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Compresores

1. Axial Flow Compressors

2. INTRODUCTION Axial compressors  are rotating,  airfoil -based  compressors  in which the working fluid principally flows parallel to the axis of rotation. This is in contrast with other rotating compressors such as centrifugal, axi -centrifugal and mixed-flow compressors where the air may enter axially but will have a significant radial component on exit. For more details on this topic, see  Centrifugal compressor . Axial flow compressors produce a continuous flow of compressed gas, and have the benefits of high efficiencies and large mass flow capacity, particularly in relation to their cross-section. They do, however, require several rows of airfoils to achieve large pressure rises making them complex and expensive relative to other designs (e.g.  centrifugal compressor ). Axial compressors are widely used in  gas turbines , such as  jet engines , high speed ship engines, and small scale power stations. They are also used in industrial applications such as large volume air separation plants, blast furnace air, fluid catalytic cracking air, and propane dehydrogenation. Axial compressors, known as  superchargers , have also been used to boost the power of automotive reciprocating engines by compressing the intake air, though these are very rare.

3. Schematic representation of an axial flow compressor It is easy to design a turbine that will work…. It requires a considerable skill to design a compressor that will work…

4. Aerofoil Geometry 1 : zero lift line 2 : leading edge 3 : nose circle 4 : camber 5 : thickness 6 : upper surface 7 : trailing edge 8 : main camber line 9 : lower surface

5. Macro Geometric Specification of An Axial Compressor The geometry of a compressor can be categorised into 3 main designs types, A Constant Outer Diameter (COD), A Constant Mean Diameter (CMD) or A Constant Hub Diameter (CID),

6. Specifications of An Axial Compressor There are several different parameters that can specify a particular compressor. The first set of input parameters are based on the running conditions for the machine. These involve mass flow, pressure ratio , rotational speed and the number of stages. Stage degree of reaction : For controlling the distribution of the load between the rotor and the stator. If this is not of importance, the outlet flow angle for the each stage must be set instead.

7. V a1 2 /c p p 1 T 1 p 01 T 01 p 03 = p 02 T 03 = T 02 V a2 2 /c p p 2 V a3 2 /c p p 3 s T Thermodynamics of An Axial flow Compressor Stage

8. Kinematics of An Axial Flow Compressor Stage Inlet Velocity Triangle Outlet Velocity Triangle

9. Kinetics of An Axial Flow Compressor Stage Rate of Change of Momentum: Inlet Velocity Triangle Outlet Velocity Triangle Power Consumed by an Ideal Moving Blade

10. Energy Analysis of An Axial Flow Compressor Stage Inlet Velocity Triangle Outlet Velocity Triangle Change in Enthalpy of fluid in moving blades :

11. Isentropic compression in Rotor Blade Degree of Reaction of A Stage, R :

12. Power input to the compressor : Current Practice : Theoretical Power input to the compressor: Inlet Velocity Triangle Outlet Velocity Triangle

13. For an isentropic compressor:

14. Axial Flow Compressors Axial Flow Compressors Stage= S+R 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.

15. Axial Flow Compressors Inside the rotor, all power is consumed. Stator only changes  K.E.  P static, To2=To3 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.

16. 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 r1=r2, u1=u2 assume Ca=const, in the direction of u. , in the direction of u.

17. Axial Flow Compressors Two dimensional analysis: Only axial ( Ca) and tangential (Cw). no radial component

18. Axial Flow Compressors

19. Axial Flow Compressors from velocity triangles assuming the power input to stage where or in terms of the axial velocity From equation (a)

20. Axial Flow Compressors Energy balance pressure ratio at a stage

21. Axial Flow Compressors Degree of reaction is the ratio of static enthalpy in rotor to static enthalpy rise in stage For incompressible isentropic flow Tds=dh-vdp dh=vdp=dp/ Tds=0 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.

22. 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)

23. Axial Flow Compressors special condition =0 ( impulse type rotor) from equation 3 1=-2 , velocities skewed left, h1=h2, T1=T2 =1.0 (impulse type stator from equation 1) =1-Ca(tan1+tan2)/2u, 2=1 velocities skewed right, C1=C2, h2=h3T2=T3 =0.5 from 2

24. 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 ratio>0.8 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.

25. Axial Flow Compressors

26. 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

27. 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.

28. 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

29. Axial Flow Compressors Determination of rotational speed and annulus dimensions: Assumptions Guidelines: Tip speed ut=350 m/s Axial velocity Ca=150-200 m/s Hub-tip ratio at entry 0.4-0.6 Calculation of tip and hub radii at inlet Assumptions Ca=150 m/s Ut=350 m/s to be corrected to 250 rev/s

30. Axial Flow Compressors Equations continuity thus

31. Axial Flow Compressors procedure

32. 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

33. Axial Flow Compressors Consider rps250 Thus rr/rt=0.5, rt=0.2262, ut=2  rt*rps=355.3 m/s Is ok. Discussed later. Results r-t=0.2262, r-r=0.1131, r-m=0.1697 m

34. Axial Flow Compressors At exit of compressor

35. Axial Flow Compressors No. of stages  To =overall = 452.5-288=164.5K rise over a stage 10-30 K for subsonic  4.5 for transonic for rise over as stage=25 thus no. of stages =164.5/25  - normally  T o5 is small at first stage de haller criterion V2/V1 > 0.72 - work factor can be taken as 0.98, 0.93, 0.88 for 1 st , 2 nd , 3 rd stage and 0.83 for rest of the stages .