Equilibrium Operating Performance of Axial-Flow Turbojet Engines by Means of Idealized Analysis Page: 2 of 12
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REPORT 987--NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS
V velocity, feet per second f
V, compressor-blade-tip velocity, feet per second
'W mass flow, slugs per second
7 ratio of specific heat at constant pressure to specific
heat at constant volume
8 ratio of compressor-inlet absolute total pressure to
NACA standard sea-level absolute pressure
0 ratio of compressor-inlet absolute total temperature
to NACA standard sea-level absolute temperature
p mass density, slugs per cubic foot
0b pressure coefficient
1 compressor inlet
2 compressor outlet
3 turbine inlet
4 turbine outlet
5 nozzle outlet
DESCRIPTION OF COMPONENT MODELS
In order to establish theoretical equilibrium operating con-
ditions, the performance of each engine component must be
described. The descriptive equations for each component
and the development of the analysis are given in the appen-
dix. The principal equations of the analysis are described
in the following paragraphs.
. . . . . . ... ... . . .=.
Compressor.-The development of the analysis requires
that a simple expression for compressor mass flow be known.
The ratio of inlet-air velocity to compressor-blade-tip veloc-
ity K, for axial-flow compressors has been found to be
approximately constant, particularly at or near design speed.
Hence, with the use of this relation, the following expression
for compressor air flow WT' can be developed:
Po c =KMc(rr) ly
The power to drive the compressor may be expressed as
550 C"' --) -70-
Turbine.-In order to obtain a simplified expression for
the gas flow through the turbine W,, the turbine and the
turbine nozzle were considered similar to a simple channel of
varying cross section. The expression for It is then written
P0 o -'jiAtT r
The turbine power may be expressed as
.lhp,=- 5 - q RT3, 1
(Herein,- A is the turbine minimum-passage area.)
Exhaust nozzle.-The exhaust nozzle is described in the
same manner as the turbine and the equation for the flow
through the exhaust nozzle becomes
Po- (.--1)R-~.L' . rKr
The assumptions are now stated and all descriptive equa-
tions are combined to define equilibrium operating perform-
ance of the complete engine.
Assumptions.-Equation (3) is used as an approximation
for W, in order to avoid complications in the equilibrium
analysis. For low-reaction turbines and turbines operating
at or near choking conditions, the approximation is of suffi-
cient accuracy. A further assumption made using equation
(3) is that the turbine pressure ratio used in calculating
equilibrium performance is considered to be the total-
pressure ratio from the inlet to the outlet of the turbine. The
turbine pressure ratio in equation (3) should be the total-to-
static pressure ratio across the turbine; however, the error
in this assumption is small. Figure 1 shows the magnitude
of error in using r, as the total-to-total pressure ratio in the
rather than the total-to-static pressure. The error reaches
a maximum of about 5 percent at low values of r,. If a
more accurate solution is required, a factor Ccan be included
in equation (3), as shown, to correct the erior.
The burner pressure ratio rb appearing in equations (3)
and (5) represents a measure of the pressure drop in the
burner. In order to make the equations as simple as pos-
sible, this pressure ratio was included rather than a more
complicated equation for the loss.
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Sanders, John C. & Chapin, Edward C. Equilibrium Operating Performance of Axial-Flow Turbojet Engines by Means of Idealized Analysis, report, February 25, 1949; (digital.library.unt.edu/ark:/67531/metadc60326/m1/2/: accessed December 12, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.