Handbook of Turbomachinery Second Edition Revised - Ventech!

This is a considerable reduction compared to the discharge pressure of
14.7 psia. From this point, the rotor exit geometry is calculated in the same
way as that presented for the case without the diffuser. The following results
are obtained:
r 2 ¼ 0:0662 lbm=ft 3
A2 ¼ 7:311 in: 2
rt2 ¼ 2:136 in:
rh2 ¼ 1:495 in:
rm2 ¼ 1:816 in:
h2 ¼ 0:641 in:
N ¼ 56;270 rpm
The tip speed is the same as the turbine without the diffuser, since the mean
blade speed is unchanged, as is the hub-to-tip radius ratio of the rotor. The
efficiency calculations also proceed in the same manner as the earlier case
with the following results (using the same inlet pressure loss assumption):
Z 0 0 2 0 ¼ 0:8882
p0 2
p0 0
¼ 0:3663
Z OA ¼ 0:8224
Since this result differs from our original assumption for overall
efficiency, further iterations would be performed to obtain a more accurate
answer. Note the almost 6% increase in overall efficiency due to the
inclusion of a diffuser. This indicates a large amount of energy is contained
in the turbine exhaust. The efficiency gain associated with a diffuser is
dependent on diffuser recovery, rotor exit Mach number, and overall
pressure ratio and is easily calculated. Figure 15 shows the efficiency
benefit associated with a diffuser for an overall turbine pressure ratio
(total-to-static) of 3. Efficiency gains are plotted as a function of rotor exit
critical Mach number and diffuser recovery. As rotor exit Mach number
increases, the advantages of including a diffuser become larger. This
tradeoff is important to consider when sizing the turbine. For a given flow
or power level, turbine rotor diameter can be reduced by accepting high
rotor exit velocities (high values of flow coefficient); however, turbine
efficiency will suffer unless a diffuser is included, adversely impacting the
axial envelope.
Copyright © 2003 Marcel Dekker, Inc.