47
Chapter 2
Basic Components
© Springer Science+Business Media Dordrecht 2015
E. Dick, Fundamentals of Turbomachines, Fluid Mechanics and Its Applications 109,
DOI 10.1007/978-94-017-9627-9_2
Abstract We learned in Chap. 1 that blades of axial machines have profiles
resembling aircraft wing profiles ( aerofoils). In machines with large spacing
between blades, this resemblance is strong, as with the hydraulic turbines and
pumps studied up to now. With axial compressors, gas and steam turbines,
blades are positioned closer together. The blades also cause a larger flow turning. A circumferential section results in a row of blade profiles with tangential
spacing comparable to, or smaller than, the largest profile dimension. We then
apply the term blade row or blade cascade. Radial machine rotor blades principally do not function as lifting objects. The blades constitute channels. The
blade profiles have no resemblance to aerofoils. Channel flows may be accelerating (turbines) or decelerating (pumps, fans, compressors). With decelerating
or diffusion flows, avoidance of separation between flow and geometry is difficult. As we have already learned, diffusers also occur as stator components of
turbines. Aerofoils, cascades, channels and diffusers constitute the basic components of turbomachines, which we study in this chapter.
2.1 Aerofoils
2.1.1 Force Generation
Figure 2.1 sketches the streamlines close to an aerofoil in an oncoming flow with
uniform velocity v ∞ and fluid density ρ ∞ far upstream. The streamlines are intended
to curve due to camber of the profile (curvature in the longitudinal direction) and
due to incidence of the flow (angle difference between the oncoming flow and the
profile). To grasp the effect of the curvature, the simplest is considering an observer
moving along with a fluid particle. This observer then stands still relative to the
flow. For the relative coordinate system with origin on the observer, with x-axis
along the flow and y-axis perpendicular to it, a centrifugal force must then be introduced with a value v
2
/R, perpendicular to the streamline, so along the y-axis, away
from the centre of curvature ( R is the radius of curvature). In the flow normal direction (y-axis) the momentum equals zero. Conservation of momentum thus means a
balance of forces. This means that the centrifugal force must be kept in balance by a
Chapter 2
Basic Components
© Springer Science+Business Media Dordrecht 2015
E. Dick, Fundamentals of Turbomachines, Fluid Mechanics and Its Applications 109,
DOI 10.1007/978-94-017-9627-9_2
Abstract We learned in Chap. 1 that blades of axial machines have profiles
resembling aircraft wing profiles ( aerofoils). In machines with large spacing
between blades, this resemblance is strong, as with the hydraulic turbines and
pumps studied up to now. With axial compressors, gas and steam turbines,
blades are positioned closer together. The blades also cause a larger flow turning. A circumferential section results in a row of blade profiles with tangential
spacing comparable to, or smaller than, the largest profile dimension. We then
apply the term blade row or blade cascade. Radial machine rotor blades principally do not function as lifting objects. The blades constitute channels. The
blade profiles have no resemblance to aerofoils. Channel flows may be accelerating (turbines) or decelerating (pumps, fans, compressors). With decelerating
or diffusion flows, avoidance of separation between flow and geometry is difficult. As we have already learned, diffusers also occur as stator components of
turbines. Aerofoils, cascades, channels and diffusers constitute the basic components of turbomachines, which we study in this chapter.
2.1 Aerofoils
2.1.1 Force Generation
Figure 2.1 sketches the streamlines close to an aerofoil in an oncoming flow with
uniform velocity v ∞ and fluid density ρ ∞ far upstream. The streamlines are intended
to curve due to camber of the profile (curvature in the longitudinal direction) and
due to incidence of the flow (angle difference between the oncoming flow and the
profile). To grasp the effect of the curvature, the simplest is considering an observer
moving along with a fluid particle. This observer then stands still relative to the
flow. For the relative coordinate system with origin on the observer, with x-axis
along the flow and y-axis perpendicular to it, a centrifugal force must then be introduced with a value v
2
/R, perpendicular to the streamline, so along the y-axis, away
from the centre of curvature ( R is the radius of curvature). In the flow normal direction (y-axis) the momentum equals zero. Conservation of momentum thus means a
balance of forces. This means that the centrifugal force must be kept in balance by a