.5 from 2
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Axial Flow Compressors
Three dimensional flow 2-D? 1. the effects due to
radial movement of the fluid are ignored. 2. It
is justified for hub-trip ratiogt0.8 3. This
occurs at later stages of compressor.
3-D? are valid due to 1. due to difference in
hub-trip ratio from inlet stages to later-stages,
the annulus will have a substantial taper. Thus
radial velocity occurs. 2. due to whirl
component, pressure increase with radius.
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Axial Flow Compressors
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Axial Flow Compressors

• Design Process of an axial compressor
• (1) Choice of rotational speed at design point
  and annulus dimensions
• (2) Determination of number of stages, using an
  assumed efficiency at design point
• (3) Calculation of the air angles for each stage
  at the mean line
• (4) Determination of the variation of the air
  angles from root to tip
• (5) Selection of compressor blades using
  experimentally obtained cascade data
• (6) Check on efficiency previously assumed using
  the cascade data
• (7) Estimation on off-design performance
• (8) Rig testing

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Axial Flow Compressors

• Design process
• Requirements
• A suitable design point under sea-level static
  conditions (with 1.01 bar and , 12000 N as take
  off thrust, may emerge as follows
• Compressor pressure ratio 4.15
• Air-mass flow 20 kg/s
• Turbine inlet temperature 1100 K
• With these data specified, it is now necessary to
  investigate the aerodynamic design of the
  compressor, turbine and other components of the
  engine. It will be assumed that the compressor
  has no inlet guide vanes, to keep weight and
  noise down. The design of the turbine will be
  considered in Chapter 7.

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Axial Flow Compressors

• Requirements
• choice of rotational speed and annulus
  dimensions
• determination of number of stages, using an
  assumed efficiency
• calculation of the air angles for each stage at
  mean radius
• determination of the variation of the air angles
  from root to tip
• investigation of compressibility effects

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Axial Flow Compressors

• Determination of rotational speed and annulus
  dimensions
• Assumptions
• Guidelines
• Tip speed ut350 m/s
• Axial velocity Ca150-200 m/s
• Hub-tip ratio at entry 0.4-0.6
• Calculation of tip and hub radii at inlet
• Assumptions Ca150 m/s
• Ut350 m/s to be corrected to 250
  rev/s

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Axial Flow Compressors

• Equations
• continuity
• thus

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Axial Flow Compressors

• procedure

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Axial Flow Compressors

• From equation (a)

N
260.6 0.2137 0.4
246.3 0.2262 0.5
227.5 0.2449 0.6
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Axial Flow Compressors

• Consider rps250
• Thus rr/rt0.5, rt0.2262, ut2?rtrps355.3 m/s

Is ok. Discussed later. Results r-t0.2262,
r-r0.1131, r-m0.1697 m
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Axial Flow Compressors

• At exit of compressor

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Axial Flow Compressors

• No. of stages
• ?To overall 452.5-288164.5K
• rise over a stage 10-30 K for subsonic
• ?4.5 for transonic
• for rise over as stage25
• thus no. of stages 164.5/25?

- normally ?To5 is small at first stage de
haller criterion V2/V1 gt 0.72 - work factor can
be taken as 0.98, 0.93, 0.88 for 1st, 2nd, 3 rd
stage and 0.83 for rest of the stages.
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Axial Flow Compressors

• Stage by stage design
• Consider middle plane
• stage 1
• for no vane at inlet

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Axial Flow Compressors

• Angles

check de Haller
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Axial Flow Compressors
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Axial Flow Compressors

• Second stage

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Axial Flow Compressors
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Axial Flow Compressors
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Axial Flow Compressors
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Axial Flow Compressors
6 5 4 Stage
2.968 2.447 1.992
405 381 357
1.199 1.213 1.228
3.560 2.968 2.447
429 405 381
0.592 0.521 0.455
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Axial Flow Compressors

• Stage 7
• At entry to the final stage the pressure and
  temperature are 3.56 bar and 429 K. the required
  compressor delivery pressure is 4.151.014.192
  bar. The pressure ratio of the seventh stage is
  thus given by

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Axial Flow Compressors

• the corresponding air angles, assuming 50 per
  cent reaction, are then ?150.98,

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Design calculations using EES

• "Determination of the rotational speed and
  annulus dimensions"
• "Known Information"
• To_1288 K Po_1101 kPa m_dot20kg/s
  U_t350 m/s
• ifnot ParametricTable
• Ca_1150m/sr_r/r_t0.5cp1005R0.287Gamma
  1.4
• endif
• GamrGamma/(Gamma-1)
• m_dotRho_1Ca_1A_1 "mass balance"
• A_1pi(r_t2-r_r2) "relation between Area and
  eye dimensions"
• U_t2pir_tN_rps
• C_1Ca_1
• T_1To_1-C_12/(2cp)
• P_1/Po_1(T_1/To_1)Gamr
• Rho_1P_1/(RT_1)
• TabStops 0.5 2 in

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Design calculations using EES
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Design calculations using EES
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Design calculations using EES
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Design calculations using EES
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Design calculations using EES
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Design calculations using EES