Handbook of Turbomachinery Second Edition Revised - Ventech!

espectively. Radial-inflow turbines supplied from a scroll, such as
turbocharger turbines, often have no vanes in the stator. For turbines
that must operate efficiently over a wide range of inlet flow conditions,
variable-geometry stators are used, typically with pivoting stator vanes. For
high-temperature applications, the stator vanes are cooled using lowertemperature
fluid, usually compressor bleed air.
The purpose of the rotor is to extract energy from the flow, converting
it to shaft power. The rotor blades are attached to a rotating disk that
transfers the torque of the rotor blades to the turbine output shaft. Like the
stator, the rotor has a number of individual curved airfoils called rotor or
turbine blades. The blades are angled to accept the flow from the stator with
minimum disturbance when the turbine is operating at design conditions.
The flow from the stator is then turned back in the opposite direction in a
controlled manner, causing a change in tangential momentum and a force to
be exerted on the blades. Figure 4shows cross sections of generic axial-flow
and radial-inflow turbine blades. Axial-flow rotors have been constructed
with blades integral with the disk and with blades individually inserted into
the disk using a dovetail arrangement. Cooling is often used for rotors in
high-temperature applications. Exotic materials are sometimes used for both
rotors and stators to withstand the high temperatures encountered in highperformance
applications.
The flow leaving the turbine rotor can have a significant amount of
kinetic energy. If this kinetic energy is converted to static pressure in an
efficient manner, the turbine can be operated with a rotor discharge static
pressure lower than the static pressure at diffuser discharge. This increases
the turbine power output for given inlet and discharge conditions. Diffusers
used with turbines are generally of the form shown in Fig. 5(a) and 5(b) and
increase the flow area gradually by changes in passage height, mean radius
of the passage, or a combination of the two. Diffusers with a change in
radius have the advantage of diffusing the swirl component of the rotor
discharge velocity as well as the throughflow component.
Turbine Energy Transfer
The combined parts of the turbine allow energy to be extracted from the
flow and converted to useful mechanical energy at the shaft. The amount of
energy extraction is some fraction of the energy available to the turbine. The
following describes the calculation of the available energy for a turbine and
assumes familiarity with thermodynamics and compressible flow.
Flow through a turbine is usually modeled as an adiabatic expansion.
The process is considered adiabatic since the amount of energy transferred
as heat is generally insignificant compared to the energy transferred as work.
Copyright © 2003 Marcel Dekker, Inc.