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National Advisory Committee for Aeronautics, Technical Notes - Construction and Use of Charts in Design Studies of Gas Turbines

A method is presented for the computation and graphic representa-
tion of a series of possible turbine designs for any specific applica-
tion. The use of a preliminary design chart is suggested for deter-
mining the number of stages and the effects of exit whirl and annular—
area divergence on possible configurations. A specific design chart
can then be made to aid in the study of the relations beimeen turbine
radius ratio, diameter, and significant design parameters such as Mach
numbers, turning angles, and blade root stresses.

In designing turbines for aircraft engines, it is desirable to
obtain units having high reliability, light weight,- low frontal area,
simplicity, and high efficiency. Because the number of variables
involved in designing turbines is large and the relations among them
are complex, determination of the best design compromises may become
quite involved. A method of computation and graphic presentation of
a series of possible turbine designs for any specific application is
presented herein to aid in determining these compromises.

Usually the first step in a design study is to obtain an estimate
of the number of stages required, the type of velocity diagram to be
used, and the need for annular flow-area divergence. A preliminary
design chart is suggested for this purpose. Such a chart indicates
whether a single-stage turbine will do the Job with any specified
centrifugal stress at the blade root, Mach number, and size limitations
that may be imposed. The preliminary chart also indicates the effects
of exit whirl and annular flow-area divergence. If a single-stage
turbine is inadequate, each stage of a multistage unit can be checked
against the design limitations so that an estimate can be made of the
number of stages required.

With the information obtained from the preliminary chart, a spe-
cific design chart can be made for each stage of the required turbine.
The specific design chart presents the parameters that describe the
flow within the stage in terms of velocities and flow angles and relates
these parameters to design variables such as size, radius ratio, cen-
trifugal stress at the blade root, rotative speed, and specific Weight
flow. This chart contains sufficient information for fixing the
dimensions of the unit.

The results obtained by the methods given in this report should be
considered as preliminary in nature because of the simplifying assump—
tions that are used. These results serve to eliminate areas in which
design lbmitations cannot be satisfied and indicate the points for
which final analyses should yield suitable designs.