
ENGINE SELECTION: PARAMETRIC CYCLE ANALYSIS lO9
Power transmission components.
For those components that merely trans-
mit mechanical power by means of shafts, gears, etc., a simple definition of me-
chanical efficiency is used to account for the losses due, for example, to windage,
bearing friction, and seal drag. In such cases
mechanical power output
TIm
=
(4.11)
mechanical power input
so that rlmn, tlmL, Omen, and t/mPL refer to the high-pressure turbine shaft, low-
pressure turbine shaft, and power takeoffs from the high-pressure shaft and low-
pressure shaft, respectively.
4.2.6 Assumptions
Before proceeding with the analysis, the underlying assumptions to be employed
are summarized as follows:
1) The flow is, on the average, steady.
2) The flow is one-dimensional at the entry and exit of each component and at
each axial station.
3) The fluid behaves as a perfect gas (but not necessarily calorically perfect) with
constant molecular weight across the diffuser, fan, compressor, turbine, nozzle, and
connecting ducts.
4) For the case of variable specific heats, the NASA Glenn thermochemical data
and the Gordon-McBride equilibrium algorithm are used to obtain thermochemical
properties of air and combustion gases at any station (see Chapter 9). For the case
of the calorically perfect gas, Cp and F are assigned one set of values from station
0 through stations 3.1 and 1.6 (denoted as Cpc and Fc), a second set of values from
station 4 through 6 (denoted as Cpt and Ft), a third set of values leaving the mixer
at 6A (denoted as CpM and FM), and a fourth set of values from station 7 through
9 (denoted as CpA8 and FAS).
5) The total pressure ratio of the diffuser or inlet is
Ygd = 7"(dmaxT]Rspec
(4.12a)
where Zrd max is the total pressure ratio caused only by wall friction effects and
OR spec is the ram recovery of military specification MIL-E-5008B (Ref. 5) as
given by
17Rspec
=
1 for M0 < 1 (4.12b)
OR~ve¢=I--O.O75(Mo--1) 1"35 forl <M0 <5 (4.12c)
800
for5 <M0 (4.12d)
1"JR spec -- M4 +
935
6) The fan and low-pressure compressor are driven by the low-pressure turbine,
which can also provide mechanical power for accessories, ProL.
7) The high-pressure compressor receives air directly from the low-pressure
compressor and is driven by the high-pressure turbine, which can also provide
mechanical power for accessories, Pron.
8) High-pressure bleed air and turbine cooling air are removed between stations
3 and 3.1.
9) The flow in the bypass duct (from station 13 to 16) is isentropic.